LUMINESCENCE IN METEORITIC AND LUNAR SILICATES

by

G. Walker M.Sc. Tech.

Thesis submitted to the University

of Manchester for the degree of

Doctor of Philosophy.

January, 1971

ABSTRACT

Investigations of proton-excited luminescence spectra

of s ilica te mineral phases present in meteoritic and lunar

material have been carried out« Low energy proton excitation

was used, orig in a lly on account of the possible relevance of

such studies to possible luminescence of the lunar surface

excited by the solar wind. Powdered samples are irradiated by

protons from a small 10 - 120 KeV proton accelerator and the

resulting luminescence emission is scanned by a photoelectric

grating spectrophotometer. Improvements to the existing apparatus

are described along with a b rie f description of the accelerator

and spectrophotometer.

The design and construction of an all-metal low energy

( 2 - 8 KeV) proton/electron accelerator is described for the

study of integrated luminescence and radiation damage. A cryostat

which attaches to this accelerator has been constructed for the

purpose of thermoluminescence studies and a linear heating-rate

temperature controller has been designed and built for programming

the sample temperature. Preliminary comparisons of the 3I 0W

curves of meteoritic and synthetic enstatites have been made.

The luminescence spectra of over 20 stony meteorites

have been measured and found to be characteristic of the class

of meteorite. The mineral phases responsible for the luminescence

emission have been iden tified and the nature of the luminescence

centres in these minerals is discussed. The estimated luminescence

-2 -5effic ien c ies are, in general, in the range 10 to 10 , the most

e ff ic ien t luminescent meteorites being the enstatite achondrites

which are an order of magnitude more e ff ic ien t than any other

class o f meteorite. The luminescent mineral phase responsible is

enstatite which gives either a predominantly red or blue emission

depending on the manganese content. The red emission (peak at

6700 A) is due to Mn2+ substituting for Mg2+ in the M0 metal

cation s ite in the la tt ic e . 'Hie blue emission is thought to be

due to an unidentified la tt ic e defect. Enstatite is also the

major luminescent component in the enstatite chondrites.

The variations in luminescence spectra of synthetic

enstatites and fo rster ites with manganese and iron content have

also been investigated. I t is shown that the Mn2+ emission is

quenched to a greater degree than the blue emission when iron is

introduced into the la tt ic e . Large iron concentrations

Fe SiOg) result in very weak luminescence, the spectral distribution

of which is practically independent of manganese content. Factors

7+governing the intensity and position of the Mn' emission band

in enstatites and fo rs ter ites are discussed.

Plagioclase is found to be the major luminescent

component in bronzite and hypersthene chondrites and pyroxene-

plagioclase achondrites, although in the former group accessory

amounts o f apatite also contribute to the luminescence in a minor

way.

The e ffe c t of proton radiation damage on the spectra

and effic iency of meteorite samples and related synthetic phosphors

has been examined. I t is shown that some workers who have

reported that the luminescence e ffic ien c ies of sulphides and

s ilica tes under KeV electron and proton irradiation d iffe r by

orders of magnitude have not appreciated the rapid in itia l

deterioration in luminescence e ffic ien cy which can occur when

12 2using a proton flux > 10 particles/cm /sec.

The major luminescent component in lunar material

has been shown to be plagioclase which exhibits two emission

bands in the v is ib le region . The predominant yellow-green

emission band (peak at 5600 t) is shown to be probably due to

2+ 2+Mn substituting for Ca' . Comparison with te rres tria l plcgioclases

shows that an infra-red emission band which is often the most

intense emission in te r re s tr ia l samples is the least intense

emission band in lunar and meteoritic plagioclase. The luminescence

efficiency of lunar material is a function of the plagioclase

content and is very low ( < 10 ) for Apollo 11 fines but usually

higher for Apollo 12 fin es. Luminescence effic ien cies of rock

-5 -4samples are in the range 10 - 2 . 10 .

C O N T E N T S

Page

PREFACE

ACKNOWLEDGEMENTS

HISTORICAL FOREWARD

CHAPTER I - INTRODUCTION.

1. The composition and structure of s ilicates 1

1.1 General description 1

1.2 Pyroxenes 2

1.3 Plagioclases 3

2. C lassification of meteoritic stones 4

3. The theory of luminescence in solids

3.1 General considerations 6

3.2 Crystal fie ld theory 12

3.3 The Mn2+ ion 16

3.4 Cathodoluminescence and Ionoluminescence 19

3.5 Ion-radiation damage in luminescentmaterials 23

3.6 Thermoluminescence 26

4. Possible luminescence and radiation damage

of the lunar surface

4.1 Possible lunar luminescence 29

4.2 Radiation damage of the lunar surface 32

CHAPTER II - INSTRUMENTATION.

1. The 120 KeV proton accelerator system

1.1 General description 35

1.2 The proton source and accelerator tube 36

Page

1.3 Power supplies 37

1.4 The sample chamber 37

1.5 Modifications to vacuum system 38

1.6 The new pumping system 40

2. The photoelectric spectrophotometer

2.1 The monochromator and photomultipliers 43

2.2 The electronic recording system 45

2.3 Spectral response calibration 47

3. The new low energy proton accelerator system

3.1 Design and general description 48

3.2 The ion source and accelerating system 49

3.3 The sample chamber and vacuum system 53

3.4 The electronic recording system 54

4. Thermoluminescence instrumentation

4.1 The cryostat 55

4.2 The linear heating-rate controller 57

4.3 Temperature measurement and recording 61

CHAPTER I I I - LUMINESCENCE IN METF.ORITIC SILICATES

1. Previous work 63

2. The luminescence spectra of enstatiteachondrite meteorites 64

3. Luminescence of synthetic enstatites andforsterites

3.1 Preparation 68

3.2 Spectra of iron-free enstatites andforsterites 69

3»? Spectra of ferromagnesian pyroxenes andolivines 71

4, The luminescence o f other classes o f stonymeteorites 72

4.1 The enstatite chondrites 73

4.2 Bronzite and hypersthene chondrites 74

4.3 The pyroxene-plagioclase achondrit.es 78

4.4 The hypersthene achondrites 79

4.5 The olivine-pigeon ite achondrites 79

4.6 The o livine-p igeon ite and carbonaceouschondrites 80

5, The e ffec t of proton irradiation on luminescenceeffic iency 81

6 . Preliminary thermoluminescence measurements. 85

7. Discussion

2+7.1 The wavelength o f the Mn emission inenstatite and fo rsterite 38

7.2 Other factors a ffecting the luminescenceemission in enstatite 91

7.3 Comparative e ff ic ien c ie s of electron andproton-excited luminescence 92

COPTER IV - LUMINESCENCE IM LUNAR SILICATES.

1. The luminescence of lunar surface material.

1.1 Luminescence spectra of lunar fines 94

1.2 Luminescence spectra of lunar rocks andbreccias 98

2. Luminescence spectra o f terrestria l plagioclases 99

3. Discussion

3.1 The Mn + emission 101

3.2 The blue and in fra-red emission bands 1 ( »

Page

APPENDIX

1. The electrostatic getter-ion ("o rb itron ") pump.

1.1 Operating principles and design

1.2 Testing of a prototype

1.3 Materials and maintenance

1.4 Power supplies

3F.FERI-.XES

107

109

112

113

PREFACE

After graduation from Glasgow University in 1962

with a B.Sc. Honours Degree in Natural Philosophy, I undertook

research in the Faculty of Technology of this University* I

was awarded a D.S.I.R. research studentship in October, 1962

and was appointed Special Research Assistant in Physics in

January, 1964,

In December, 1965, I was awarded the degree of

M.Sc* Tech, for a thesis on "Variations of Fluorescence E fficiencies

and Energy Transfer in Organic Systems»" I was appointed Assistant

Lecturer in Physics in October, 1966 and in October, 1969 I was

appointed Lecturer in Physics*

None of the research work presented in this thesis has

been submitted in support of any degree at this or any other

University.

January, 1971

Relevant publications!- see references 6 - 1 1

ACKNOWLEDGEMENTS

F irstly , I wish to thank my Supervisor, Dr. J.E. Geake,

fo r his encouragement and guidance throughout this work. Thanks

are also due to Professor H. Lipson for the fa c ilit ie s offered by

the Physics Department.

I am grateful to my colleague, Dr. M.D. Lumb and former

colleague Mr. C.J. Derham fo r discussions regarding the design and

operation of the original equipment.

I would also like to express my gratitude to Mr. J. McConnell,

Mr. A. Manwaring and Mr. M. Gould for technical assistance at various

stages of the project.

I am also grateful to my associates in other Departments

in th is and other Universities who have either carried out analyses

on my behalf or prepared samples used in th is work or helped in any

way. They are gratefully acknowledged at the appropriate points

in the text.

In addition, I would like to thank Dr. C.H. Kemp for

help with some of the diagrams and Mr. M. Gould for help with the

photographs included in this thesis.

I also wish to convey my thanks to my wife, Susan, for

typing this thesis and for her forbearance whilst it was being written.

Finally, thanks are due to the United States A ir Force

and the Science Research Council who provided funds for much of the

equipment used, to the British Museum (Natural History) for the loan

of a large variety of samples of meteorltic stones and fo r helpful

advice on their selection, end to N.A.S.A. fo r the generous provision

of lunar samples returned by the Apollo 11 end 12 missions.

HISTORICAL FORWARD

The work described in this thesis is basically a

continuation o f a project which was orig in a lly embarked upon in

1959 by J.E. Geake and M.D. Lumb\ This project, which was formerly

financed by the U.S.A.F., aimed at the investigation o f the luminescence

o f mineral rock samples believed to be similar to those existing on

the lunar surface. The f i r s t four years were concerned with the design

2 3and construction of a photo-electric spectrophotometer * and a proton4

accelerator . The f ir s t results to emerge from this project were

published by C.J. Derham and J.E. Geake in 1964 giving the emission

spectra of enstatite achondrite meteorites under proton irradiation .

I t is from this point that the work w ill be described in

th is thesis.

During the years 1964-9, the main interest in the work

became centred around the luminescence properties of meteoritic stones^’^’

including an investigation of the spectra o f natural and synthetico

manganese activated magnesium metasilicates .

A second proton accelerator system with metal seals was

constructed in the years 1967-70 for the study of radiation damage

effects and thermoluminescence of silicate samples, and this system

and its associated instrumentation w ill be described in deta il.

During 1969 the original proton accelerator and associated

equipment were extensively overhauled and modified in readiness for

the lunar samples which were received later that year. In particular,

the vacuum system was modified and the punping equipment entirely

replaced in the interests of providing a contamination free environment

f o r th e lunar sam ples under in v e s t i g a t io n .

In 1969-70, following the successful Apollo 11 and 12

missions, luminescence studies were carried out on lunar dust and

rock chip sample

1.

1• The composition and structure o f s ilic a te s .

1.1 General Description.

The basic building block o f a l l s ilica tes is the

(SiO^)4 tetrahedron. This tetrahedron is practically a close-

packed structure o f four oxygen ions since the central silicon

12ion is very small by comparison . I t is generally assumed that

s ilica tes are predominantly ionic structures consisting of metal

cations and s ilic a tetrahedral anions with each oxygen carrying

some negative charge1

The simplest s ilica te structure is that o f the

orthosilicates in which the Si04 tetrahedra are regularly stacked

and linked by divalent metal ions which l ie between them. The

o liv in e group o f minerals, (Mg, Fe)2 Si04 , is an example of this

type of structure (see Fig. l ) 14a. The metal ions are in six-fo ld

co-ordination in two possible sites, and Mg, with slightly

15d iffe r in g symmetries and metal-oxygen distances •

Si04 tetrahedra may, however, share oxygen ions and

linear chains or rings o f tetrahedra are formed when two oxygens

are shared. The group o f minerals known as pyroxenes consist of

such linear chains of tetrahedra, the chains being linked together

by divalent metal ions, as in the mineral enstatite MgSiO^.

Chains o f tetrahedra may join together in pairs to form

double chain structures known as amphiboles. Furthermore, i f three

oxygens in a ll tetrahedra are shared then the tetrahedra form

continuous layers or sheets which are again linked by various

cations to give the necessary charge balance. This layered

structure is found in the mica group o f minerals.

2

I f a l l four oxygen Ions are shared between adjacent

tetrahedra then a three-dimensional framework structure results«

In this case no metal cations are necessary for charge balance

and thus pure s ilica minerals such as quartz have this type of

structure.

In almost a ll these types o f basic s ilica te structures

which occur naturally, the small silicon ion is often replaced

3+to some extent by A1 , which is only s ligh tly larger in size,

12to form the aluminous s ilica tes • This substitution obviously

leads to charge imbalance which is compensated fo r by the

introduction o f further metal ions into the structure. The

3+ 4^substitution o f A1 for Si in a framework structure leads to

the introduction o f metal ions where none existed previously.

In fact, the very important group of minerals known as feldspars

are formed in th is way. The plagioclase feldspars are aluminous

s ilica tes of calcium and sodium. Potassium feldspars (e .g . orthoclase

KAlSi^Og) are also important rock forming minerals but these w ill

not be discussed here. Of main interest here are the non-oluminous

pyroxenes, the plagioclases and to a lesser degree, the olivines.

Pyroxenes.

Pyroxenes are major constituents in many types of

igneous rocks. They occur in almost a ll stony meteorites and in

lunar rocks and fines. Investigations presented here are mainly

restricted to the enstatite MgSiO^ - ferrosilite FeSiO^ series,

which is to be found in meteoritic stones. This type of ferromagnesium

silicate is referred to as enstatite i f it contains less than 10$

iron silicate . In an equilibrated sample the iron is uniformly

distributed throughout the crystal. I f the iron content is between

10 and 20 mole $ then the mineral is referred to as bronzite. When

the iron content is greater than 20 mole $ the mineral is called

3

hypersthene. These crite ria are those of meteoric!sts1^8 and,

in fact, some mineralogists and petrologists have somewhat

d ifferen t boundary cr ite ria . However, the meteoricists boundary

values w ill be used here.

There are three possible crysta l structures fo r th is

series} two o f which are orthorhombic and one monoclinic. In

the case of enstatite, they are referred to as orthoenstatite,

protoenstatite and clinoenstatite, the last named being monoclinic.

Orthopyroxene is the most common type and its structure is shown

14bin Fig. 3 . The symmetries of the two possible cation s ites

and are also shown and the metal-oxygen distances quoted

are for a hypersthene with about 50 mole % iron . The M1 site

is approximately octahedral symmetry but the M2 s ite is considerably

distorted being non-centrosymmetriC and elongated along the

b - axis. Since i t has been shown that metal-oxygen distances

18increase sligh tly with an increase in iron content , the metal-

oxygen distances for orthoenstatite are lik e ly to be s ligh tly

less than those quoted in Fig. 3.

The two other types of structure are similar and occur

on accovxit of relative shifts of the chains of tetrahedra along

the c - axis. The present state of knowledge with regard to the

crystal chemistry of pyroxenes has recently been reviewed by

Zussman1 .

1.3 Plagioclases.

Plagioclase is found as a major constituent in most

igneous rocks, including lunar rocks and fines, and is present in

many stony meteorites. Plagioclases are named according to the

proportion of albite (Ab), NaAlSigOg, and anorthite (An),

CaAl2Si20g, in their composition as followst-

4

less than 1C?$ An - a lbite - more than 90?$ Ab

10-30?$ An - oligoclase - 70-90?$ Ab

30-50?$ An - andesine - 70-50?$ Ab

50-70?$ An - labradorlte - 50-30?$ Ab

70-90?$ An - bytownite - 30-10?$ Ab

over 90?$ An - anorthite - less than 10?$ Ab2+ +

Cations other than Ca or Na may, o f course, be present as small

impurities but in basic igneous rocks, plagioclase is often the

only mineral which is re la t iv e ly iron free. (The term "basic"

applied to igneous rocks means that the rock has a low s ilic a

content and the s ilica present is only found in the various s ilica te

phases and not in the free state)#

The structures o f a lbite and anorthite are both tr ic lin ic

20 21and have been determined in deta il ' • In a lb ite , one quarter

o f the silicon has been replaced by triva len t aluminium and

monovalent sodium provides charge compensation. In anorthite,

3+every alternate tetrahedra has an A1 core and thus exactly half

the silicon is replaced. Divalent calcium restores the charge

balance. In the anorthite structure the calcium is very irregularly

co-ordinated (see Fig. 2 ). There are four s ligh tly d ifferen t cation

s ites which are seven-fold co-ordinated i f metal-oxygen distances 0

up to 3A are counted. The average metal-oxygen distance is aboutO

2.5A which is somewhat larger than in orthopyroxenes and o liv ines

(2.1 - 2.2A).

2. C lassification o f meteoritic stones.

Stony meteorites consist essentia lly o f s ilica te material

with smaller amounts o f n ickel-iron, sulphides, and other minerals.

Except for some carbonaceous meteorites, the s ilic a te takes the form

of pyroxene, olivine or plagioclase. The generally accepted

classification of meteorites was firs t proposed by Prior in 1920

and is based mainly on mineralogical composition. The f ir s t major

division of stony meteorites is between chondrites and achondrites.

A chondrite contains spheroidal stones called chondrules usually

of olivine or pyroxene composition which are usually of the order

of a mm or so in diameter. Achondrites are characterized by the

absence of such chondrules. This division seems at firs t to be

one of texture rather than of mineralogy. However, there are

well-defined differences in mineralogy between say a hypersthene

chondrite and a hypersthene achondrite, the former containing

considerably more olivine than the latter.

Further division of the achondrites results in calcium-

rich achondrites and calcium-poor achondrites. The former usually

contain appreciable amounts of plagioclase or calcium-rich pyroxene;

the latter contain li t t le or no plagioclase and consist mainly of

non-calcic or calcium-poor pyroxenes.

Both chondrites and achondrites are named according to

the predominant or characteristic mineral phase or quite often

according to the two most predominant minerals e .g . the pyroxene-

plagioclase achondrites. The only exception to this rule is the

carbonaceous chondrites which are so called on account of the

presence of organic material although chemical analysis shows a carbon

content of less than 4#. A fu ll discussion of the classification of

meteorites has been given by Mason1 * and the classification of a

particular meteorite can be ascertained by reference to the Prior-

22Hey catalogue .

The peculiar names associated with meteorites are the

result of naming a particular meteorite according to the place name

of the location in which it f e l l or was found.

5 .

The olivine structure projected onto (100). The two possible

metel co-ordination sites and are shown and metal-oxygenO

distances for each site indicated in A on the left*

(A fter Bums 14* ) .

The anorthite structure* This is an inclintd projection on

(010) of parts of structure bounded by the planes y ■ 1 0.3*

Heavy lines indicate the upper part of layer shown*

(A fter Kempster et a l 2^ ).

T^B 8’fKVCTt'RK OK ANOHTHITK, C«AI/i,<>,

Fig. 3. Th» orthopyroxene structure. The upper part of the figure i *

the structure projected onto (100) showing clearly the elisini

of s ilica tetrahedra. The middle part of the figure shows the

structure projected onto (001), and the lower part shows the

two possible metal co-ordinations and M , projected onto

(100), with metal-oxygen distances indicsted in A.

(Modified from Bums ^ * * ) .

The theory of luminescence In solids.

General considerations»

Luminescent solids are generally electrical insulators

or semi-conductors. For a crystalline solid to show luminescence

there must be special sites in the lattice in which any absorbed

energy has a good probability of being radiatively emitted

rather than dissipated into lattice vibrations. These sites are

called luminescence centres. Such centres are usually provided

by either lattice imperfections (e .g . vacancies) or impurity ions

which are referred to as activators. However, not a ll such

imperfections or impurities w ill provide luminescence centres

and in fact such defects may give rise to "k ille r" centres which

quench luminescence.

Obvious possib ilities for impurity activators are

elements in which transitions of electrons in an incomplete electron

shell are shielded to some extent by outer bonding electrons.

Elements of the transition series are of this type and indeed many

ions from these series do act as luminescence centres in many solids.

The divalent manganese ion, which w ill be discussed in detail

la ter, is the most important activator in silicates and in many

other minerals (e .g . ca lc ite , spinels, apatites, e tc . ). Rare earth

ion activators often give rise to quite narrow band emission owing to

an optical electronic transition in the 4 f shell which is particularly

well-shielded.

i \ *The emission band usually occurs at longer wavelengths

than the corresponding absorption band (Stokes' law) and the

wavelength shift between absorption and emission is termed the

Stokes' sh ift. The reason fo r this shift is easy to see on a

configuration co-ordinate-energy diagram (see Fig. 4 ). In such

< id ■ 1 5

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7

a diagram the potential energy is plotted as a function of the

distance of the activator ion from the surrounding la t t ic e ions for

d ifferen t electronic states. In general, the equilibrium position

( i . e . the minimum of the P.E. curve) is different for d ifferen t

electronic states. Absorption and emission take place in accordance

with the Franck-Condon princip le i . e . the electronic configuration

changes before the heavier ions have time to a lter th e ir positions.

Therefore, such transitions appear as vertical lines on the diagram.

Since at normal temperatures the ion is usually in the lowest

vibrational leve l o f the ground state, absorption resu lts in a

high vibrational level o f the electronic excited state. This excess

vibrational energy is , however, rapidly dissipated into the la ttic e

and emission takes place from the lowest vibrational le v e l of the

excited state to a high vibrational level of the ground state. The

Franck-Condon principle is not exact but, nevertheless, transitions

which conform to i t are more probable.

I f an activator centre is in an excited state then the

probability of a radiative e le c tr ic dipole transition to the

ground state depends on the e lec tr ic transition dipole moment

23a 2M|cn end is indeed proportional to|Mj{nJ

where - eJ - g r. ^ (1 .1 )

where is the position vector of the i th particle o f charge

z^e in the luminescence centre. I f k • n then M is simply the

dipole moment in state \jr» Since M)cn is a reel physical property

of the centre it must be invariant to symmetry operations* Thus

cannot change sign on account of such an operation. I f for

any pair of wave-functions the integral does change sign on account

of a symmetry operation, then i t must be zero and the transition

between these states is said to be forbidden.

8

The "allowedness" of a transition is usually indicated by

quoting the oscillator strength of the transition f kn.

« " • ” 'k„ ■ ,0” ' i k„ I !knl 2 « • «

««here V kn is the average wavenumber of the transition* The

oscillator strength is a ratio comparing the intensity of a given

transition with that of an allo««ed transition at the same frequency

for a three dimensional harmonic oscillator. For strongly allowed

transitions f is usually slightly less than unity* Alternatively

the radiative lifetime *^rad may be used as a measure of

forbiddennesst since ^ rad - (1 .3)

For f ■ 1 (strongly allowed) and y ■ 20,000 cm“1 (5000 A),

X rad is approximately 4.10“^ sec*

The radiative lifetime is the reciprocal of the radiative

transition probability for spontaneous emission

I S k n l 2 »•< >

I f the decay of the excited state of the activator is the only

rate determining step in the luminescence process, the luminescence

decay w ill be exponential . I f N is the number of emission

centres in the excited state at time t then the intensity of

emission I . “£ [ - (K ♦ K .) N (1.5)dt 1 1

Integration yields N ■ Noe" ^Kf + t

or I - ! V < *f \ * 0 '(1.6)

where XQ is the luminescence intensity at t ■ 0, when centres

are in the excited state* is the probability of non-radiative

de-activation* The actual observed lifetime of the emission

1Kf + Ki

which tends to i f is small compared

1- ni:‘ V; - » ' 7 ' • •>' f. ; V

l f •• (

«1«

II

t * x X

7 ?

. *ir

phí:«ÍY n< I

*xc

yj ; ■! ; . ' J.t

] bf*

o j c » i o

9

with K_. The efficiency of the luminescence process within a

Kfcentre i s *h » _ _ _ _ _ _ which obviously decreases as K.I Kf ♦ Kj

increases. Kj is temperature dependent but it Kf

the efficiency w ill be sensibly temperature independent. With

increasing temperature, however, Kj w ill increase until i t becomes

comparable with Kf and thus a reduction of efficiency w ill then

be apparent. Temperature quenching can be understood by

reference once again to the configuration co-ordinate diagram

(F ig. 4 ) . At the point E where the P.E. curves for ground state

and excited state closely approach each other, a non-radiative

transition from the excited state to the ground state is possible.

The activation energy W can be supplied thermally and the

probability of non-radiative de-activation w ill be given by

_ I»Kj ■ Ce where C is some constant. I f the point E occurs

very near the minimus of the excited state P.E. curve then it is

obvious that luminescence w ill be improbable unless the temperature

is lowered* I f the point E occurred at a position co-ordinate

between that for the minima for the ground state and excited state

23cthen luminescence would not be possible • In general, a large

displacement of equilibrium position between ground state and

excited state results in a high probability of non-radiative

transitions. Thus the shielding of inner unfilled electron shells

in transition metals is not necessarily the reason why luminescence

occurs at a given temperature, but i t is rather the position of

24the point E which appears to be the determining factor •

The luminescence decay may not be exponential i f the

transition probability within the centre is not the only rate -

determining step. I f the probability of emission also depends on

c j. i I

o Jti d. ' ' ' ' - *l 51 + vN

*; ;+ j, i i f< ] '• i;;V

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of* f " : < i

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t n ,,i • . V 'H IV ; i ‘ i ’I ' i Ylimi, r e ; n i ( r i ‘ i t n* • ’ . i 1J .u’

to nolttvoq erii ledisr ai i i ,

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eiit ’ll Ir i ti'on<x,xe od i f n y «m Y1 ofh ‘.«r uecs niii'iii e<r r

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MO f Iv.'» ;?>l) cals nolai'lme to y f l • gnlnl»neit»h

10,

the number of available centres, then the kinetics would be o fOOW jij O

second order i .e . I * ’ — Qf N t It is easy to show

that on integration this leads to a decay which is hyperbolic*

In practice the decay of fluorescence may be complex involving

a mixture of different components of firs t or second order kinetics.

It i 8 necessary to distinguish between fluorescence and

phosphorescence. Phosphorescence is defined as that emission

which has occurred when the excited electron has resided for a

time after excitation in a metastable "trapping" state before

transferring to the emitting state. The term fluorescence is

reserved for emission which occurs without such prior "trapping"

of the excited electron. In general, the lifetime of phosphorescence

is longer than for fluorescence for a particular phosphor, but

very long fluorescence lifetimes can occur when the emission is

the result of a transition which is forbidden for electric dipole

radiation.

Models used to describe possible mechanisms of luminescence

in the so-called photoconducting phosphors invariably use the

23d 24well-known band theory of solids as a basis ’ , Luminescence

centres, quencher or "k ille r" centres and trapping centres are

assumed to give rise to localised levels between the valence and

conduction bands (see Fig, 6 ), An impurity or defect centre may

introduce an empty localised energy level below the conduction band

which hss a high probability of capturing an electron from the

conduction band but negligible probability of subsequently capturing

a hole from the valence band. Such a level is known as an electron

trap i f normally empty and as a donor level i f it is normally

occupied. Similarly, f i l le d localised levels lying above the valance

Fig. 4 .

Fig. 5»

Fig. 6.

Configuration co-ordinate - energy diagram for a luminescence

centre* AB is an absorption transition and CD an emission

transition* E is a close approach of ground state and excited

state P.E. curves and x represents the distance from the

24centre to nearest neighbour atoms. (After Garlick * ) .

Configuration co-ordinate - energy diagram for the Mn2* ion

in calcite. (A fte r Medlin25)»

Energy band theory model for a photoconducting phosphor*

Emission centre leve ls are indicated by A and trapping levels

by C* (A fter Garlick^*).

band which have a high probability o f capturing a hole from the

valence band are termed hole traps» or, i f such a level is normally

empty, an acceptor le ve l. The position o f the Fermi leve l governs

whether a particular localised leve l is normally occupied or not.

Intermediate energy leve ls which have comparable capture

probabilities for electrons and holes constitute recombination

centres. I f an electronic transition involved in the recombination

process is radiative then the centre is a luminescence centre. A

luminescence centre may involve more than one intermediate level

and may be formed by an associated pair o f donor and acceptor leve ls .

Whilst manganese activated phosphors are not usually

considered as photo-conducting phosphors, excitation of the

manganese centres without excitation o f electrons into the

conduction band may be d if f ic u lt on account of the forbiddenness

o f possible absorption transitions in the divalent manganese ion.

Certain transition metal ions (Fe2+, F e^ , Ni2+, Co2+)

¿o not usually act as luminescence centres in solids and in fact

often act as "k ille r " centres. The quenching mechanism of these

ions does not appear to be fu lly understood. Early attempts to

explain the k ille r effect of these ions assuned that they

introduced hole traps into the solid and that energy transfer was23e 25

by positive hole migration in the valence band ’ • More recently,

several workers have concluded that k i l le r ions give rise to deep

electron trapping levels which on account of their depth do not usually

give rise to phosphorescence or show up in thermoluminescence ’ #

However, such transition metal ions may function as emission centres

in a spectral region outside the visib le range. Since the absorption

transitions between ground state and the lower excited states

invariably occur in the infra-red for these ions in various lattices,

12

one might expect luminescence emission in the in fra-red.

24 2+Garlick has in fact shown that. Co gives r is e to infra-red

29emission in zinc sulphide and recently Reynolds and Garlick

2+have found infra-red emission due to Mi in various phosphors

at low temperatures. At higher temperatures non-radiative

de-octivation predominates.

3.2 Crystal Field Theory.

On account of the shielding e ffect o f the outer bonding

electrons on the un filled 3d shell of the f i r s t transition series

elements, the energy leve ls of such a transition-metal ion in a

la ttic e can be described as the energy levels o f the free ion which

are subject to a perturbation on account of the presence o f the

surrounding anions or ligands.

In predominantly ionically-bonded structures the transition-

metal ion is considered to be in a resultant e lec trosta tic f ie ld

due to the surrounding anions which, in the f ir s t approximation, are

considered to be point charges. This simple theory has met with

considerable success in qu alita tive ly explaining many aspects of

30transition-metal chemistry • The e ffe c t on the energy leve ls of

the transition-metal ion, which substitutes in a la tt ic e cation

position, depends on the symmetry and intensity o f the crysta lline

fie ld and therefore, in particular, on the type and position o f the

surrounding anions.

The d-orbitals o f an atom or ion are fiv e - fo ld degenerate

(neglecting spin) and can accommodate up to ten electrons. Each

orbita l has, in general, four lobes extending along two mutually

perpendicular axes. The sign of the wave-function for lobes

extending in opposite directions is the same and opposite to that

13.

for lobes extending in a perpendicular direction. With respect to

a set o f cartesian axes x, y, z along which the p-orbitals l i e ,

three o f the d-orbita ls designated d , d , d have lobesxy’ yz ' xz

projecting in directions making equal angles with the axes specified .

degeneracy and the subscript 2 indicates that the orb ita l does not

change sign on rotation about axes diagonal to the cartesian axes

x, y, z. The subscript g refers to inversion symmetry and w il l

be explained la te r. The remaining two d-orbitals, designated

o f these orbita ls as possible with parallel spins since this gives

lowest energy i f the electron interaction energy is greater than the

energy o f interaction with the crystal f ie ld . In a perfectly

spherical f ie ld the energy of the d-orbitals would simply be increased

but in a non-spherically symmetry environment the d-orbita l energy

levels are sp lit and at least some degeneracy is lo s t. In a simple

cubic, tetrahedral or octahedral co-ordination the leve ls sp lit

into two leve ls , the eo group forming one and the t? group the other.

In octahedral co-ordination with six identical anions or ligands

situated on the cartesian axes x, y, z, the lobes of the e^ orb ita ls

w ill be repelled to a greater extent than tp^ lobes since e^ lobes

are directed towards the ligands. Hence e orb ita ls w ill have higher

designated by a crystal f ie ld parameter A for octahedral co-ordination,o

These orb ita ls are designed tp o rb ita ls : t re fers to the three-fold

^x*._ y a.and ^za. » have lobes directed along the cartesian axes

and are described as e orb ita ls, e referring to the two-fold

degeneracy.

According to Hund's rule, electrons w ill occupy as many

In a tetrahedral or cubic co-ordination3

* - t 0 energy separation is

14

A t for cubic, etc» The sp lit levels obey a 'centre of g rav ity '

rule about the energy leve l that a ll d-orbitals would have i f

the intensity o f the crystal f ie ld was identical but spherically

symmetric, Tn octahedral co-ordination, therefore, the e levels3 . 9

are — above the leve l for the spherically symmetric case and2

the t 0 levels _ A below i t . Since the t „ levels are lower ¿g 5 o 2g

in energy they w ill be occupied before the e leve ls . Each electron

in a t^^ orbita l therefore stabilizes the transition-metal ion by

? A c whilst each electron in an e orbital de-stabilizes the ion

3 A 9by 5 . The nett stabilization energy of an ion in a particular

environment is called the crystal fie ld stabilization energy

(C.F.S.E.) and re la tive values o f this parameter for d ifferen t sites

give an indication as to what co-ordination a particular ion prefers.

However, transition-metal ions with the d configuration (h a lf-

f i l le d shell) such as Mn?+ or Fe^+ have zero C.F.S.E. in a l l

possible co-ordinations.

The crystal f ie ld sp litting parameter A is not the same

for a ll co-ordination s ites } in fact

A (octahedral) * A. (tetrahedral) A„ (cubic)c 4 t o C

The minus sign indicates reversal o f the order o f t ?rj and e^ groups.

I f the co-ordination is distorted from a simple symmetry,

as is often the case, further sp littin g of both the t j and e^

leve ls may occur and each d-orbita l energy level may become non­

degenerate, neglecting spin degeneracy (see Fig. 8) .

2+Calculations o f the C.F.S.E. fo r Fe ions in M1 and

sites in orthopyroxenes have been made from the positions o f the

2+absorption bands which show that Fe ' has a slight preference fo r

the site in spite of the fact that A is estimated to be

l . r 3. r fo r ,h* “1 14C- H « .v . r , th . F .»* h, vln3 . d6

configuration gains additional C.F.S.E. in a distorted octahedral

site re la tive to an undistorted one since the sixth electron enters

the lowest t ^ leve l which has sp lit to an even lower energy. Thus4 A J

the s ite which is more distorted than the s ite is preferred •

17 *31X-ray d iffraction studies and MWssbauer measurements' on

orthopyroxenes have confirmed this s ite preference.

15.

The size of A for a transition-metal ion depends on

several factors. I t is larger for trivalent ions than for divalent

ions and its magnitude w ill also depend on the type o f ligand with

which it is co-ordinated. I t depends c r it ic a lly on the metal-ligand

distance R since A oC —c, 1 i f other variables mentioned remain R3

fixed, where Q is the charge on the ligands. F inally, the magnitude

o f the crystal f ie ld sp littin g parameter depends on the co-ordination

symmetry, as mentioned above, although for highly distorted sites

A loses some of its significance since many d ifferen t energy

separations w ill then ex is t.

In simple crystal fie ld theory it is assumed that there

is no overlap o f metal and ligand orbitals which is never absolutely

true for structures in which at least some degree o f covalent

bonding occurs. In fact, theoretically calculated values of the

crystal f ie ld sp littin g parameter using the simple e lec trosta tic

theory are invariably smaller than values derived experimentally

R2from absorption spectra • There is also evidence from electron

spin resonance measurements that even in fluorides such as MnFp some

delocalisation of d-electrons occur and 'd '-o rb ita ls may have 1 C$

33or more o f ligand orb ita l character' ’ . The modified theory which

takes account of such orb ita l overlap is referred to as ligand fie ld

theory’ • In this modified theory, A is taken as the sum of

ionic and covalent components and is an adjustable parameter to be

determined by experiment. For structures in which covalent bonding

J

16

is predominant a molecular orbital approach 5s used in which A

s t i l l represents the energy separation of t ^ orbita ls and

antibonding er, orbita ls ahd Is indicative o f the strength o f the

metal-ligand bonds#

3.3 Hie ion.

2+The Mn ion prefers the s ligh tly larger s ite in

14^pyroxenes rather than the site , but this is on account of

its larger s ize compared with Fe and Mg and not because o f

crystal f ie ld e ffec ts since it has zero C.F.S.E,

2+In it s ground state the Mn ion is in a high spin state

(sextet) since a ll five d-eloctrons have para lle l spins in

accordance with Hund's ru le, and since it has one electron in each

d-orbita l, i t is also spherically symmetric. Obviously, any single

electron excitation in the 3d shell is going to lead to a quartet

state necessitating a sp in -flip . In fact, the f ir s t excited state

4 34 5is a G state . The ground state o f a d configuration is not

sp lit to any significant extent by the crystal f ie ld . However, the 4G state is sp lit into four components as shown in Fig. 7j the lower

component approaches the ground state in energy as the crystal

fie ld sp littin g increases. The luminescence emission o* the Mn ion

in many la ttic es has been postulated therefore as being due to the4 g

transition which w ill tend to be o f lower energy

(longer wavelength) as the crystal f ie ld sp litting Increases. In

fact, the wavelength o f the emission peak varies from about 5200 A in Zn^SiO - Mn to about 6700 A in MgSiO - Mn.

I t is surprising that a strong emission occurs since

the transition involved is forbidden by the following selection

rules. F irs tly , i t is spin forbidden since i t involves a change

17

o f m u ltip lic ity . Nevertheless, transitions involving a spin

change are only s tr ic t ly forbidden when there is neglig ib le

spin-orbit coupling and th is is only true for atoms o f low atomic

number. In heavy atoms such as mercury, transitions involving

spin change give rise to strong emission lines (e .g . the 2537 A

Hg lin e ). In hydrocarbon molecules where spin forbidden transitions

have a very low probability, heavier halogen atoms are often

introduced to induce s in g le t- tr ip le t transitions.

2+The Mn emission is also forbidden by the Laporte rule

for an e lec tr ic dipole transition . I f in a particular state the

electronic wave-function is symmetric with respect to inversion

about a centre o f symmetry, i t is termed a g-state (gerade). On

the other hand i f the state is anti-symmetric with respect to

inversion in a centre o f symmetry i t is termed a u-state (ungerade).

The Laporte rule states that only transitions between u - and

g - states are allowed since transitions involving states o f

similar symmetry would mean that the transition dipole moment

integral would change sign on inversion and must, therefore, be

2+zero. However, i f the s ite in which the Mn ion is situated is

not s tr ic t ly centro-symmetric, the assymmetrical ligand f ie ld

perturbs the d-electron wave-function such that d-orbitals no

longer have s tr ic t ly g character and some p-orbital character is

introduced. Since a p d transition is strongly allowed such

mixing o f d - and p-orh itals causes some relaxation o f the

forbiddenness. Even in a centro-symmetric environment the

forbiddenness may be relaxed owing to vibronic coupling i . e . by an

appropriate combination o f electronic and vibrational wave-functions.

I f during a vibration the centre o f symmetry ceases to ex ist then

again the s tr ic t ly g-character o f the d-orbitals is lost«

13,

For allowed electric dipole transitions the lifetim e is

-*8of the order o f 10 secs (see Section 3,1) but i t is noteworthy

2+that the lifetim e o f the Mn excited state is seldom shorter_3

than about 10 secs and in a highly symmetric environment can be■ 2 4

as long as 0.1 sec (CaF^) . In this la tter case the emission is

thought to be o f magnetic dipole character. The osc illa to r strength_5

o f a magnetic dipole transition is o f the order o f 10 . I t is

important to note that for e lec tric rjuadrupole or magnetic dipole

radiation the Laporte rule is exactly the reverse o f the e lec tr ic

dipole case ( i . e . g ■<->• u forbidden instead o f g*-*-u allowed).

Fonda has reviewed the emission characteristics o f a

number of manganese-activated phosphors and found good correlation

2+between the wavelength of emission and ?ln' environment! the

more crowded the environment the longer is the wavelength o f 35

emission • The average metal-oxygen distance fo r the oxide was

quoted as being applicable to the s ilica te since such data was not

available for s ilic a te s at the time (1957).

However, although the Metal-oxygen distance way be the

■oat important factor governing the wavelength of sad. ss ion, there are

many other factore to be considered which affect the value of the

crystal fie ld sp litting parameter, as indicated in Soction 3.2.

Moreover, i f the Mn2+ environment is of low symmetry the degenerate

4T^g level w ill be sp lit into three components. Very recently,

Palumbo and Brown have found that the excitation spectrum of

ZnjSiO^ i Mn shows such splitting of the 4T ^ level

A discussion o f such factors w ill be given later with

reference to the wavelength of emission in the s ilic a te s under

investigation.

2 5Partial energy level diagram for the Mn ion (d configuration)

showning the variation in energy of excited states with the

crystal fie ld sp litting parameter A . Only quartet states

are shown and energies are measured from the ground state

(not shown) which is not split to any significant extent by

the crystal f ie ld . (A fter O rge l'* ;.

Energy level diagram for 3d orbitals of transition metal ions

in lattice sites of different symmetries.

(a ) octahedral) (b ) tetragonal (elongated along the tetrad

axis)) (c ) trigonal (compressed along triad axis)) (d ) monoclinic.

The metal-oxygen distances are assumed to be the same for each

site . (A fter Bums 14^),

<«) <»> W M

19

3.4 Cathodolumlnescence and Ionolumlnescence.

The mechanisms involved in particle-excited luminescence

are complex involving many possible stages. F irstly , considerable

backscattering of the incident particles (particu larly electrons)

usually occurs without loss in energy ( i . e . elastic scattering)

and this reduces the overall efficiency of the luminescence

37process • Once a particle has penetrated into the so lid , ionisation

w ill occur producing secondary electrons. Further losses may,

therefore, be involved by loss of some of the faster secondaries

from the surface. I f the incident particle is a heavy particle

(e .g . a proton), ionisation and excitation w ill be confined to a

narrow cylindrical channel along a reasonably straight path.

However, for an incident electron large angle scattering may occur

and the depth of penetration into the solid may bear l i t t le

resemblance to the actual length o f the electron path. Ehrenberg

and Franks have shown that the excitation volume is approximately

spherical fo r low energy electrons. For higher energy electrons

I MeV) the excitation volume is a cylindrical channel ending in

a nearly spherical volume of diffused electrons, both primary and

38secondary.

The term 'ionisation* applied to solids usually means

that an electron has been raised from the valence band to the

conduction band leaving a hole in the valence band. Garlick estimates

that this process in zinc sulphide takes a mean energy from the

37primary electron of approximately three times the band gap.

Radiative recombination of electrons and holes can then occur via

luminescence centres as in photoluminescence.

In organic phosphors, sulphides and alkali-halides

experimental evidence exists of the formation of bound eloctron-hole

20

pairs known as axcitons. These excitons are mobile and since

the electron is not completely dissociated from the hole, less

energy is expended in its formation than for excitation to the

conduction band» In pure defect-free crystals, exciton migration

can be a very effic ien t means of energy transfer without charge

transfer» However, an electron in the conduction band may become

associated with a hols in the valence band to form an exciton

which may then become trapped by a defect or luminescence centre*

Since conductivity is excited in this second mode of formation

i t is d iffic u lt to separate experimentally from a dissociated

electron-hole pair.

In manganese-activated phosphors, excitation of the

centre may occur directly or by capture of an exciton, or by

resonance transfer of the excitation energy from another centre

known as a sensitizer. Alternatively, the centre may be ionised

and la ter recombination of the ionised centre with an electron from

the conduction band may occur giving emission,

l - io oFor ions in the^KeV region the incident particle loses

energy by two processes on entering the so lid . Energy may be lost

by intsraction with the electrons of the solid or by slastlc

collisions with the lattice ions. The former process may give rise

to luminescence amission and the la tte r to phonon omission or

la ttice ion displacements* The recoiling ion may also cause

electronic excitation but th is process is probably not important

for incident protons*

The process of excitation o f luminescence by charged

particles has received much attention particularly with respect

to the detection of such p srtic lss using organic and alkali halide

sc in tilla to rs (See e*g* Birks39 and Garlick‘d ) . Of particular

concern is the efficiency o f the luminescence process for d iffe

20,

pairs known as sxcitons. These excitons are mobile and since

the electron is not completely dissociated from the hole, less

energy is expended in its formation than for excitation to the

conduction band* In pure defect-free crystals* exciton migration

can be a very effic ien t means of energy transfer without charge

transfer* However, an electron in the conduction band may become

associated with a hole in the valence band to form an exciton

which may then become trapped by a defect or luminescence centre*

Since conductivity is excited in this second mode of formation

i t is d ifficu lt to separate experimentally from a dissociated

electron-hole pair.

In manganese-activated phosphors, excitation of the

centre may occur directly or by capture of an exciton, or by

resonance transfer of the excitation energy from another centre

known as a sensitizer. Alternatively, the centre may be ionised

and later recombination of the ionised centre with an electron from

the conduction band may occur giving emission.

l - l o oFor ions in the^KeV region the incident particle loses

energy by two processes on entering the solid* Energy nay be lost

by interaction with the electrons o f the solid or by elastic

collisions with the lattice ions. The former process may give rise

to luminescence amission and the la tte r to phonon emission or

la ttice ion displacements* The recoiling ion may also cause

electronic excitation but this process is probably not important

for incident protons*

The process of excitation o f luminescence by charged

particles has received much attention particularly with respect

to the detection of such particles using organic and alkali halide

scin tillators (Sea e*g* Birke39 and Garlick3^ ). Of particular

concern is the efficiency of the luminescence process for different

21

particles of d iffarsnt energies* Silver-activated zinc sulphide

has a remarkably high efficiency for 20 KeV electrons (25#)40,

for oc -partic les ( ~ 2 0 * ) 239, and for 25 KeV - ions ( — 23g6)41.

Manganese-activated zinc s ilica te also appears to have similar

40luminescence effic iencies under irradiation by 20 KeV electrons

and 25 KeV H2+ ions41, although according to Hanle and Rau41 the

efficiency of both ZnS-Ag and Zn^SiO^ - Mn is considerably reduced

when much heavier ions (e *g . argon) are used for excitation.

Organic scin tillators» however» show a considerably

different luminescence efficiency for electrons» protons and

42oc-partic les of similar energy* Data collected by Brooks

for anthracene show relative luminescence yields for these particles

at different energies (lOKeV - 10MeV). For example» at a particle

energy of 100 KeV» electrons are about five times more e ffic ien t

in producing luminescence than protons and about seven times as

e ffic ien t as oc -partic les* These differences may be thought to

be a consequence o f the fact that the heavier the partic le the

greater the proportion of kinetic energy expended in nuclear

co llis ions rather than electronic excitation. However» for protons

and oc -partic les the proportion of the kinetic energy expended

in such collisions at energies above a few KeV is re lative ly small

and cannot explain the large differences in luminescence efficiency

compared with electron excitation* These differences are»

therefore» attributed to the different ionisation densities

prevailing and thus to the different values of the specific energy

loss . For low energy electrons or for heavier particles the dx

specific density of ionised or excited centres w ill be much higher

than fo r higher energy electrons and interaction between ionised

and excited centres w ill occur. Blrks has postulated that such

22

interaction give» rise to a quenching of the primary excitation .

Thallium-activated a lka li halides also have different

luminescence effic iencies for d ifferent particles of the same

energy, although unlike organic sc in tilla to rs a plot o f luminescence

efficiency against ^ usually exhibits a maxlain at a

particular • The f a l l in luminescence efficiency for high dx

specific energy losses is again thought to be an ionisation

37quenching effect .

However, for other phosphors it appears that ionisation

quenching is not an important process for protons or ©C -partic les

of energy greater than a few KeV. Van Wijngaarden at a l have

found that the luminescence efficiency of Zn2Si04 - Mn is

independent of energy in the range 3 - 1 0 0 KeV for proton excitation43.

For excitation by heavier lone (A *, Kr+ , N+) the lumineacence

efficiency which is lower than for proton excitation, increased

with energy in this range. These authors attribute this phenomenon

to the fact that the stopping cross-section for "nuclear" collisions

SR ( i . e . co llisions with lattice ions) is comparable with the

stopping cross-section for electronic excitation S# at lowor

energies for heavy ions. As tho energy increases SR fa l ls and S#

increases and thus a higher-proportion o f the kinetic energy of

the particle is available fo r electronic excitation. Using

theoretical values for S and S_ derived from the work of Lindhard

et a l44*49. Van Nijngaarden at a l aasune that the total luminescence

output L produced by one particle along its entire path is

proportional to the total energy lost to electrons and obtain the

expression

L ■ C

I

39

23

where the integration ie performed numerically and the constant C

is then chosen to give the best f i t with experimental data.

For heavier ions (A+ , N+ e tc .) they found that even with a

'best f i t ' value of C the experimental plot of log L against

log E had a s ligh tly larger gradient than the corresponding

theoretical curve. Moreover, i f the value of C evaluated to

give the best f i t for A+ ions is used for the theoretical curve

for protons i t is found to predict a lower luninescence efficiency

for proton excitation than is found by experiment. Thus i t

appears that some ionisation quenching may be operative for heavier

ions which produce a larger specific ionisation. This would have

the effect of making C dependent on energy and on the mass of the

particle. However, for excitation by lighter ions such as protons

it appears that the luminescence efficiency of Zn2Si04 - Mn may

approach that fo r electron excitation in the KeV energy range.

3.5 Ion-radiation damage in luminescent materials.

In general, when a luminescent material i s bombarded by

a large flux of ions the luminescence intensity under steady

excitation fa lls appreciably with time of irradiation . The decrease

of luminescence efficiency which is more rapid in it ia l ly and

decreases with time, is probably due to the introduction of defects

into the structure which act as quenching centres*

Even protons of KeV energy are capable o f producing

many vacancy-interstitial pairs in a lattice whereas the energy

threshold for incident electrons to produce such atomic displacements23h

is of the order of a few hundred KeV • According to Qirie

one H2+ ion of 1GKeV energy produces about 20 vacancy-interstitial

pairs in germanium. The production of vacancy-interstitial pairs

and possible aggregation of such defects are the major offsets of

24

ion-radiation damage in crystals4^.

The number of lattice displacements w ill increase as

the mass of the incident particle approaches that of the lattice

ion since a greater fraction of the particle momentum w ill then

be transferred to the lattice ion. In this case the knocked-on

lattice ion may i t s e l f produce further secondary defects. The

number of such secondary knock-ons is a function of the incident

particle mass and energy, being large for heavy high energy

particles. When the mean free path between displacement collisions

is comparable with the lattice spacing the aggregation of defects

is inevitable and a 'displacement spike' may occur47. However,

for protons th is process is unlikely to occur except perhaps near

the end of its path.

Hanle and Rau4 have studied the degradation o f

luminescence in ZnS t Ag, Zn2Si04 t Mn and MgW04 when irradiated by

25 KeV hydrogen and inert gas ions. They found that the heavier

the ion, the faster the phosphor deteriorated for a given rate of

incidence of ions per unit area. The variation of luminescence

intensity I with tota l number o f incident ions N was found to follow

the relation Z ■ ..--1 °.____ (1 .7 )1 ♦ CN

where ZQ is the in it ia l luminescence intensity and C is a constant,

known as the damage constant, fo r a given phosphor and incident

ion. The magnitude of this constant is a measure of the rate of

degradation of luminescence. The above relation was f i r s t derived

to describe the degradation of luminescence in anthracene crystals

48when irradiated by oC - particles .

G iIfrich has found the above relation to hold over

certain ranges of N for a variety of phosphors when irradiated by

10 KeV protons4* . The value of the damage constant C was found

25

to increase generally with type of phosphor in the following

order} a lkali-halides, oxygen-dominated phosphors (s ilic a te s ,

tungstates, e tc »), sulphides, organic phosphors» Values of C

ranged from 0.05 . 10~14 cm2 for NaCl t Ag to 2 0 0 .10"14 cm2 for

anthracene, and appear to be indicetive to some extent of the

relative strength of the chemical bonds in these materials* As

might be expected, the stronger the bonding, the more resistant

is the material to radiation damage* However, other factors also

influence the value of the damage constant since G ilfrich found

that the degradation of luminescence in ZnS t Mn was much slower

than in ZnS i Ag although varying the concentration of Ag in

ZnS t Ag had l i t t le e ffect on the value of C* Martin has shown

that the value of C is practically independent of incident particle

energy fo r Hg* ions in various phosphors in the renge 4 - 4 0 KeV

50although i t is marginally larger at lower energies • However,

more recently, Van Nigngaarden and Hastings have pointed out that

the values of the deterioration constant C determined by previous

workers are for Integrated light output over the ion energy range

from the incident particle energy to zero ' • These authors

have, therefore, determined values of the damage constant for

particular ion energies in ZnO i Zn. They find that for incident

protons, C, evaluated for particular energies, decreases with

increasing particle energy in a similar way to the predicted

variation of the cross-section for nuclear collisions S .n

The probability of s direct collision with a lattice ion

must be dependent on the direction of travel of the particle with

respect to the crystallographic axes* The phenomenon of proton

"tunnelling" along a path through the crystal where its probability

of direct collision is very small, i s now well-knoMt. However, for

26

randomly orientated microcrystalline powders, this effect is

not lik e ly to be important* The mean penetration depth of protons in

powdered phosphor material has been estimated in an ingenious way by

53Young . A phosphor screen was irradiated with a given quantity

of heavy ions and then a variable-energy electron beam used as a

probe to determine the degradation of luminescence and the depth

of such damage. The penetration depth of 20 KeV protons was

estimated to be about 0*1 microns (10**^ cm) and was shown to

increase approximately linearly with incident ion energy in the

range 5 - 2 5 KeV* Annealing the damaged phosphor by hesting to

a high enough temperature for the migration of defecta to become

appreciable restores the luminescence efficiency to some extent*

Young found that the luminescence efficiency of ZnS t Ag could be

completely resuscitated after damage by hydrogen ions by baking

at 450°C for several hours* Grosser end Scharmenn have uaed

Young's technique to investigate the deterioration of tungstates

54and phosphates with similar results •

3*6 Thermoluminescence«

Thermoluminescence, better termed 'thermslly stimulated

phosphorescence', occurs when a phosphor is hasted in the dark

after excitation at a low temperature. During this excitation

electron trapping states are f i l le d and during subsequent heating

the probability of trapped electrons being relessed is incressed

and emlasion is observed as released electrons undergo recombination

via luminescence centres* The rate determining step is the

probability P of release of s trapped electron according to a

Boltzmann type law, P - I ■ •• U ,B '

where X is the trap lifetiaw and s is a constant termed the

'frequency of attempted escape'• A plot of light output against

27

temperature yields a 'glow-curve' which is essentially an energy

spectrum of the depth of trapping states* The light output

at a given instant I ■dt

" E/kT (I* 9)

where n is the number of electrons s t i l l trapped at this instant*

Replacing dt by where f l is the heating rate and separatingf i '

the variables gives on integration

or

■ « / s exp ( - £J r kT

) £ (1.10)

ft

{ - fT' • < - I t >j )

(1.11)no #XP

where nQ is the in it ia l number of trapped electrons at temperature

Tq* Substitution for n in equation (1*9) yields

nQ s exp ( - j*T ) exp {- / • «■> <- It > f] "•1J)

A plot of this function yields a smooth curve climbing to a

maximum at some temperature Ta and fa llin g again approaching zero

as T becomes ^ T a * At the peak of amission £ ■ 0

andkT_

s exp ( - L ) m

(1*13)

Thus i f s is known, E can be calculated* This simple theory

in which only one trap depth is considered was f ir s t fom ilated

by Randall & Wilkins95 and later developed by Garlick & Gibson9*

who also considered the possibility o f retrspping of relessed

electrons* I f such rstrapping is considered to occur, the kinetics

becosM second order and the thermoluminescence intensity

dn _ - £ • ndt (N • n) t n

„2 2■ - n

N't- s exp ( • ) (1.14)

where N is the number of traps and n the number of electrons in

28

traps. Here i t is assumed that traps and centres have equal

cross-sections for electron capture* Completing the formalism

as for 1st order kinetics gives for 2nd order kinetics

The shape of this function is sim ilar to equation (1.12) but with

a longer ta il on the high temperature side. The position of Tm

is approximately the same as in (1 .12 ). Recent development of

the formalism of thermoluminescence mechanisms have considered

different values of a retrapping factor R which is the ratio of

57the cross-section for retrapping to that for radiative recombination

Simple theory with no retrapping is the special case when R ■ 0

whereas R ■ 1 is the special case of second order kinetics with

equal cross-sections fo r the two processes. For R 1, retrapping

predominates i f the number of traps is of the same order as the

number of recombination centres.

Methods of determining trap depths from glow curve data

are now numerous but usually fa l l into one of the following basicKQ

categories* (1 ) Quick approximation methods • (2 ) The 'in i t ia l -

r is e ' method^*. (3 ) Methods which use the emission peak shape^.

(4 ) Methods which use the variation of Ta with heating rate .

A ll methods assume the type of kinetics involved although method (2 )

is independent of type of kinetics provided the number of empty

recombination centres is much greater than the number of electrons

in traps. Brlunlich has shown that i f this provisor is not

fu lf i l le d then method (2 ) is only accurate for Rj{157. Booth's*0

method (4 ) as also used by Hoogenstraaten is useful since Ta

is not appreciably altered from that given by the simple Randall &

Wilkins formulation unless R ^ 1. Thus for several different

I - (1.15)

29

heating rates, a consideration of equation (1.13) shows that a

plot of log# A against _2_ w ill give a straight line ofm

Egradient - . The great advantage of methods (2 ) and (4 ) is the

elimination of s which otherwise has to be estimated.

However, in geological applications calculation of

trap depths is seldom attempted and thermoluminescence is used

either as a "finger-printing" technique or as a method of dating.

4. Possible luminescence and radiation damage of the lunar surface.

4.1 Possible lunar luminescence.

For some years now the question of whether measurable

luminescence occurs at the lunar surface has been a controversial

issue and many excitation mechanisms have been proposed which

might account for luminescence of the surface material. The crux

of th is controversy has been whether luminescence could be excited

by various components of the solar 'wind' or of solar flares

which would be bright enough to be measurable against the

background of reflected solar light. The lunar surface is , in

fact, a very poor re flector, reflecting only about T% of incident

ligh t. The remaining fraction is absorbed and converted into heat,

giving rise to in fra-red emission. Evidence for luminescence

emission comes from three sources*- (1 ) Transient 'events'

involving localised changes in colour or brightness of a particular

area on the lunar surface, (2 ) The brightness of the surface during

soam lunar eclipses, and (3 ) light emission other than reflected

light as revealed by measurement of Fraunhofer line depths.

Transient 'events ', whatever their origin, are certainly

not uncommon aince in 1966 Burley and Middlehurst listed soam 238

such reported occurences including about 50 in the period 1960-6^ •

Nearly half of a ll auch events concern the region of the crater

Ariatarchus but this nay be due partly to the fact that

Aristarchus has been observed much more than other areas* Many

mechanisms have been suggested in an attempt to explain such

events and these include volcanic activity , fluorescence of gaseous

emission, surface fluorescence excited by solar radiation and

tharmoluminescenes of the surface*

Many reports involve sightings of red glowing areas and

a well-authenticated sighting of this type was witnessed by

several astronomers in 1963 (Greenacre )• Kopal and Rackham

photographed through a red f i l t e r what they claimed to be red

glow near the crater Kepler, by comparison with a photograph taken

through a green filte r* They suggested that this glow could be

correlated with a solar fla re whioh occurred 8 hours previously*

The time lag was assumed to be the transit time of the solar

particles which i t is claimed produced red luminescence over a

large area of the surface* However, some doubt has been expressed

concerning their photographs and more recent systematic observations

using similar techniques fa iled to detect any such luminescence^4*

Moreover, Middlehurst^ found that soom 103 events up to 1964 were

not correlated with times of solar activity*

66Link was possibly the f ir s t to suggest that lunar

luminescence might be caused by particles trttich resulted from

solar activity* This suggestion was an attempt to explain the

anomalous brightness of the moon during some eclipses* According

to Link light refracted by the earth 's atmosphere is absorbed by67

ozone before i t can reach the eclipsed moon* However, Ney et a l

believe that bright eclipses are caused by such refracted light

and dark eclipses by the scattering of this light by dust particles

31

in the atmosphere such as would be experienced after volcanic

eruptions* Link presumably invokes the earth 's magnetic fie ld

to deflect charged particles from the sun onto the eclipsed

moon. In the latest contribution to this subject, Dubois and Link

invoke the UV and x - radiation from the outer corona as the

excitation*

The possib ility of detecting steady luminescence

excited by either short UV or quiet solar wind protons using

the Fraunhofer line depth method was orig inally suggested by

Link^ and several workers have obtained positive results by this

method * ^ 71* A Fraunhofer line p ro file is scanned viewing

the sun d irectly and then by viewing sunlight reflected at the

lunar surface* The difference in line depth is attributed to

light originating from the Moon's surface* This method has

the advantage that any luminescence emission can be measured in

the presence o f reflected sunlight and unlike observations on

eclipses and transient events, i t is not prone to atmospheric

interference* However, Grainger & Ring71 have found some peculiar

time fluctuations in the line depth and show that when instrumental

errors are eliminated the luminescence background is less than 3K

of the continuum.

By considerations of the energy available, Nash has

shown that transient 'events' such as described by Kopal & Rackham

cannot be ascribed to luminescence induced even by strong solar

72flare protons • More recently he has also shown that proton-

excited luminescence of lunar surface material of the type returned

by the Apollo 11 mission could not even be detected by the 'lin e

depth' method73* In fact, i t seems unlikely that luminescence

excited d irectly by solar ions can be responsible for any

measurable effect* There remains the possib ility of short UV or

68

32

X-ray excitation but again the energetics o f the process« although

■ore favorable than particle excitation« are s t i l l unlikely to

give enough luminescence intensity* Since thermoluminescence

involves an energy storage mechanism it has been suggested that

this process may be responsible for some transient events* Sidran

has discussed this possib ility and has pointed out that many

transient events are correlated with lunar dawn • However«

excitation mechanisms do not appear to be fu lly considered and some

mechanism must be found of f i l l in g traps when radiation from the

sun is not reaching the surface i*e* during lunar night* Moreover«

recent results on thermoluminescence of actual lunar surface

material are not encouraging although samples are as yet from only

two particular locations* Thermoluminescence of these samples

has proved to be of very low intensity and the traps responsible

have been shown to be 'leaky ' i.e * the stored energy decreases11,76,77

rapidly with time since excitation.

In conclusion, i t appears that many effects concerning

the luminosity of the lunar surface cannot be explained by

luminescence o f the surface material*

Nevertheless, the study o f the luminescence of minerals

can yield much valuable information and is of interest for it s

own sake, particularly when comparisons are made with synthetic

samples of carefully controlled composition*

4.2 Radiation damage of the lunar surfact.

Although, the lunar surface is not weathered in the way

the earth's surface is , i t is not shielded by an atmosphere from

meteors, or the solar wind* Thus the lunar surface w ill be

bombarded by micro-meteoritic particles as well as larger meteors*

Moreover, after many aeons of solar wind proton irradiation the

33

surface layer would be expected to be extensively radiation

damaged and» therefore» appreciable luminescence would not be

75expected* However, Geake has suggested that disturbances such

as the arriva l of a meteor may expose fresh undamaged material*

Since the energy of protons in the quiet solar wind is

of the order of 2 KeV or less7®’ 7^ the penetration depth w ill be

very small* For solar fla res , in which particles of a few MeV

may be present, the penetration depth w ill be much greater but

then the integrated flux is much smaller* Laboratory experiments

concerned with bombardment of rock samples by large doses of low

80 81 82energy protons have been carried out by Wehner et a l * , Hapke

83and Nash in order to ascertain the possible condition of the

lunar surface*

Wehner et al found that many powdered samples darkened

appreciably under simulated solar wind boabardment and suggested

that many of the unusual properties of the lunar surface such as its

low re flec tiv ity could be explained by the action of the solar

wind* Dollfus has measured the degree of polarisation o f moonlight

for d ifferent phase angles for several regions of the Moon using

84a Lyot polarimeter • He has shown that i t is very d i f f ic u lt to

match the moon's polarisation characteristics using te rre str ia l

rock samples* The closest match requires a fine dark coloured

85basaltic rock powder. However, Dollfus & Geake have made

polarisation measurements on a powdered enstatlte achondrite

meteorite sample which was lntansively proton irradiated by the

author and found that such treatment resulted in a very goot match

with the moon's polarisation characteristica* Hapke also found

that a number o f rock powders had polarisation versus phase angle

curves similar to the lunar surface after being irradiated by large

34.

doses of 2 KeV protons* In fact, Hapke concluded that the

composition of irradiated rock powders did not seem to be very

critica l in determining their photometric properties.

Nash, however, has shown that darkening of silicate

rock powders under proton irradiation is considerably reduced

i f precautions are taken to reduce various possible contaminants.

Carbon from irradiation-decomposed hydrocarbons and sputtered metal

from ion source components are suggested as the contaminants

responsible for the darkening e ffec t. He found that proton

darkening was appreciable only i f the proton flux was high enough

to produce a sample surface temperature of above 150°C. Nash,

therefore, concludes that the hypothesis of a solar-wind darkened

lunar surface is supported by experimente which are unrealistic

in reproducing the conditions of solar-wind bombardment of the

-__ - - --- ----

lunar surface

35

1. The 120 KeV proton accelerator system.

The accelerator, in it s original form, has been4

described in detail elsewhere and, therefore, a more brie f

description w ill be given here but fu ll deta ils of recent

modifications w ill be included*

1.1 General Description*

The general arrangement is shown in Figs. 9 and 10.

Spectro-grade X hydrogen (BOC) is leaked into the evacuated ion

bottle by means of a needle valve. Here i t is ionised by an

R.F. fie ld and the positive ions (mainly protons) pass through

a canal and are accelerated by the potential difference (usually

about 60 KV) between the canal and an earthed cylinder. In order

to reduce ion recombination, electrons are extracted from the

plasma by an electrode in the top of the ion bottle which is

held at a potential of a few hundred volts positive with respect

to the canal. After acceleration the protons coast along the

axis of the earthed cylinder and a further extension tube until

they h it the sample area which is approximately three feet from the

canal. At th is point the proton beam is about one inch in diameter.

This is not merely a shadow image of the canal as previously stated4

since a degree of focussing is attained by the electrode arrangement.

The whole system was orig inally evacuated via the sample

chamber by a 2" o il d iffusion pump (Edwards F 203) with liquid

nitrogen trap backed by a single stage 50 1/min rotary pump

(Edwards 15C50B). The ultimate pressure of the system was about

5. 10'6 torr and the working pressure with hydrogen leak about

5 • IQ“5 torr.

General arrangement of the 120 KeV proton accelerator

system after modifications*

Fig, 10. General view of the lower part of the 120 KeV proton accelerator

system after modifications*

H *

Ir

m

Fir]. 10. General view of the lower part of the 120 KeV proton accelerator

system after modifications.

>

Fig. 10A Close-up view of sample chamber and pumping system.

The comparison channel photomultiplier housing is seen

in the le ft foreground. The monochromator, which ie

on ra ils , has been moved away from the sample chamber

to enable a change of samples to be made. This

photograph was taken when the orbitron pump was

connected directly to the accelerator tube. I t is

now connected via a large bore valve.

Fig» 10A Close-up view of sample chamber and pumping system.

The comparison channel photomultiplier housing is seen

in the l e f t foreground. The monochromator, which is

on r a ils , has been moved away from the sample chamber

to enable a change of samples to be made. This

photograph was taken when the orbitron pump was

connected d irectly to the accelerator tube. I t is

now connected via a large bore valve.

Th* proton source of the 120 K*V *cc*l*r*tor.

(A fter Derham and Geak*4)*

The sample chamber in its original form. The sorption pump

line has now replaced the flex ib le coupling which previously

connected a diffusion pump to the chamber* The flap valve

has also now been modified (see text)* (A fter Derham and Geake'

1.2 The proton source. and scctlarstor tub«

The eource and associated electronic equipment is

enclosed in the top box the whole of which is , under operating

conditions, at a high positive potential (usually 60 KV) with

respect to earth« This box is supported by three ebonite legs

(about 1 f t . long) and an evacuated 6" diameter Quickfit Pyrex

glass tube (about 18" long)>which encloses the accelerating

electrodes, provides s fourth support (see Fig. 9 ) . For safety,

the box is caged o ff and a trap door in the mesh, which gives

access for servicing, is fitted with a micro-switch which is in

the mains supply to the H.T. power unit.

The proton source it s e lf is shown in F ig. 11. The

design of the ion bottle is due to Mr. K.R. Chapman of Birmingham

University. The bottle is of Pyrex glass and has a narrow neck

connecting the main part with the bulb at the top in which the

electron extractor electrode is situated. The narrow neck

minimises the possib ility of proton bombardment of this slectrode.

The aluminium plug in which the canal is d rilled

(V i g diameter, j * long) is protected from ion bombardment by a

ceramic shield. A canal by»pass pipe enables the ion bottle to be

evscuated more quickly than it could be through the canal and a

valve in this pipe is closed prior to switching on the H.T. and

opening the hydrogen-leak needle valve. The needle valve is

opersted remotely using nylon cords. The source of hydrogen is a

glass flask which in it ia lly contains 1 l i t r e of gss at atmospheric

pressure.

Acceleration of the protons takes plsee scross a 1" gap

between the top plate and the top end of an earthed stainless steel

cylinder. This gap is shielded from the (Xiickfit glass tube by a

37

cylindrical skirt attached to the top plate.

1.3 Power Supplies.

The H.T. power supply for the accelerator (Brandenburg

MR 120) consists of a high frequency oscillator followed by a

Cockroft and Walton m ultiplier stack and gives a variable

stabilized D.C. output of 10 - 120 KV with less than 0.1# ripple.

The polarity can be reversed by inverting the multiplier stack.

The accelerator top box contains a 25W R.F. osc illator

(ex - Air Ministry) and a 0 - 4 KV stabilized H.T. unit (Labgear).

The R.F. osc illator is tunable around 50 MHz and the output tank

circuit is connected to two co ils wound round the ion bottle .

The oscillator is tuned for maximum output as indicated on the

output meter. The output of the 0 - 4 KV H.T. unit which is

connected to the electron extractor electrode is adjusted for

maximum proton beam current.

Since both the R.F. osc ille to r and extractor H.T. unit

are mains operated and at a high potential when the accelerator is

in operation, power for these is derived from a self-exciting

alternator in the top box which is driven along a 1 ft long

ebonite shaft coupled by a flex ib le drive to a 1. H.P. single­

phase induction motor at earth potential.

A ll adjustments to the electronic units in the top

box are carried out remotely by means o f nylon cords fastened

to the various switches. Meters are read by means of illuminated

mirrors.

1.4 The sample chamber.

The sample chamber is machined out of a solid block of

dural and has five ports. A detailed diagram is shown in Fig.12.

38

To on« port is connected the vacuum roughing line and to another

a Penning gauge head* The protons enter via the top port which

can be closed using a flap valve in order to le t the chamber

up to atmospheric pressure for sample changing whilst keeping the

accelerator section at a reasonable vacuum* The flap valve is

insulated e lectrica lly from the chamber and connected via a spot

microammeter to earth so that i t can be used to intercept the

proton beam and thus measure the beam current*

The spring strip arm of the flap valve proved unsatisfactory

and has now been replaced by a rig id arm and b a ll jo int.

The sample holder locates in the base of the chamber

with a bayonet fitting and luminescence from the sample is

reflected through the lower window in the chamber by a plane

aluminised mirror* A second plane mirror mounted outside the

chamber re flects the light into the entrance s l i t of the monochromator*

The upper window fac ilita tes visual observation of the

sample and can be replaced by a re-entrant window which allows

part of the sample to be viewed using a microscope with a long

focal length objective*

In order to interchange samples without breaking the

vacuum, a sample holder has been made incorporating a rotatable

turret which holds four samples in 4* diameter stainless steel trays

(see Fig. 15). This has made the comparison of luminescence

efficiencies of various samples much easier*

Modifications to the vacuum system.

Common to a l l accelerator systems which are pumped by

o il diffusion and rotary pumps there is a danger that cracked

organic material may be deposited on the sample by the bombarding

39

particles« This is a well-known phenomena in electron microscopes«

Moreover, extensive proton bombardment of meteorite samples hasQC

led to pronounced darkening of grain surfaces and i t is

suspected that such deposition could have at least contributed

to the darkening. It is d if f ic u lt to determine how much of this

darkening is due to deposition and how much to genuine radiation

damage of the sample.

It was, therefore, thought desirable that for

investigations on actual lunar samples that organic vapours be

eradicated as far as possible from the vacuum system. The

existing pumping system was, therefore, replaced by an ion pump

for high vacuum pumping and sorption pumps for roughing.

Furthermore, a l l the existing seals in the system were of n itr ile

rubber and as an added precaution these were replaced by elastomer

"viton" seals which have a much lower vapour pressure. This

necessitated a complete dismantling and rebuilding o f the

accelerator vacuum system.

The dural plate at the base of the Quickfit pyrex tube

section and the original copper extension tube between this plate

and the sample chamber were replaced by a 1 f t diameter stainless

steel flange welded to a 2J-" bore stainless steel tube. This

new tube section incorporates a side-arm also of bore to which

the getter«ion pump is connected via a 2^" bore isolation valve

using copper gasket seals (see Fig. 9 ). The ion-pumping line is

thus placed above the flap valve so that the accelerator men be

pumped even when the sample chamber is at atmospheric pressure.

The sorption pump roughing line is connected to the

sample chamber using the port of the former connection to the

diffusion pump. The roughing line is a horizontal 'T* of 1" bore

40,

stainless steel tube« two ends of which are connected via isolation

valves to two sorption pumps using copper gasket seals. A ll valves

both in the roughing line and ion-pump line are of the bellows

type sealing onto a viton ’O' ring (Vacuum Generators L td .).

The pressure in the original system before modifications

was monitored by two Penning gauge heads (Edwards MH5). One

head was connected to the sample chamber and the other monitored

the pressure at the base of the Quickfit glass tube. These

Penning gauges« however, cannot measure accurately pressures below

-6about 10 torr, and since pressures below 10"° torr were

anticipated using the new pumping system, another type of gauge

was required.

The Penning gauge on the sample chamber was retained

but the gauge monitoring the pressure in the accelerator tube was

replaced by a Bayard-Alpert type ionisation gauge head (Veeco RG 17).

This gauge head has a non-burnout filament (lanthanum) and can be

used to measure pressure in the range 10~2 - 10~10 torr. A TC 5 (VG)

ionisation gauge control unit supplies the necessary voltages for

this gauge head and amplifies the collector eurrent.

1.6 The new pumping system.

Sorption pumps have the advantages over mechanical rotary

pumps of cleanliness and freedom from vibration since there are no

moving parte* Even when operated with a fora-line trap, rotary pumps

are s t i l l a possible source o f o il vapour contamination.

Sorption pumps do, however, have certain disadvantages

compared with a gas-ballast rotary pumpi certain gases (particu larly

hydrogen) are not pumped very effic ien tly and water vapour soon

saturates the sieve material. The sieve material can, however, be

resuscitated by baking the pump at 200 - 300°C using heating tape*

41

Porous sodium aluminium silicate pellets (BDH type 13X)

are used as sieve material in the sorption pumps which are of

multi-ttibular design (Applied Research & Engineering L td .). The

pellets have a very large surface area ( ^ 500 m2/g) on which

gas is adsorbed when the pump is cooled. Liquid nitrogen is used

as coolant and is contained in expanded polystyrene tubs which

act as dewars.

A pump is pre-cooled for 5 - 1 0 minutes before being

introduced to the system and a pressure of around 10 torr can be

achieved starting from atmospheric pressure in about a further

10 minutes. I f th is pump is then isolated and a second pre-cooled

pump is connected to the system a vacuum better than 10-3 torr can

be achieved in a further 10 - 15 minutes. This procedure is

•4referred to as sequential pumping. A pressure of 2 • 10 torr

has been achieved merely by sorption pumping using freshly ,baked-out

pumps.

After a pump is isolated from the system, the coolant is

removed from around the pump which then is allowed to warm up to

room temperature. I f the pump hafc hdsorbed a considerable quantity

of gas, pressure w ill build up inside the pump as it warms. This

is released by allowing the gas to blow out a chained rubber bung

which seals a side tube*

The ion pump used for high vacuum pumping is an

electrostatic getter-ion ("orbitron") pump (Applied Research &

Engineering Ltd*)* This pump could s t i l l be described aa being

developmental in design. Like a l l ion pumps i t does not require

"basking", as a diffusion pump does, once it is in operation. It

was considered ideal fo r our needs since i t gives a large pumping

speed for it s compact size, does not have large associated magnets,

42

and was cheaper to obtain than a magnetic type ion pump of

comparable pumping speed. Since a considerable amount of testing

of this type of pump was carried out, a complete description and

appraisal of this pump and an ea rlie r prototype, together with

constructional details and power supply requirements w ill be

given in the Appendix. The pump used here is a 2¿" diameter

triode pump and is water cooled. I t is normally kept under high

vacuum when not in use and is put into operation before being

introduced to the system. It w ill then pump the system from a

-3pressure of around 10 torr down to eventually an ultimate of_7

about 5 • 10 torr, which is an order of magnitude better than

that attained previously using a diffusion pump. This ultimate

is restricted not by the pump but by the outgassing of the walls

of the system particularly from the dural which unfortunately

cannot be reduced appreciably by baking and then cooling to

ambient temperature as can outgassing from materials such as

stainless steel and glass.

Pump down time is variable and depends on many factors

such as dryness of the samples, the length of time the sample

chamber was at atmospheric pressure, the length of time since the

chamber was at atmospheric pressure, etc ., and, of course, the

condition of the pump. In optimum conditions, the getter-ion pump

-6 -3w ill pump the system down to around 10 torr (from 10 torr) in

less then sn hour, elthough usually in conditions less than-5

optimum i t obviously tskes longer. A pressure of 10 torr can

sometimes be attained in 5 - 10 minutes. The hydrogen leak is not

opened until the pressure is less than 2 . 10 ^ torr and operating

pressure with the optimum hydrogen leek rate is about 2 • 10 ® to rr.

43.

In a l l . the performance of this pump ia superior to a

2" diffusion pump using "convalex 10" o il (CVC) and a liquid

nitrogen trap. However, greater care is needed in it s operation

whilst i t also requires more maintenance.

2. The Photoelectric Spectrophotometer.

Full details of the construction, calibration and

performance of this instrument have been given by Lumb and2

Geake & Lumb • Only a b rie f description w i l l , therefore, be given

here but including details of recent minor modifications. An

improved spectral calibration technique w ill also be described.

2.1 The monochromator and photomultipliers.

An optical diagram of the monochromator is shown in

Fig. 13. This is of the Ebert type and has as the dispersive

element a 5" x 4” Bausch & Lomb reflection grating (1200 lines/mm)O

blazed at 7000A in the f ir s t order. A 14" concave mirror of 72"

radius of curvature both renders undispersed light from the entrance

s l it para lle l and focuses dispersed light on the exit s l i t . The

system has f/9 optics and the reciprocal linear dispersion in

firs t order is 8 A/mm. Interchangeable fixed s lit s are available

but for most purposes where high resolution is not required but

rather a high light-gathering power, 3 mm s lit s are used giving a

static bandwidth of 24A. Light passing through the exit s l i t is

monitored by a photomultiplier, the output of which is amplified

and recorded. A s ilica disc reflects a small proportion of

undispersed light onto the photocathode of a second photomultiplier

(see Fig. 13), the output of which is used to compensate for time

variations in the total luminescence output of the sample.

_________ _ .

A spectrum is scanned across the exit s l i t by tilting

the grating via a sine drive using a synchronous motor. The

scanning speed can be changed by altering the gear ratio between

the motor and the drive shaft. A micrometer on the drive shaft

is calibrated against wavelength, and the wavelength range can be

altered by changing the length of the stub at the end of the arm

which t i l t s the grating. Usually only one range is used, namelyO

3000 - 8000A but recently measurements have been extended to the

infra-red and a longer stub used to give a wavelength range

5500 - 10.000A.

In the v is ib le region, EMI 9558B photomultipliers are

used. These have a 2" diameter t r i-a lk a li S20 cathode

Sb(NaK)Cs }w ith a pyrex window. Although having higher dark

current and noise characteristics than the usual S11 (CsSbO)

cathode, they have a much higher sensitivity in the red extending

beyond 7500A.

For infra-red scans a Mullard 150 CVP photomultiplier

with an S1 cathode (AgOCs) is used for monitoring dispersed light.

(This was kindly loaned to us by Prof. Garllck of Hull University).

This type of photomultiplier has to be cooled to around - 50°C in

order to reduce the dark current to a tolerable level and obtain

an optimum signal-to-noise ra tio . The cooling is achieved by

passing cold dry nitrogen gas through the housing of the

photomultiplier tube. In order to avoid frosting the housing is

thermally insulated with expanded polystyrene sheet and foam rubber.

Light from the exit s l i t passes through an evacuated double silica

window (spectrosil) in the housing to reach the photocathode.

13« Optical diagram of the monochromator; PM1 and PM2 are

photomultipliers for spectral scanning and compensation

respectively. (A fter Geake and Lunb2) .

Fi2«__1£. Block diagram of the electronic system.

(A fter Geake and Lumb2) .

UOMT FROM WIT KIT

--------------*

UHDllftflUnUOMT

-------------►

Turret sample holder with four samples in position* The

shield locates on four p illa rs and exposes only one sample

to the proton beam.

Turret sample holder with four samples in position,

shield locates on four p illa rs and exposes only one

to the proton beam.

The

sample

> V t

M r r > r , ,*fi • <■>/ • K -

f e i i t e

Fig. 16 Vi*** of th* 2$-" diameter triode electrostatic

getter-ion pump. Not* th* water jacket for cooling*

»

View of the 2?" diameter triode electrostatic

getter-ion pump. Note the water jacket for cooling.

Fin, 16

2.2 The Electronic recording system.

A block diagram of the recording system is shown in

Fig. 14. Since the intensity o f luminescence may f a l l appreciably

during a spectral scan owing to proton-irradiation damage some

means of compensation for this e ffect is necessary i f the ’ true'

spectrum is to be determined. This is achieved by electronically

taking the ratio o f the amplified signal from the 's ign a l'

photomultiplier monitoring dispersed light to that from the

'comparison' photomultiplier monitoring undispersed ligh t. Thus

a change in total light causes no change in this ra t io . This

system works admirably provided the spectrum its e lf does not

change drastically during a scan. However, although in some cases

spectral changes do occur owing to radiation damage, these changes

usually proceed at a much slower rate than the change in overall

luminescence intensity.

Photomultiplier currents are amplified by high stability

D.C. amplifiers (AVO 1388B) of the electrometer valve type

employing high negative feedback. The photocurrent produces a

7 13voltage across a high value input resistor (10 - 10 ohms) and

the current range is changed by selecting the appropriate value

of input resistor. The amplifier has voltage gain unity and acts

essentially as an impedance matching device and f i l t e r between the

photomultiplier and the recorder* It has a 100 mV output suitable

for recorder operation which is tapped from across a 100 ohm

resistor in the output resistor chain. The time constant is

nominally 1 second on the current ranges normally used.

A potentiometric pen recorder (Philips 4069M/04) modified

by Lumb3 records the ratio of tha outputs of the amplifiers. This

instrument is essentially a self-balancing potentiometer. The

46

output from tht 'comparison' amplifier is fad across the slide

wire of the recorder instead of the usual internal stabilized

voltage supply. The output of the 's ign a l' amplifier is then

applied between one end of the slide wire and the pen carriage

in the usual way and the servo-system then finds the 'n u l l '

position such that th is applied voltage is equal to the voltage

existing between the end of the slide wire and the position of the

pen carriage on the slide wire. The response time of the servo­

mechanism is nominally 1 sec for fu l l scale movement. The pen

carriage also travels along a para lle l 's lave ' slide wire which

was supplied by an independent constant voltage so that the

recorder trace could be repeated on one channel of a three channel

recording milliammeter (E & V).

The mains supplies to the recorder chart drive motor

and the monochromator scan motor pass through a master switch so

that they can be started synchronously.

The outputs of the amplifiers were also connected via

buffer amplifiers to two channels of the recording milliammeter

so that these could be separately monitored. However, operation of

this rather old instrument has late ly been unreliable and i t has

now been repleced by a two-channel potentiometric recorder

(Rikadenki B241). Buffer amplifiers are, therefore, no longer

necessary.

A backing-off voltage is available for backing o f f a

steady dark current arising from tha 's ign a l' photomultiplier when

used for monitoring very low light levels. A high impedance current

source is derived from a photocell run at a low anode-cathode

voltage (9 v .) which monitors light from a small bulb supplied by

a variabla but stabilized voltage source.

47,

2.3 Spectral Response Calibration.

The recorder trace gives the luminescence spectrum of

the sample as modified by the spectral response characteristics

of the monochromator and the photomultiplier monitoring dispersed

light. Thus the spectral response curve for the instrument as a

whole must be determined so that the necessary correction factors

can be applied.

The instrumental spectral response profile is determined

by shining a tungsten lamp of known colour temperature onto a

magnesium oxide coated sample tray which is located in the usual

sample position, and it s spectrum scanned. The 12V, 36W tungsten

lamp was standardised by G.E.C. Research Laboratories who give the

colour temperature as 2854°K when run at a current of 2.90 amps.

The energy spectrum of a black body radiating at

2854°K is determined from tables. The required spectral response

profile can thus be found by dividing the recorded spectrum by

the calculated 'true ' spectrum. D ifficu lties were experienced

during this calibration procedure owing to grating anomalies, common

to a ll grating instruments, which manifest themselves ss kinks in

the spectral response curve. The shape and size of these kinks3

depend very much on the polarisation of incident light • I t is

assumed that luminescence from randomly orientated micro-crystalline

powders w ill be unpolarised. However, i f light scattered from the

magnesium oxide powder used for calibrotion is not completely

unpolerised, the spectrel response curve w ill not be correct in

the region of the anomalies. Such a situation results in the

sppearance of spurious features at the same wavelengths for a l l

corrected spectra. This problem was overcome by inserting e piece

of Polaroid before the entrance s l i t to ensure that both luminescence

48

and tha scattered light used for calibration are equally

polarised. I f the plane of polarisation of the light admitted

to the monochromator is made parallel to the grating rulings»

the kinks in the spectral response curve are also practically

eliminated. The only disadvantage of th is procedure is that

necessarily more than half the incident light is lost.

Both spectral response calibration and luminescence

spectral scans were carried out for the v is ib le region using a

Wratten 2B f i l t e r which absorbs light below 4000A in order to avoid

any possible U.V. in the second order from making a contribution

to the measured intensity of red light in 1st order. For example»

2nd order of wavelength 3500A may be registered along with 1st0

order 7OOOA in the absence of such a f i lt e r * However» fo r theO o

spectral range below 4000A (down to around 3500A) no f i l t e r or

Polaroid is used.

For infra-red scans a Wratten 25 f i lt e r is used to

avoid ambiguity which would arise owing to 2nd order v is ib le light*

3. The new low energy proton accelerator system.

3.1 Design and general description.

This system» which has recently been completed and tested,

was o rig ina lly intended simply to irradiate various rock samples

with protons to produce radiation damage similar to that which

the lunar surface was supposed to have had owing to aeons of solar

wind bombardment.

This project was originally planned as e Joint venture

with Dr. G. Fielder and Dr. L. Wilson of London Observatory who

were also interested in proton damage e ffects on lava samples

produced in their laboratory, but later another accelerator system

was bu ilt in their laboratory. Originally, Dr. L. Wilson was

49

responsible for the R.F. oscillator and ion source whilst the

author was responsible fo r vacuum system design and construction

and anc illia ry instrumentation.

In order to try to eliminate the contamination problem

during irradiation as described earlier the system was designed

to be evacuated by sorption pumps and a prototype electrostatic

getter-ion pump which is described in the appendix. A ll vacuum

seals are metal wire seals of indium or gold and the system is

entirely free from organic material.

The accelerator voltage was kept low ( < 10KV) for

several reasons. F irstly , the energy of protons in the 'qu iet'

solar wind is now known to be about 2 KeV . Secondly, insulation

and power supply problems are very much less i f the voltage is less

than about 10 KV. Furthermore, i f the proton current was to be

considerably higher than that of the original accelerator, as was

anticipated, then the energy of the particles should not be such

as to produce characteristic line X-radiation from the elements

under bombardment otherwise careful radiation shielding would be

required. Although the protons in the original 100KV accelerator

do produce characteristic X-radiation the flux is very low. However,

in the new accelerator the flux is rather higher but the low energy

'white' X-radiation produced is mostly absorbed by the walls of

the system.

The new system wss elso designed so that a cryostat for

thermoluainescence studies could be incorporated.

3.2 The ion source and accelerating system.

Diagrams of the system sre given in Figs. 17 and 18.

The design of the pyrex ion bottle and the extractor plates sre

due to A .E.R.E., Harwell who donated to us the extractor plates

50.

which are machined from dural. The ion bottle, which is about 2"

in diameter, has a silica disc mounted at its upper end to prevent

electron bombardment of the pyrex caused by the acceleration of

electrons in the reverse direction. The ion bottle is connected

to the accelerating region between the extractor plates by a

diameter hole in the high voltage extractor plate. Another

s ilica disc with a central hole of -J" diameter prevents ion

bombardment of this plate. The ion bottle is clamped onto the

top extractor plate using a viton *0’ ring holding the edge of

the bottle onto an indium seal. Acceleration is across a W

gap between upper and lower extractor plates and the accelerated

particles pass through a canal ( 3>/32 diameter, -J" long) in the

lower plate which is at earth potential. The extractor plates

are specially machined into smooth shapes to give the desired

fie ld distribution between them. Although the separation in the

accelerating region is quite small, the separation at the outer

rim is about 1". The insulating ring is of glass and is 1H in

depth and i " thick. It is ground f la t on both edges and sealed

by indium seals to both top and botton plates. The original

Harwell design used rubber 'O' rings recessed into grooves in a

perspex insulating ring, to which both top and bottom plates were

fastened using screws tapped into the perspex ring. In the present

design, the top and bottom plates were originally compressed

together by tapping bolts into insulating p illa rs outside the glass

ring. However, this method proved unsatisfactory since to get

enough compression of the indium to form two adequate seals the

bolts had to be tightened very hard resulting in either stripping

of the thread in nylon p illa rs or fracture of the p illa r it s e lf

when perspex p illa rs were used* Therefore, steel nuts and bolts

had to be used to clamp the plates together onto the glass ring.

These bolts are at earth potential and insulated from the top

plate by nylon sleeves and washers.

Hydrogen (BOC grade X) is leaked into the system via a

fine bore in the lower extractor plate from a small cylindrical

reservoir using a needle valve. Hydrogen is contained in this

reservoir at a pressure not exceeding 2 atmospheres ( i . e .

1 atmosphere w .r .t outside). This reservoir is replenished

periodically via a fine adjustment valve by gas from a small

cylinder. These cylinders are supplied at a gas pressure of about

14 atmospheres (200 lbs/sq.n) . Nitrogen can also be leaked into

the system from a somewhat larger reservoir in order to raise the

system to atmospheric pressure without admitting dirty or damp a ir.

Ionisation of the hydrogen is accomplished by an R.F.

c o il around the bottle which is the centre-tapped inductance of a

conventional Hartley osc illa to r. This oscillator was constructed

by Or. L. Wilson around a large tetrode valve (type 813) the anode

potential of which can be up to 2KV. The maximum power output of

th is oscillator is about 100W and is varied by adjustment of the

valve anode potential. The frequency is around 30 MHi and the

circu it has been pre-tuned for max. plasma density by adjusting the

spacing of a simple paralle l plate condenser connected across the

R.F. coil*

In order to concentrate the ions in the region of the

canal to obtain a larger proton current and to try to provide some

degree of focussing, co ils of D.C.C. copper-wire wouAd on insulating

fonaers are positioned around the base of the ion bottle around the

extractor plates and around the tubuletlon immediately below the

lower extractor plate. A D.C. current of up to 6 amps can be used

giving over a thousand ampere-turns. Whilst these coils have been

51.

52

successful in producing a much higher proton beam current, the

focussing e ffect on protons is only marginal*

One advantage of this type of extraction and acceleration

of the ions is that by simply reversing the polarity of the H.T.,

an electron beam can be obtained* The focussing e ffect of the

coils on electrons is reasonably good, since the e^a ratio is

considerably larger than for protons, and a spot of a few mms

diameter can be achieved at a distance of some 10" from the canal*

The accelerator H.T. is provided by a 0 - 15KV stabilized

D.C. supply (Brandenburg 705) which employs a Cockroft & Walton

stack fed by a high frequency oscillator. Ion beam current

measurement is achieved by intercepting the beam by a diameter

copper cup located about 1" from the canal. This cup is supported

by a copper wire which passes through a ceramic-insulated vacuum

lead-through in the centre of a stainless steel disc (see Fig. 17).

This disc is connected to the system by a stainless steel bellows

section which allows the copper cup to be moved into and out of

the ion beam. The proton current is monitored by allowing the

charge collected by the cup to flow to earth through a spot

microammeter* This bellows device was incorporated instead of a

simple 'O ' ring sealed mechanical feed-through in order to eliminate

the use of rubber or elastomer seals* Edge-welded bellows are used

(Palatine Precision Ltd.) since they have a greater extension and

compression per unit length than the usual folded bellows* The

spring rate was chosen so that the bellows just compress fu lly under

a pressure difference of one atmosphere and the natural length

of the bellows chosen so that the compression stroke was Just under

i " . The arrangement is such that when the syetem is under vacuum

the cup is in the ion beam intercepting position* In order to move

53,

the cup out of the ion beam, the bellows are stretched to their

natural length and held by inserting a metal U - c lip between the

stainless steel end disc and the wall of the system» The movement

of the bellows is constrained by two guide rods which pass through

brass bushes in the end disc»

3.3 The sample chamber and vacuua system.

The sample chamber is fabricated from thick stainless

steel tubing. It has several ports fitted with Mullard-type f la t

flanges suitable fo r gold-wire sealing. The chamber is 5" in

diameter and has a 1" diameter observation window through which the

central area of the base plate may be viewed. The stainless steel

base plate can be completely removed allowing access to the chamber.

It can also be replaced by a cryostat which w ill be described in the

next section. A nude Bayard-Alpert ionisation gauge head (Leybold)

is inserted in a 2" diameter port of the chamber fo r pressure

measurement. The electrostatic getter-ion pump is connected to

another 2" port via a 2" bore all-m etal, bellows-type valve

(Vacuum Generators L td .). The roughing vacuum line is connected to

a £" diameter port and can be isolated by a bore all-metal valve

(Mullard L td .). These valves seal by passing a gold or copper padA

connected to a bellows movement onto a circular stainless steel

knife edge. Originally two single tube sorption pumps were used for

roughing but were found inadequate to deal with the outgsssing of

the gauge head a fter the gauge had been at atmospheric pressure.

One of them has, therefore, been replaced by s larger multi-tubular

pump of similar design to the sorption pumps now fitted to the

120 KV proton accelerator system. The small single tube pump is

now used to evacuate the system from atmospheric pressure to around

10"1 to 10*2 torr and is then isolated. The multi-tubular pump is

f ia » 17 Sectional diagram of the new low energy accelerator syet<

(front view) with cryoetat attached*

IONB O TTLE

.UPPEREXTRACTOR, p l a t e(A T H . T . V \

LOWER

(A T E A R TH ) ' tG LA SS RING

CANAL

BEAM C U R R EN T

m e a s u r i n g c u p

b e l l o w s

T O SO RPTIO N PUM PS

PR TCO N N ECTIO N S

C R YO S TA T

Sectional diagram of new law energy accelerator ayatea

with cryoatat attached (aide view)*

D.C. F O C U S IN G - COILS

HYDROGENLEAKLINE

m ir r o r

p h o t o m u l t i p l i e r

H O USING

IONGAUGE

SAM PLE

COPPERROD LIQ UID

NITROGENTAN K

Flq. 20 Clow up vi#w of low «ntrgy acc#l*r«tor and iw p l* ch««b«r

with cryo»t»tt

ItliAilr i t i f i l i

•I •' I!'If

>« — » *

Fin. 20 Close up view of low energy accelerator and sample chamber

with cryostat.

View showing the ’ orbitron' pump with water cooling coil#

The cryostat is on the le ft and the hydrogen reservoir with

pressure gauge can be seen in the top right of the picture.

The cryostat#

The sample holder haa since been modified to reduce thermal

time lag between heater and eenaor# The heater shielding hae also

been improved#

F in . 21

F i n . 22

View showing the 'orbitron ' pump with water cooling c o il.

The cryostat is on the le f t and the hydrogen reservoir with

pressure gauge can be seen in the top right of the picture.

The cryostat.

The sample holder has since been modified to reduce thermal

time lag between heater and sensor. The heater shielding has also

been improved.

I

54.

then introduced to the system and pumps down to about 10“J torr.

At th is point the roughing line is isolated by the all-metal valve

and the getter-ion pump, which has been started whilst isolated

from the system, is introduced. The ultimate pressure of themn

system is around 5 . 10 torr (a t room temperature) and is again

limited to some extent by the dural in the system. However, whenIf

liquid nitrogen is pumped into the cryostat, i t is fitted to theA

system, the cooled surfaces of the liquid nitrogen tank adsorb

residual gas and sosm degree of cryopumping takes place resulting

in an ultimate of 10~7 torr or better. This effect is not eery

beneficia l, however, since subsequent heating causes adsorbed gas

to be released slowly at f i r s t and then quite rapidly in the

temperature region around 150 - 200°C indicating that the gas is

probably mainly water vapour. This rapid outgassing may cause

the pressure to r ise to almost 10*"4 torr. Above 200°C, however,

such outgassing is reduced and the pressure fa l ls again.

3*4 The Electronic recording system.

Integrated luminescence intensities of samples, mounted

at the level of the base plate, in the proton or electron beam,

can be measured by a photomultiplier (EMI 9558B, S20 cathode)

which ie attached to the viewing port. Various f i lt e r s can be

placed ever the window and luminescence from the sample paaaea

through the window and f i l t e r and is reflected by a plane mirror

onto tho photocathode (see F ig. 18).

A high stab ility H.T. unit (Brandenburg 471R) supplies

the dynode-biasing resistor chain of the photomultiplier and

the output of this photomultiplier ie fed to an electrometer

valve D.C. amplifier (Keithley picoammeter 416). This amplifier

• i ■ >i < ■ 1 i . . i . .

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boa isliqliJumoJ'riq erii \a nlsrio 10i»l*n i*ii?*ld-ebonyb erii

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haa a faat raaponaa (laaa than 1 n illisec on aoaa currant rangaa)

and» tharafora» a fa ir ly broad bandwidth. On the lower currant•8

rangaa (10 aaipa and below)» however, a variable daaiping control

ia operative which can lengthen the rise tiae up to a maximum of

3 sacs. The rise time can thua be optiaiaed to give the fastest

response compatible with an acceptable aignal-to-noiae ratio . The

current range ia determined by selecting the input resistor value.

Unlike the AVO amplifiers used previously this amplifier haa

appreciable voltage gain since the input resistor values are auch

lower for a given current range in order to make tha tiae constant

of the input circuit considerably shorter. The output for recorder

operation ia 3V maximum with an output impedance of about 1 K jl.

An XY recorder (Hewlett-Packard Moseley 703SB) is

available for recording luminescence intensity as a function of

any other parameter (e .g . temperature) and a plug-in variable

tism-base (Moseley 17106M) enables variations of luminescence

intensity with time to be recorded.

Thffaolyminf«9fnce InstrumentsIon.

Tfrt CTY9tW»

The requirement ia for the sample to be heated in vacuo

from around - t90°C to about 400°C. Seme type of dawar arrangement

ia necessary to contain the coolant* While the sample must be in

good thermal contact with the coolant, i t must bo reasonably

thermally insulated from the surroundings* Host losses or gains

by the cample holder are Inevitable without complex design* However

such heat flows can be reduced to a tolerable level by simple design

Conduction losaos or gains are reduced by making the conduction path

as long as peaaiblo and as small in ereoa-aectional area as peaaible

The material In the conduction path should, of course, be of lew

• *

(•e ‘i,*x JnfiTxrr aooa no oe«.lJHr. f nc.ii.' e iu i) «art* |S a? Jest a S6ii

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conductivity. Convaction lossaa ara reduced by having the apecinen

and heater in a good vacuum and radiation loaaaa by having poliahad

surfaces of low emisaivity. The design of the cryostat used hare

is an axtanaivaly aodiflad version of a design which Dr. M.L. Reynolds,

formerly of Hull University, used soma years ago. The sample is

placed on the and of a i " diameter copper rod which it brazed

through the top of a polished stainless steal cylindrical tank.

Thia tank is 2" in diaiaettr, 2" deep, and of wall thickness.t '

The copper rod protrudes 1-J* into the tank which can be f i l le d with

liquid nitrogen via a bora stainless steal tuba uhieh enters the

tank horizontally near its top (aaa F ig. 18). The tank la contained

in a 4" diameter stainless steal cylindrical vassal and is

supported only by the nitrogen f i l l in g tuba ahlch pasaaa through

the outer cylindrical wall. The outer cylindrical vassal has a large

flange which mates with the baaa-plata flange of the sample chamber

and is normally aaalad to it using an indium wire seal. Thus the0

apace around the liquid nitrogen tank forms part of the vacuum

aystarn.

Mounted on the copper rod batwaan the sample and the

top of the tank i t a 100W heater* The heater co il (20 S.N.G. nichrome)

is wound on a a llie s alaova and the whole haatar assembly la

sheathed by stainless ataal tub* to prevent light from the haatar

raaohlng the photomultiplier. The haatar la operated at a law A.C*

voltage ( <^*10V max) and the currant supply (10 A max) paaaaa through

twin oeramio vacuum leadthroughs In the outer wall*

The temperature of tha sample is monitored by a miniature

platinum rasiatanea element (Roamaount Engineering Co* Ltd*) whioh ia

positioned In a groove at tha and of tha eappar rod and paaaaa

through tha sample-retaining copper annulus* Before insertion i t was

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painted lavishly with colloidal ailvar paint to anavira good thanaal

contact with both tho copper rod and the »ample. The element l t M l f

ia encapsulated in a ceramic rod V 16" in diameter and $" long

although the element only occupies i Ns the rest is a sheath for the

silver lead connections* The platinum resistance thermometer leada

are connected via a glass multiple vacuus leadthrough in the outer

wall.

A platinum resistance element is used in preference

to a thermocouple on account of reasonably good linearity over the

entire temperature range.

4.2 Thf l i n e huU.nfl-J.ltf S°«feo,lle.r.

This controlling system was devised so that a specimen

could be heated at a linear rata from about -150°C to 400°C for

the purpose of obtaining thermolumineseence 'glow ' curves. A

linear heating rata is desirable i f 'glow' curves obtained by

different workers are to be compared and also fac ilita tes calculation

of trap depths.

Most ways o f producing a lin ea r heating rata use one of

two basic approaches. The f i r s t approaoh is to programme the supply

o f heat to the specimen holder so that a lin e a r rata o f temperature

r is e i s produced. This i s done em pirically by t r ia l and e rro r

u n til the correct programme i s determined. Often the required

programme can be achieved quite simply, sametimes even manual

operation o f a Variae in the heater supply may su ff ic e . A common

method ia to use a Variae driven by a meter v ia a earn which le out

te a shape which has been found w i l l g ive a lin h a f heating ra te ** .

The second approach ia to control the temperature by

a aervo-ayatem in which the camper icon standard o f resistance

(o r vo ltage ) la increased in such a may as te fe llo w with time the

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58.

resistance (o r voltage) tanperatura curve of the senaor. I f

this curve is nearly linear the experlMntal technique is very much

simplified. Some type of proportional controller is required since

simple on-off devices tend to produce over-correction particularly

in systems «here there is an appreciable thermal time lag between

heater and aenaor. The temperature range of a proportional

controller in which the current ia varied from zero to some maximum

is referred to as the temperature bandwidth. The required

temperature which the controller is trying to sustain is usually

situated about the middle of th is band. This second method of

producing a linear heating rate ia more frought with d ifficu lt ie s

owing to possible over-correction by the eervo-system on account

of the time lag involved between temperature measurement and the

Implementation of the correction. Howevert it does have certain

advantages over the f i r s t msthod. F irstly , i t la much leas

empirical and hence more flex ib le in that i t is easily adaptable to

differing conditions. Secondly, since i t uses a temperature

controller, the temperature can be held accurately at any value

above the ambient. In fact, this method i s the basis of the linear

heating rate device used here.

A miniature precision platinum resistance element

(resistance 90«/2.at 0 °C ) is used as ssnser and is osnnscted in one

arm e f a simple D.C. Mheatstone bridge . The reference resistance, a

lin ea rly wound, 15 tu rn , 0 - 1 5 0 ohms, h e lic a l potentiometer

(Golvem L td .) I s connected in the balancing arm and two high

tolerance, high s t a b i l i t y 100 ohm re s is to rs ^rq connected in the

two remaining arms (s e e P ig . 23 ). Since the resistance/tempersture

curve fe r the platinum resistance thermometer (P .R .T .) i s not

exactly linear over the complete temperature range, the h e lic a l

potentiometer is shunted between tappings a t 50 J l , 100 JL end 1 3 0 J I.

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A 1.2K til resistor connsctad bstwesn ths iOJl and 100Jt tappings

raducas tha raaistanca interval to 48 jland a 470 J t raaiator batman

tha 100 JL and 150 «/L tappings raducas this 50 J7, interval to

45 JL • This ensures that tha teaperature/tiae curve ganarstad

is a good reproduction of the rasistance/teaperature curve of tha

P.R.T. Tha helical potentiomter is motor driven vis a aulti-speed

gear box to give a range of d ifferent hasting rates*

I f tha resistance of tha P.R.T. is different fro « tha

resistance of tha he lica l potentiosMter, an out~of-belsnce voltage

w ill appear across tha input of a contact-aodulator type D.C.

amplifier (Pye). Tha output of th is anplifiar is than fad via a

biasing circuit to a thyristor firin g module (Neston Engineering

Co. L td .) which generates voltage pulses synchronous with the aains

at 100 Hz ( i . e . one per half cycle) but at a phase angle, with

reference to the mains, from 0 * 180° depending on the input voltage.

Them pulses are applied to the gate of a triac connected in the

heater supply. (A triac is a two way silicon-controlled re c t ifie r

which «r i l l conduct on both halves of the cycle from tha instant a

voltage pulse is applied to the gate until the end of that half

cycle. I t is equivalent to two thyristors In inverse p a ra lle l).

The A.C. current to the heater i s , therefore, contro lled by the

proportion o f a h a lf cycle which the t r ia c conducts. Zere f ir in g

angle g ives maximum current w h ilst a f i r in g angle c f 180° gives

sere current. A t r ie s hae the advantage that i t responds much

fa ste r than a serve motor. The time response o f th is e lectron ic

serve-ays tea depends mainly on the time constant o f the am plifier

which in th is ease io 1 sec max. '

The D.C. am plifie r used here mechanically chops the

input vo ltage , am plifies i t and woes phase sensitive re c t ific a t io n

to give a D.C. output o f -10V to +10V maximal depending an the

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polarity of th« Input. Voltaga gain« of 102 to 105 ara

available in decado «taps. The input impedance is 3.3K ohms.

The biasing circuit simply converts the voltage range

-10V to +10V into approximately 0 to 1V (see Fig. 23). Adjustment

of variable resistances allows a voltage range less than this to

be converted into 0 - 1V i f desired and also allows for zero output

from the amplifier to be converted into other than ¿V. A ze'ner diode

restricts the maximum voltage supplied to the thyristor firing

module input to ju st over 1V, thereby protecting the triac from

large amplitude pulses which may destroy it*

The temperature bandwidth of proportionality for the

control system can be varied by changing the amplifier gain, by

adjusting the variable resistance in series with the amplifier

input and to a lesser extent by adjusting the biasing circuit

resistances. The control system is , therefore, versatile and can

cope adequately with a range o f d iffering conditions including

quite large thermal time lags between heater and sensor.

■ten used in conjunction with the previously described

cryostat accurately linear heating rates were obtained in the.

temperature range above ambient temperature but in i t ia l ly some

deviations from lin e a r ity were experienced at low temperatures.

These were due to two e ffe c ts . One was a thermal time lag e ffe c t

oaused by switching on the heater and con tro lle r together which

resu lted in in i t ia l over-eerreotien . This a ffeo t could be

elim inated, however, by switching on the ^ea^ap f i r s t and waiting

fo r a few seconds before switching on the h e lie a l potentiometer

motor d rive . The other o ffse t was due to the be llin g o f f o f liqu id

nitrogen . Mien the lev e l o f liq u id nitrogen dropped below the end

e f the eepper red a major source o f heat le a s , namely the latent

B IA S C IR C U IT

Fig, 24 View of the electronic recording equipnent* The linear

heating rate controller ia aeen in the lower le ft of the picture.

View o f the electronic recording equipment. The linear

heating rate controller is seen in the lower le f t of the picture.

• »

mi

heat of vaporisation, was suddenly lost causing the temperature

to rise very quickly, and this again caused some overshoot«

However, this problem was overcome by pumping o ff most of the

liquid nitrogen prior to heating. The specimen holder is

sufficiently well insulated from the outside to hold its

temperature reasonably well for a few minutes.

4,3 Temperature measurement and recording.

Originally two P.R.T. elements were used« one for

temperature control and the other for temperature measurement.

After minor modifications to the specimen holder, made in order

to reduce the thermal lag, i t was found that temperature control

was so good that measurement and control could be Achieved using

only one P.R.T. element without appreciable error. It is possible

to keep the error voltage across the bridge to less than 0.2 mV

below ambient and less than 1 mV up to 350°C when controlling the

temperature using the P.R.T. in contact with the sample. Thus a

direct measurement of the voltage across the P.R.T. using the

X-axis of a high impedance XY recorder y ie lds a temperature

measurement which is accurate to better than 4# at low temperatures

and better than 3ft at high temperatures. Previously, temperature

measurement whilst somewhat more accurate ( ^ 2ft) required an

additional constant current supply.

The precision P.R.T. elements used obey the Callander-

Van Dusen equation relating resistance with temperature which

is accurately linear over temperature ranges of the order of

100C deg. but s t i l l ressonably linear over much longer temperature

ranges. The manufacturers quota raaistance values at 10 C deg

intervals with tolerances and the type used here (2050S) has a

resistance at 0°C of 50 - 0.00 ohms. Accurate meaeurement of lta

61.

62

resistance gives a temperature tolerance of Í 0.1 C deg at

-100°C, and Î 1.4 C deg. at 400°C. Thermal time response to

a 100 C deg. temperature step is of the order of a second.

CHAPTER I I I

UJMINE9CENCE IN METEORITIC SILICATES

63

1. Previous work«

Very l i t t le evidence o f studies of luminescence in

meteorites existed prior to 1964 and what l i t t le work had been done

was concerned mainly with thermoluminescence95' 96' 97. However,

the possible relevance of such studies to the lunar surface

stimulated further interest, particu larly a fte r the publication of

the luminescence spectra of enstatite achondrites in 19645’6.98 99

Reid et a l ’ were interested in the difference between red and

blue luminescent enstatite and o rig in a lly suggested that the

difference might be an order-disorder phenomenon, but later confirmed

the conclusions presented here as did Grflgler and Liener .

Houtermans & Liener101 and Liener et a l100' 102 determined the

natural and a r t i f ic ia l 'glow' curves for a large number of meteoritic

samples up to 400°C but only above room temperature* Sun & Gonzales,

however, determined the 'glow ' curves for enstatite achondrites in

the temperature range -150 to 250°C, for both blue and red emission

103bands separately using f i lte rs .

Using 10 KeV proton excitation, Nash has measured the

luminescence spectra and efficiency of a number of minerals and

72finda the efficiency, in general, very low •

104The use of cathodoluminescence as a petrological tool ’

105,106 ravi vtd interest in the nature of luminescence centres

in nsturally occurring minerals* However, most studies to date

have been phenomenological in approach and l i t t le or no discussion

of the nature of the luminescence centres has been attempted. This

is not surprising since the systems involved ere very complex both2+

in composition end structure* One exception has been the Mn7

emission in iron-free enststltes which was orig inally suspected

as a result of s comparison with synthetic MgSiO^ t Mn107*

64

Manganese-activated magnesium metasilicate is a well-known108

phosphor of known spectral emission and cathodoluminescence

efficiency4^.

Previous studies of the luminescence of plagioclases

have been limited mainly to thermoluminescence properties since

feldspars are probably responsible for thermoluminescence in most 109

igneous rocks • Detailed studies of the luminescence spectra

of plagioclases have only been attempted very recently when this

mineral was found to be the major luminescent component in lunar

surface m ateria l^*^»^3,110^

2. The Luminescence spectra of enstatlte achondrite meteorites.

The luminescence spectra of these meteorites consist of

a red emission band with a peak at about 6700A and a broad blueO

emission band of much lower intensity with a peak at about 4100A.

The proton-excited luminescence spectra of average samples of five

meteorites of this class are shown on the le ft of F ig.27. Variations

in the ratio of red to blue peak height occur between different

samples of the same meteorite as well as between different meteorites.

However, it is evident that average samples of Pesysnoe show a

larger red to blue peak height ratio and, of Bishopville a smaller

red to blue peak height ratio than the others.

*Following the original publication of the proton-excited

luminescence spectre of three specimens of th is class of meteorite,

Gerlick suggested that the red emission wee probably due to manganese

centres in the enstatite107. He showed that the position and shape of

the red emission peak of the meteorite Khor Temiki was almost

identical to the eathodoluminascenes spectrum of synthetic MgSi03

with 0.19 mole per cent manganese. It was not known at that time

what mineral phase was responsible for the blue emission.

In order to determine what mineral phases were

responsible for the two emission bands, a crushed sample of the

Bustee meteorite was separated by visual appearance under a

microscope into its different mineral constituents. This was

possible only on account of the coarse-grained texture of specimens

of this class of meteorite. White crystalline enstatite appeared

to be the major constituent. Gray crystals were identified, with

the help of Prof. MacKenzie of the Geology Department, as Diopside

(CaMgSigOg) which is a monoclinic pyroxene. This mineral, however,

usually showed ex-solution lamellae of enstatite and was, therefore,

d ifficu lt to obtain enstatite free . Brown crystals of oldhamite (CaS)

were present and rare minute golden octahedral crystals of osbornite

(TiN) were identified.

Proton-excited luminescence spectra of a l l these

separated fractions were determined with the exception of osbornite

which was non-luminescent. The emission spectrum of enstatite

was very similar to the emission spectrum of the meteorite as a

whole and was of similar luminescence efficiency. The luminescence

spectrum of diopaide, however, appeared to be similar but of lower

efficiency. It waa, therefore, concluded that this luminescence was

due to enstatite contamination. Oldhamite also appeared to be

sim ilarly contaminated but its luminescsnce spectrum showed an

additional paak at about 5800X, However, since this mineral is

present only in small amounts and is also more susceptible to

radiation damage than enstatite, this peak is not usually evident

in the spectrum of the meteorite as a »tools. The luminescence

spectrum of a laboratory sample of CaS was found to have a similar

yellow peak, and the luminescence efficiency was also found to

66

deteriorate rapidly*

It is concluded) therefor«) that the enstatite is

responsible for a l l the observed luminescence including the blue

emission*

By viewing the proton-excited luminescence of separated

enstatite grains under a low-power microscope it was noted that

some grains visually appeared to luminesce distinctly red and

others d istinctly blue whilst some appeared bluish redf or purple*

The d istinctly blue and red luminescent grains were separated out

and their luminescence spectra determined* The luminescence

spectrum of a visually red luminescent enstatite grain was found to

be sim ilar to the spectrum of an average sample of the meteorite

as a whole but with a s ligh tly reduced blue peak*

The luminescence spectrum of a visually blue luminescent

enstatite grain from the Bustee meteorite is shown in Fig* 26*

Although the blue emission band is now predominant) the red emission

peak is « t i l l present and the blue to red peak height ratio is

about 1*5* X-ray powder analysis has shown both red and blue

luminescent grains to be orthoenstatite*

Observations on a thin section of the Bustee meteorite

under proton irradiation revealed that a region of enstatite showed

mainly red luminescence with a small zone of predominantly blue

luminescence* Studies of the optical properties of this zone

under a mioroscope did not reveal any consistent differences from

the properties of the surrounding enstatite*

Grains of enstatite from the Khor Temikl meteorite

separated according to the colour of luminescence by the above

procedure were subjected to neutron activation analysis* Using the

"rabbit" fa c ility on the Universities Research Reactor at Rlsleyf

67

these samples were irradiated with thermal neutrons by

Mr« G. Hemingway. ]£-ray spectroscopy was then carried out on

the irradiated samples« Other samples were also subjected to

this type of analysis including a sample of the phosphor

MgSiOg - Mn supplied by Prof. Garlick; a synthetic clinoenstatite

(with no added Mn) supplied by Dr« Pollack; a blue luminescent

grain of the shallowater meteorite also supplied by Dr. Pollack;

and two samples of terrestria l enstatite which had been found

to have a very low luminescence efficiency under proton irradiation.

Table I l is t s the manganese content of these samples as obtained

by this analysis. The uncertainties listed in the values of

manganese content re fer to the statistics of £-ray spectroscopy.

The actual values depend on the analysis of the MgSiO^-Mn phosphor

carried out by Prof. Garlick and Dr. Steigmann using E.S.R. and

X-ray fluorescence analysis. The uncertainty in this value may

be as much as 2Q$. This uncertainty applies to absolute values but

not to relative values of manganese content. It is evident that

there is a definite correlation between manganese content and the

99"redness" of the luminescence. More recently, Raid & Cohen , and

GrBgler & Liener100 have also found such a correlation between

"redness" of luminescence and manganese content in anstatite

from anstatite achondrites using electron microprobe techniques.

The colour of luminescence in pure iron-free enstatite

is e measure of the reletive amount of manganese present (as Mn )

in magnesium lattice sites. The variations of colour of luminescence

in some crystals show that the manganese is very heterogeneously

distributed whilst in other crystals» uniformity of colour indicates

a homogenous manganese distribution. The luminescence efficiencies

of te rrestria l samples of enstatite are very low in spite of a high

68

manganese concentration* However, since enstatite may contain an

appreciable amount of iron, it is probable that the luminescence

in these samples is quenched on this account* Terrestria l sources

of enstatite are seldom iron-free, whereas Reid and Cohen" estimate

that enstatite in enstatite achondrites normally contains less

than 500 ppm iron.

By comparison with standard phosphors (ZnS I Ag, Cl and

Zn2Si04 i Mn) the luminescence efficiency of the enstatite

achondrites under proton excitation was estimated to be o f the

order of 1$. Luminescence efficiency is here taken as the ratio

of luminous energy emitted to the incident energy. Estimates of

efficiency have been based on the absolute measurements o f Bril

40and Klasens for 20 KeV electron excitation* Whether the luminescence

efficiency under proton excitation is similar to that under electron

excitation has been questioned” ' ^ and a discussion o f this

point w ill be given later. Nevertheless, the luminescence

efficiencies under proton and electron excitation are here assumed

to be similar on the basis of the evidence presented in Chapter I ,

section 3*4*

3. Luminescence of synthetic enstatites and forsterites*

3*1 Preparation.

The mineralogy of enstatite achondrite meteorites gives

evidence that the formation must have been in a highly reducing

environment* Any iron present is either in the metallic state

or in the sulphide phase ( t r o i l i t e ) ^ 0* In order to prepare

synthetic enstatita in similar conditions, Dr. James, formerly of

the Geology Department, heated prepared gels in a carbon crucible

using a radio frequency heater to a temperature of ebout 1700°C

and small crystals of anstatite were formed from the melt on cooling*

69

X-ray powder analysis showed the enstatite to be clinoenstatite.

It was found that rapid cooling resulted in forsterite (Mg2Si04)

crystallizing out, particularly when manganese was present. In

order to produce a pure orthoenstatite hydrothermal techniques

had to be used. This was a lengthy procedure involving heating

for up to 20 days at around 800°C at high pressure. On account

of the time involved in producing orthoenstatite, investigations

were carried out using clinoenstatite. Clinoenstatites were

prepared with nominally zero, 100, 300 and 1000 ppm Mn by weight.

Iron bearing clinoenstatites to which 1000 ppm manganese had been

added, were also prepared in this manner. The resulting compositions

nominally contained 1, 3 and 10 weight per cent FeSiO^. A sample

with nominally 10 weight per cent FeSiO^ was also prepared without

added manganese.

Forsterite was also prepared in the above manner with

no added manganese and with 1000 ppm manganese. Preparation of

olivines with 1000 ppm manganese and nominally 10, 20 and 30 weight

per cent Fe2Si04 was attempted by the above method but this proved

unsatisfactory since a ll the iron was reduced to the metal phase.

Compositions were then tried at subsolidus temperatures by heating

in a carbon crucible at red heat for one or more hours. The

compositions produced formed mixtures of olivine plus iron oxide

and the resulting composition of the olivines were estimated by

112X-ray d iffraction using the method described by Yoder and Sahama •

A ll preparations and X-ray analysis were carried out by Dr. James.

3.2 Spectra of iron -free enstatites and fo rsterltes.

The proton-excited luminescence spectra of synthetic

clinoenstatites are shown in Figs. 29 and 30. Pure iron-free

enstatite prepared with as l i t t le manganese as possible ( ~ 20 ppm)

70,

appears visually blue under proton irradiation. However, the

spectrum obtained showed that the characteristic red emission peak

2+of the Mn ion was s t i l l contributing to the luminescence, and the

red to blue peak height ratio was about 0.9. The blue emission

appears to be a property of pure enstatite it s e lf and the

luminescence centres are probably due to imperfections (e .g . vacancies)

in the la tt ice . As w ill become evident throughout this work,

iron-free s ilicates in general usually exhibit a blue emission band

o«fin addition to Mn or other activator bands. Reduced s ilica also

113shows a strong blue emission band which suggests that the centre

may be an oxygen vacancy. The sub&tution of Al3* for silicon has

also been suggested as giving r ise to a blue emission114. Such a

substitution would make oxygen vacancies more probable owing to

charge compensation effects.

The efficiency of the intrinsic blue luminescence does

not vary drastica lly with manganese concentration! the blue peak

height of a sample containing 20 ppm Mn was about twice the blue

peak height of a sample containing 1000 ppm Mn. However, the red

peak height for the latter sample was some forty times that for

the sample with minimal Mn.

Fig. 33 shows the variation of efficiency of red

luminescence of synthetic clinoenstatite with Mn concentration.

The proportion of the added manganese which substitutes for

magnesium probably depends on the method of preparation and, in

particular, on the degree of reduction attained. It appears that

for a given manganese content the luminescence of synthetic enstatite

is in general "redder" than meteoritic enstatite thus suggesting

that a greater proportion of manganese has substituted in cation

sites.

71

The luminescence of clino and orthoenstatite d iffe r

mainly in the position of the blue peak. For clinoenstatite the

blue peak is shifted about 200X to longer wavelengths with respect

to orthoenstatite. This fact is demonstrated c learly in Fig. 32

which shows uncorrected spectral profiles of the blue emission of

synthetic clino- and orthoenstatite along with that of Khor Temiki.

The blue emission peak for Khor Temiki occurs close to that of

synthetic orthoenstatite showing that the enstatite present is

mainly orthoenstatite with possibly a l i t t le clinoenstatite, which

agrees well with mineralogical determinations. The position of

the red emission peak appears to be similar for both polymorphs but

with the emission peak for clinoenstatite at possibly slightly

longer wavelengths ( ^ 5 0 - 100A).

Synthetic forsterite shows similar luminescence propertiesO.

to synthetic enstatite (see Fig. 31). The Mn emission occurs

at shorter wavelength (peak at about 6400A) but there is a similar

dependence of red peak height on manganese concentration. The

blue emission occurs at longer wavelength than fo r clinoenstatite

with a peak at around 4600A. There is also a pronounced difference

in radiation damage properties in that the intensity of the red

emission of synthetic forsterite f a l l s much more rapidly with

respect to the blue emission during proton irradiation than i t does

in synthetic enstatites.

3.3 Spectra of ferromagneslan pyroxenes and o liv ine».

The isomorphous substitution of iron fo r magnesium in

these silicetes has an important effect on their luminescence

even when only s small proportion of magnesium is thus replaced.

As might be expected the luminescence efficiency is reduced es a

72

result o f the quenching e ffect of iron (presumably Fe2+) .

2+However, the red Mn emission is quenched to a greater degree

than the blue emission thus changing the shape of the spectral

profile in a manner similar to that caused by reducing the

manganese content (see Fig. 30). In addition, the substitution

of a small amount of iron causes a sh ift in the position of the

blue peak for clinoenstatite from about 4300A to about 4600A.

Fig. 31 shows the effect on the spectrum of forsterite of partial

substitution of iron for magnesium.

Relative effic iencies of blue and red emission for

synthetic pyroxenes and olivines with different iron contents

are shown in table II where they are compared with meteorite samples.

The fact emerges that pyroxenes and olivines In which nominally

10# or more of the magnesium has been replaced by iron have

exceedingly low luminescence e ffic iencies. Moreover, the

concentration of manganese is no longer an important factor either

in determining the efficiency or colour of the very weak emission.

4. The luminescence of other classes of stony meteorites.

No type of meteorite examined has a luminescence

efficiency that is as high as that of the enstatite achondrites.

In fact, a l l other stony meteorites have a luminescence efficiency

under proton excitation which is at least an order of magnitude

lower and often two orders of magnitude lower. For most types

of stony meteorites, the proton-excited luminescence spectrum

changes shape to some extent under prolonged irradiation. The way in

which i t changes also appears to be characteristic of the class of

meteorite. The luminescence properties of various classes of stony

meteorites w ill now be described.

Fig, 25

Fig. 26

Block diagram of electronic recording system for

thermoluminescence measurements»

Proton-excited emission spectrum of a blue

luminescent grain from the Bustee meteorite*

TABLE I

wavelength (A)

Mn content(p/M by at.) luminescencu approx, red/blue

•ampin f-------- colour peek ratiosynthetic elinoeiiHtatito < 10 ± blue 0 «Shallowater 8A 10 blue 1

blue, 2 grains / I7U Irtl isor. ior 1 blue 0-fl to 2

Khor Tcmiki blue/not 425 10 blus/rod —

red, 2 grains 1 770 40l 11450 301

rod « t o 10

terrestrial »nutet ite; Moravia 1020 20 nil —

H. Carnlinn 1970 20 nil —

Mg8iO,-Mn 4500 »0 rod 00

I

Fig. 25 Block diagram of electronic recording system for

thermoluminescence measurements*

Proton-excited emission spectrum of a blue

luminescent grain from the Bustee meteorite*

TABLE I

Mn content (p/M by a t . ) lunumaeanoo approx, rml/blue

•ampin -----------«---------- colour pivik ratio

•ynthotia alinonnHtatita < 10 ± blun 0-5ShallowutJT 85 10 bliin 1

blue, 2 grain«(170 151 1305 lO f

1 blue 0 5 to 2

Khor TVcniki blue/red 425 10 blun/rcd —

mH, 2 grain*( 770 40l 11450 30)

rod • to 10

tornntriiil »nutatito: Moravia 1520 20 nil —H. Camlinn 1070 20 nil —

MgSiO.-Mn 4500 »0 rml 00

Proton-excited luminescence spectre of echondrite meteorites*

The fu ll curves are for relatively undemaged samples and the

dashed curves« where shown, ere spectra after subsequent

irradietion damage* The ordinates represent intensity on scales

which are arbitrary but linear with the zero on the wavelength

ax is , which is graduated in thousands of A units* The spectra of

damaged samples have been scaled to be below those of the undamaged

samples but the actual intensity scales are unrelated* The

instrumental bandwidth is in a ll cases about 3QA*

Johnstown Hypersthene Ach.

i— r

t — t— t— *

Khairpur Laka LabyrinthEnatatila Ch. Hyparathana Ch.

à.

I

Fig» 29 Luminescence spectra o f i -

1* Khor Temiki (enstetite achondrite)

2. Synthetic clinoenstatite t 316 ppm Mn

3. Synthetic clinoenstatite t 18 ppm Mn.

The linear intensity scales of these curves are arbitrary

and unrelated.

F io. 30 Luminescence spectra o f i -

1. Synthetic clinoenstatite i 1000 ppm Mn|

3 mole per cent FeSiOg

2. Synthetic clinoenstatite i 1000 ppm Mnj

10 mole per cent FeSiO^

The linear intensity scales are arbitrary and unrelated.

FIq . 31 Luminescence spectra o f i -

1. Synthetic forsterite i 1000 ppm Mn.

2. Synthetic olivine I 1000 ppm Mn|

4 mole per cent Fe2Si04

3. Synthetic clinoenstatite.

The linear intensity scales are arbitrary and unrelated.

Fig, 32 Uncorrected spectra of the blue emission o f« -

1, Synthetic orthoenstatite

2* Synthetic clinoenstatite

3, Khor Temiki (enstatite achondrite)

Fla* 33 Relationship between red peak height and manganese content

for synthetic clinoenstatite

TABLE I I

Fe x 100* Intensity of

red emission

at 6600A

Intensity of

blue emission

at 4300A

Sample (Fe + Mg)

in s ilicate

pyroxene 0 (<100 ppm)Fe

1

1000 + 15(Mg,Fe)Si03 250 5

+ 1000 ppm Mn 3 11 0.3 (0 .8*)

10 0.9 0.4 (0 .7*)

pyroxene 0 24 30

+ 20 ppm Mn 10 - 0.5

olivine 0 1200 13 (40»)

(Mg,Fe)2Si04 4 14 0.5 (1 .5*)

+ 1000 ppm Mn 10 0.3 < 0 .5

Meteorites (composition of samoleis variable)

Khor Temiki 0 ( < 0.05) 230 - 300 13 - 25(enatatite

chondrite)

Holbrook '>20 1 - 1.5 3 - 4(hyperethane

chondrite)

Khairpur ? ( < 3 ) 12 - 13 3 - 4(enstatite

chondrite)

f taking th ia arbitrary valua (abaoluta efficiency-\$0

and measuring other valuaa ralativa to thia*

* at 4600A

73,

4.1 The Enstatite Chondrites.

Proton-excited spectra of three specimens of this class

of meteorite are shown at the top le ft of Fig. 28. Khairpur and

2+Daniels Kuil show quite distinctly the Mn emission from the

enstatite, which constitutes about 50# of these meteorites, whilst

in the emission spectrum of Abee the Mn emission peak is very weak.

As in the case of the enstatite achondrites, the luminescence

efficiency is higher when the red to blue peak height ratio is

larger. The spectrum of Indarch has also been determined and found

to be similar to Daniels Kuil but with a slightly lower red/blue

peak ratio . Comparisons of the in it ia l luminescence efficiency

of the red and blue emissions of various samples of Khairpur have

been made with the corresponding emissions of Khor Temiki ( an

enstatite achondrite), and these show that the luminescence

efficiency of Khairpur is of the order of 0.06$. The comparative

figures are given in table I I . These comparative figures are in

good agreement with those more recently determined by B lair and

115Edgington using 146 MeV proton irradiation . Comparisons of

estimates of absolute efficiencies with those estimated by these

workers are d if f ic u lt on account of the grossly different proton

energies and fluxes involved.

This class of mstsorite is also characterized by a high

degree of reduction. MnS has been reported in Abee so that i t is

possible that the manganese concentration is very low in the s ilicate

phases of this meteorite. However» the luminescence efficiency of

the blue emission in enststite chondrites is considerably less

than that for enstatite achondrites. This fact suggests that the

snstatits is not ss pure in the chondrites as i t is in the achondritee.

This contention is supported by very recent work by Greer1 A

74

small amount of iron in the enstatite would, in addition to

reducing the overall luminescence efficiency, also reduce the

red to blue peak height ratio with respect to iron-free enstatite

with a similar manganese concentration, as shown in the last

section* Thus we have a qualitative explanation of why the red to blu«

peak height ratios are lower in the enstatite chondrites than in

the achondrites* However, proton irradiation damage effects in

enstatite chondrites lead to a very noticeable difference in the

rates of deterioration of blue and red luminescence. The intensity

2+of the red Mn emission deteriorates much faster than that of

the blue emission although the blue emission peak also sh ifts to

longer wavelengths* Excepting Bishopville which shows a similar

effect, enstatite achondrites and synthetic enstatites do not show

such a pronounced d ifferentia l deterioration of the two luminescence

emissions* The reason for this difference in luminescence properties

between enstatite from the chondrites and synthetic enstatites with

a small proportion of added iron is not yet clear* It is possible

that other re lative ly iron-free mineral phases, which are present

in small amounts, may be contributing to the luminescence* The

most likely is plagioclase usually present in amounts up to 1CJ6

and of oligoclase composition*

4,2 Bronzlte and Hypersthene chondrites*

These are by far the most common types of stony meteorites*

The Prior-Hay catalogue lis t s about 250 of them and according to

Mason many of the unclassified meteoritic stones probably belong

to this group1 *

The proton-oxcited luminescence spectra of several

specimens of these types of meteorites are shown in Fig* 28* The

75

overall colour of luminescence could be described as bluish-white

or pale blue» These meteorites consist mainly of pyroxene and

olivine with usually more of the latter. Plagioclase of

oligoclase composition is usually present in amounts up to about

1($. The pyroxene contains more than 14 mole per cent FeSi03 in

the bronzite chondrites and more than 20 mole per cent in the

hypersthene chondrites; the olivine contains a similar proportion

of iron. Therefore, provided the pyroxenes and olivines present

are equilibrated ( i . e . the distribution of iron fa ir ly uniform)

then l i t t le or no luminescence would be expected from either of

these minerals according to the results of section 3.3. The

efficiency of luminescence in these meteorites is of the order of_4

10 but even this is considerably higher than the efficiency of

synthetic enstatites and olivines containing nominally 10 mole

per cent iron silicate (see table I I ) . The spectral p ro file usually

shows a maximum at around 4500^ in it ia lly but the peak becomes less

pronounced and moves to longer wavelengths on prolonged exposure

to proton irradiation; the overall luminescence efficiency decreases

115in the usual fashion. B lair and Edgington suggest that the

ferromagnesian minerals in these meteorites are responsible for

the luminescence emission but according to the results presented

here, this seems unlikely. Moreover, studies of thermoluminescence

of these meteorites by Liener and Houtermans have shown that

there is a correlation between the intensity of thermoluminescence

and calcium content. Since most of the calcium w ill be in the

feldspar they conclude that the feldspar ia responsible fo r the

observed thermoluminescence. In order to determine whether the

proton-excited luminescence was due to the feldspar (plagioclase)

component, mineral separation was attampted on Holbrook, a

hypersthene chondrite,using the fa c ilit ie s kindly provided by

76

Prof. McKenzie of the Geology Department. The technique used

was the usual one of separation by density using heavy organic

liquids of d ifferent densities. Plagioclase is considerably less

dense than the ferromagnesian minerals and w ill tend to float in a

liquid of intermediate density. Repeated separation of the light

fraction was carried out and the refractive index of the grains

thus separated estimated by immersing in liquids of known refractive

index on a microscope stage and then observing the Becke effect.

This separation technique proved very lengthy and did not yield

very pure separated minerals, owing to the existence of composite

grains. Further grinding to a smaller grain size was, therefore,

required but separation with very fine grains proved d iffic u lt

owing to surface tension and suspension effects. However, the

luminescence intensity of the light and heavy separated mineral

fractions from Holbrook were compared. It was found that although

the heavy fraction s t i l l showed some luminescence (possibly due to

unseparated plagioclase) the emission from the light feldspar

fraction was about two or three times brighter. Viewing the

luminescence of these grains through a low power microscope revealed

a fa ir ly large grain showing an orange coloured luminescence.

Dr. M ills of Leicester University has produced colour

photo-micrographs of a section of the Barwell meteorite under

electron bombardment. These show areas of blue luminescence and

occasional small areas of yellow-red or orange emission with large

areas showing negligible luminescence.

The recognition of minerals is much easier in thin

section and simultaneous analysis of the mineral phase and

observation of i t s luminescence is possible, using an electron

microprobe analyser. Therefore, with the helpful cooperation of

Mr. P. Suddaby of the Geology Department, Imperial College of London,

sections of the equilibrated hypersthene chondrites Appley Bridge

77

and Mangwendi were studied using an electron microprobe analyser.

An interesting area of the section was selected and

the scanning electron beam fa c ility used. This fa c ility allows

a fine electron beam to raster across the selected area. The

resulting luminescence emission can either be photographed in

colour or visually observed. Colour photography, however, requires

very long exposures of the order of an hour or more. The X-ray

emission from the area under bombardment is monitored using an

X-ray crystal spectrometer and geiger tubes. The spectrometer was

set on the K — line for a particular element and the output of

the scaler used to modulate the intensity of the electron beam

of a C.R.O. The C.R.O. electron beam was rastered in synchronism

with the microprobe electron beam and the resulting pattern

photographed. This pattern showed the distribution of the

particular element over the area of the section under bombardment.

Element distribution photographs were obtained for several elements

including iron, aluminium, calcium and magnesium. A definite

correlation was observed between areas of low iron count and areas

of luminescence. Areas of low iron density usually coincided with

areas of appreciable aluminium density indicating that these areas

were plagioclase. One small area of a section of Appley Bridge

showed the yellow-orange luminescence mentioned previously. This

area had a very low iron and aluminium count but a very high

calcium count. Mineralogical analyses of these meteorites show

that they often contain accessory amounts (about 1$) of apatite,

which is a calcium halophosphate. An X-ray picture of the

phosphorus distribution confirmed that the orange luminescence

was an area of apatite. The colour of this luminescence is

reminiscent of manganese-activated calcium halophosphate. Areas

of high megnesiiaa and iron count were usually associated with non-

78

luminescent areas and, therefore, it appears that the plagioclase

present is responsible for almost a l l the observed luminescence

with a small contribution from the minor amounts of apatite.

4,3 The Pyroxene - Plaoioclase achondrites.

These meteorites are the commonest type of achondrites

and there are now more than 40 recorded. They are characterized

by a re lative ly large proportion (usually of plagioclase

which is near anorthite in composition. This class of meteorites

is usually subdivided into eucrites and howardites according to

the nature of the pyroxene. In the howardites i t is mainly

hypersthene and in the eucrites i t is mainly pigeonite which is

a clinopyroxene containing a small amount of calcium. Both the

hypersthene and the pigeonite contain more than 20 mbit per cent

iron silicate and would not, therefore, be expected to contribute

much to the luminescence of these meteorites. The proton-excited

luminescence spectra of a typical howardite and eucrite are shown

in Fig, 27. Both show a prominent peak at about 560oX which

becomes less prominent after prolonged irradiation. This peak is

almost certainly the same as that which occurs in the luminescence

spectrum of lunar plagioclases as w ill be discussed in the next

chapter. A number of terrestria l samples of plagioclase have also

been found to exhibit the same peak in the proton-excited

luminescence spectra. Moreover, there is also a hint of this

emission peak in the spectrum of some bronzite and hypersthene

chondrites (see Fig. 28). A discussion of the possible origins of

plagioclase luminescence is deferred until the next chspter.

The efficiency of luminescence in Juvinas (a eucrite)

and Kapoeta (a howardite) is somewhat higher than for hypersthene

and bronzite chondrites but of the same order of magnitude ( ~ 1 0 “4) .

79

4.4 The Hypersthene achondrites.

These meteorites, which are small in number, consist

almost entirely of hypersthene with only small amounts of other

minerals such as olivine and plagioclase. Of the three meteorites

of th is class which were examined, only Johnstown exhibited

any measurable luminescence, Shalka and Tatahouine had luminescence

-5efficiencies of less than 10 . The proton-excited luminescence

spectrum of Johnstown is shown in Fig. 27 and it is probable that

this luminescence originates mainly from the small amount of

plagioclase present which is bytownite in composition* The

luminescence efficiency of Johnstown is rather lower than that

of a typical hypersthene chondrite, but of the same order of

magnitude ( about 10 ) ,

4.5 The Olivine - Pigeonlte achondrites.

There are only three known specimens of this rare class

of meteorite and only two of any appreciable size. Of these

two, one Novo-Urei is kept in the U.S.S.R, and so only Goalpara

was available fo r study. An analysis of Novo-Urei has been made

117by Ringwood who did not detect any feldspar. These meteorites

consist of olivine and clinopyroxene (pigeonite) in a matrix of

carbonaceous material. Very small diamonds have been found in

Goalpara and Novo-Urei. Ringwood measured the refractive indices

of the olivine and pigeonite and deduced that they contained more

than 20 mole per cent iron silicate . In view of the mineralogical

composition of these meteorites i t might be predicted that no

luminescence would be observed. However, Goslpara shows a red

luminescence under proton (or electron) excitation and the

proton-excited luminescence spectrum is shown in Fig. 27, Moreover,

the efficiency o f luminescence is compsrable with that of the

80,

-4 -3enstatite chondrites i .e . in the range 10 to 10 . The

emission is almost to ta lly confined to the orange-red region

of the spectrum and the peak occurs about 6400A. The origin

of this emission remains an unsolved problem. Neither of the

major constituent minerals would be expected to show luminescence

with so much iron present, particularly i f the emission is due to

2+Mn . Moreover, the lack of any appreciable blue emission is

also unusual. It is possible, however, that the olivine and the

pigeonite are unequilibrated and that small regions of low iron

content occur which may coincide with regions of high manganese

content. Both iron and manganese have a preference for the same

lattice site (M2) in o livines and pyroxenes. This possib ility is

suggested by the fact that very recently, red emission has been

118observed in an unequilibrated hypersthene chondrite . However,

such an explanation can only be very tentative without further

information.

4.6 The Ollvlne-Pigeonlte and Carbonaceous Chondrites.

Several meteorites of these types were examined

(Karoonda, Kaba* Orgueil and Cold Bokkeweld) and none showed

any appreciable luminescence) i .e . luminescence efficiencies

-5were les6 than about 10 • Olivine-pigeonite chondrites often

contain up to 1Q6 plagioclase but either this must have an

appreciable iron impurity content or else most of the luminescence

is absorbed by other mineral phases. These meteorites are almost

black in colour, and consist mainly of olivine with more than

30 mole per cent faya lite (Fe2Si04) . The s ilica te in the

carbonaceous chondrites is usually hydrated and sometimes amorphous.

It is stated that some carbonaceous chondrites contain chondrules

16eof forsterite or enstatite which would show some luminescence,

but such chondrules were not evident in the samples examined.

81

5. The effect of proton irradiation on luminescence effic iency«

As discussed in Chapter I (section 3 , 5 ) , proton

irradiation of phosphors using high flux densities leads to a

rapid decrease in luminescence efficiency* Measurement o f the

in it ia l intrinsic luminescence efficiency of the undamaged phosphor

must, therefore, be made before enough particles have been incident

to cause a noticeable deterioration. When fa ir ly high flux

densities ( ^ > 10^2 particles cm“2 sec"*) are used, the measurement

must be carried out in the f i r s t fraction of a second of exposure.

Failure to do this may result in large errors in the estimation

of intrinsic luminescence efficiency, particularly for phosphors

which show the quickest deterioration such as organic phosphors

and sulphides. The usual type of recording system involves a time

response of the order of a second which is determined either by

the amplifier time constant or the response time of a pen recorder.

12 -2 -1For proton flux densities higher than about 10 cm sec a time

response of the order of a second or more could lead to serious

error in estimating the intrinsic luminescence efficiency*

Comparisons of efficiencies were, therefore, made using as small

a proton flux density as possible. The smallest workable beam

current density was about 0.04 ^A cm“2 which corresponds to around

2 . 1011 protons cm“2 sec“^.

12According to G ilfrich about 2 . 10 incident protons«2

dm result in the luminescence efficiency of ZnS - Ag (hexagonal)

fa llin g to half its in it ia l value**. In order to examine the in it ia l

rate of deterioration of luminescence under the conditions of

irradiation in the present accelerator the luminescence decay of

ZnS - Ag with time was recorded using a fast response recording

system. This system used a Keithley high speed picoammeter

82

type 416 which is an electrometer D.C. amplifier having a

minimum rise time of the order of a millisecond» The output

was then fed to a C.R.O, (Hewlett-Packard 140A) which was set to

trigger a single time base sweep on the incoming pulse» The

resulting trace could then be photographed using a Polaroid 'scope

camera with the shutter held open» The results are shown in

Fig. 34, The 'scope trigger was armed for a single sweep at

either 0.1 or 1 sec/cm and the flap valve, which was intercepting

the proton beam, quickly moved away to expose the sample* The

luminescence was monitored in a particular wavelength band by a

photomultiplier and the signal from this triggered the 'scope

time base. The major limitation of this method is the speed with

which the flap valve can be moved out of the proton beam. This

takes about a ^ 20th to V-|oth of a second. Measurements of the

change ( i f any) in the luminescence efficiency in the f ir s t V^qq th

of a second of exposure to irradiation would be possible using a

photographic shutter in the proton beam. However, the ultimate

limitation is one of signs1-to-noise ratio when using large

bandwidth amplifiers. Results obtained here for ZnS t Ag are in

reasonable agreement with those obtained by G ilfrich .

72However, Nash claims that the luminescence effic iency

of ZnS i Ag under 5 KeV proton irradiation is some 300 times less

than the luminescence efficiency under 20 KeV electron excitation

as measured by B r il & Klasens4®. In view of the measurements of

Henle & Rau41 and the feet that th is phosphor is often used fo r

the detection of heavy particles this result seems anomalous*

However, Nash quotes this efficiency as that measured after one

second of exposure to the irradiation. Since the proton current

density is around 10jih/cn , the efficiency quoted is likely to be

83

considerably lower than the in it ia l intrinsic efficiency. In

fact, assuming that the formula I ■ *o is applicable1 + CN

at such a current density where C has a sim ilar value to that

obtained by G ilfr ich and by the author, then the intensity after

one second of exposure would have fa llen by a factor of the order

of 50 - 100. Nash's figures for luminescence efficiency of

ZnS i Ag after various times of exposure show that the above

formula appears to be applicable to his resu lts but with a value

of C which is about 100 times smaller than that determined by

either G ilfrich or the author. It would, therefore, appear that

C may be dose rate dependent particularly fo r high dose rates.

However, experiments carried out by the author indicate that C

changes very l i t t le fo r an order of magnitude change in dose rate

but i t has not been possible so fa r to carry out measurements at

proton current densities similar to those used by Nash.

111Schütten & Van Dijk have also claimed that an

appreciable difference in luminescence efficiency occurs between

electron and proton excitation. Their measurements were carried

out on willemite (Zn2Si04 t Mn) and suggest a factor of 103( I )

for the ratio of e ffic iencies under electron and proton bombardment.

However, it has since been ascertained that proton current densities

of the order of 50 «A/cm were used and it is not known just how

long a fte r in it ia l exposure these measurements were made. It

seems reasonable, therefore, to discount such measurements as

invalid .

It is found that the deterioration of the luminescence

efficiency of enstatite is considerably slower than that of

ZnS i Ag. This is consistent with the findings of previous

workers4* ’ regarding the higher stab ility of silicatea.

A comparison of the in it ia l deterioration of luminescence

intensity of ZnS t Ag with MgSiOg t Mn under 40 KeV proton

irradiation is shown in Fig, 34.

The deterioration of luminescence over several minutes

of proton irradiation was monitored using the usual recording

system (time constant sec) and the results for ZnS i Ag and

the meteorite Khor Temiki are plotted in Fig. 35 and 36. I f

the formula I ■ — is applicable, a plot of 2 /N yields1 + CN I

C 1a straight line of gradient ___ and intercept 1 on the*o I 0

1 axis. From Fig. 35, the damage constant C was found to be

-14 2 2+about 1.5 . 10 cm. sec.for the Mn emission of the meteorite

Khor Temiki. On account of the rapid deterioration of the

luminescence intensity of ZnS i Ag under proton irradiation, the

measurements for this phosphor are best plotted logarithmically

(F ig, 36). A plot of log 2 against log N yields a straight

line of gradient unity when CN 5 1, i f the formula given above

is applicable. When CN o»1, the plot yields a curve but by

extending the straight line until the value of 2 on this line

is 'iia lf the actual value on the curve we find the value of N when

-14CN ■ 1. This procedure gives a value for C of about 140 . 102

cm . sec. for ZnS 1 Ag. This is rather higher than the value

obtained by G ilfrich but of similar order o f magnitude.

Recovery of the luminescence efficiency of enstatite

has been obtained by heating for an hour at a temperature of hbout

800°C. A degree of recovery at lower temperatures was achieved by

heating for longer periods but very l i t t le recovery was evident

after heating for several days at temperatures below about 350°C.

I

/

Fig. 34 Oscilloscope traces of the in it ia l fa l l of luminescence

intensity on f ir s t exposure to 40 KeV proton irradiation#

(a ) ZnS t Ag, sweep speeds 0*1 sec/cm.2

proton current density« 0.15 ^A/cm .

(b ) ZnS s Ag, sweep speeds 1 sec/cm.2

proton current densitys 0.04 ^A/cm .

(c ) MgSiOg s Mn (1000 ppm)

sweep speeds 1 sec/cm.2

proton current densitys O .M p A / c m •

'

( a )

( b )

I

Fig» 35 Graph showing the deterioration of luminescence intensity,, o

I , with total number of incident protons /cm , N, for

ZnS i Ag and for the red emission of the meteorite

Khor Temiki (K .T.)

F ig, 36 A logarithmic plot of the reciprocal of luminescence

intensity, I , against total number of incident

2protons /cm , N, for ZnS t Ag. The gradient is

approximately unity except in the region where CN « 1,

85

Such heating was carried out in a tube furnace through which

a continuous stream of 'white-spot' nitrogen was passing. Great

care had to be taken to avoid jostling the powder grains on

transfer from the accelerator to the furnace and back since a

disturbance of the grains would give rise to an apparent recovery

of luminescence efficiency owing to undamaged material being

exposed*

Very recently some preliminary direct comparisons of

the in it ia l luminescence efficiency of ZnS t Ag and MgSiO^ i Mn

under 5 KeV protons and electrons have been attempted using the

recently completed low energy accelerator. However, identical

conditions of focussing and beam current density are d iffic u lt to

achieve. Preliminary results suggest that the luminescence

efficiency of ZnS i Ag and MgSi03 t Mn under 5 KeV proton

excitation is about a factor of 5 lower than for 5 KeV electron

excitation after corrections have been made for radiation damage

effects. However, further investigations are necessary particularly

with regard to the accurate measurement of beam current density.

Extensive proton irradiation of a sample of MgSiO^ i Mn

has been carried out in the new accelerator to determine whether

any radiation darkening affects occur. Irradiation with 5 KeV2

protons for an hour at a beam currant denaity of about 5 p A/cm

did not induce any viaually distinguishable differences in

re flectiv ity or colour between the area of the sample irradiated

and the surrounding unirradiated area. \ '

6. Preliminary, thermo lu m in e s c e mef^urementf.

Thermoluminescence 'glow ' curves have been measured for

enstatite achondrlte meteorites and synthetic enstatltes in the

B

r pci ;e >: ■

i nloi Islreiiim b*r>funfcbou of

V i ... .-iv •! ; :o 2 yjf r in Jtaiq epos -

a

o«*

firJOCjX®

nt-, t .O i;». hm- •> . •’ n ; ( » .1 : ■< ; ■. ( fV

eftt ;>nlan 1-s.tqwsli b flftOtl !>V6u 1 iK.'iloei* ..iu- poet* i'i Ve . c ?*bnu

, • * ' ( ■ ' ■ t 1 JOi 00 YJ C '

. f ' ! 'D ili,'1 ‘>"f y*' Uni> dm f i . i f j bftOO

. C.il tJ J •Hit h a s • 'V< ’fir e

, f. - iV* AroitFiq Ve > 'ibhnu n. i • •!. 0-H bn* !ph i c'n to i/OI 1 T 9

I 'liueJ*' Vp.\ ■ l flf ¡1$ ' • *1 no2 j J

9i ■<, i i in. 1 ji'ibei .< . r !»< •v • ■; i , ' .! t * 1 >¿¿0X9

•it Juri iTtii; ; >■ s-o i- f’<

,

nM i _ U K c-: ' l o » J q n f.e

. • .tfc'i. • ; . i t . .: -• ‘it i'.i -

»sill noioiq pvien-iix:

.»n orft ni fix tr,J i«a nee'i i ni

, • . : . ' ■ f ‘ . • ;C« A i . . t j-i.M-.iB lo f '.’i b Jnerxuo *M K i 1 #• l x rt nr io t $r\"tcn<\

ipriierlv» «niii’i&Jsi» o) tc

VeX C rlflw (u U slbs 'n

noVu \

ni seoneietftb »IdrrUiugnititlb (H rurIv yne eoubni ion bib

Imirll'* | ri r.*I<ir ■ »i.j *.*wj‘. fii' io;, r- Y#lv; Jooltyi

. i

• .li- ' i

••I l h<< L1 ' r ' ■: f ! • ‘ Vl. • l 1 ' ' . ' .|J

erii n l x e i l f s f a n s oJtor ilnY« bn* e * f h tlitmoridi

86

temperature range -190 to 400°C after excitation at liquid

nitrogen temperature using 7 KeV electrons. Previous measurements

by other workers have not extended over the complete temperature

range (see section 1 of this chapter). Sun S. Gonzales used

different heating rates for measurements below and above room

temperature, whilst the published results of B lair & Edgington

are qualitative in nature although they are the only previous

measurements carried out with the sample in vacuo. Only the

measurements of Liener et al extend above 200°C. Excitation was

achieved by 2 MeV electrons (S & C ) } MeV electrons from Sr (L )

and 150 MeV protons (B & E ). Sun & Gonzales state that

thermoluminescence from enstatite achondrites shows vivid flashes

of blue and red when observed visually . However, during the

measurements presented here no visually detectable emission was

observed. This is probably because of the low electron energy

used for excitation which gives only a small volume of excitation

on account of low penetrating power. Also, since the photomultiplier

had of necessity to be a considerable distance from the sample

( * v 7 tt) , the solid angle of light collected was small. Some

d ifficu lty was also experienced in adequately shielding the heater

so that v is ib le or near infra-red light did not reach the

photocathode. A stainless steel tube was, therefore, used to 'pipe*

light from the sample and shield the photocathode from extraneous

sources (see Pig. 18). The thermoluminescence intensity recorded

is now adequate for a ll but the weakest of thermoluminescent samples -

such as lunar samples. Heating rates up 4o‘ SO cdeg/min can be

used and most measurements have been made using this heating rate.

The recording system is shown in Fig. 25. The XY recorder plots

intensity against temperature but as a check on the lineanty of

heating rate the temperature is also monitored as a function of

time by a separate recorder (not shown in Pig. 25).

• '

bin ' ! *0 .~ CX ' - c> - »(/nsi ewdsisqciei

•j IUM • :«•' ' 0BfS Vs V (W ;p«i m u ;• if)' i st report.'.n

met e.i 9ftj isvc bebnejxe Join %V5fi/ a ■ •Jt- r w rerl to yd

beeU 7*r/' rw a to f m01 to9, 99* ) speer

4 m< l »vcdf- rib woleri 8tnw»MU'iSWi *10\ ¡nlissd tr .»r« lllb

nC' + ,,nj llr .I 10 tilur 91 beriellonq eni j r c

6uoiv I nr edi e'tb yer!J rtpuodJIe sTuJsn nl e vlie flltu p eia

•fit . 1I el'-maa srW tttlw JLK fail

esw nci J evods brioixi. 1* is n* i <• in i •> npf.n:

( j 'it l< 9 VsM f (.: • . jm <- VsM 2 yd bevslrlos

JGflj e + ifo J. c, c a; “ ; . i . n •'q Ve Oc!f bn6

■ ,n i•xviv i ,c’ *,(' cp «.* *leje: no'x t ■ • i: :i

sr!J 4 « vv •< • . ■' v xk) n«r.' •' ;-x r siilr l fi

. / i ' itatilr# ■ .!. f Jo*Jeh yllsuelv on etei i he.+r'-•e»n ' einivneruassHi

yr 1 *30: looi« wni art .i ui iKO - '!■ • . ■ 7 C I

no i 11C -■ :jXS lo sr iriov Ji : ■ Jnr r »vi >irfw rwiltailoxe rol beau

it i <■ it; Bi •0 ' ,• « ' c tr Jr-uoccs no

o- < Oi li .1 i h no; > r.- C > ■ ■; . 't<.

r ( .1.11ft» sew h>eJo*JIoo til"11 " «-.1 [in. , , "V . )

rl edt t-nii /!«1J, i»i.»• ‘'I nl been ■y; y.9 ( ; .. 'uo 't'-::.

C| * •mm < 1 Iv Jrrit oa

■ • i.. .V , ■<•.(> . ■ Tf.' J , < • slnlati » - > cr.'r i>

V ( .1 |neif X® M ■ < .11-.; n i l ; ! i ns oiqi trlpll

DSn/'lli Ofi'X ' <r. J i : .1 •••' ..I. ♦ ■ ■ . / ■■■ ’ C ?

leiqmr.* fn-o. ■ i i.'ulrnnerii to treslse».' srit Jud lie toI eiau]>ebs won al

ed nlm\ tibo OC <>t qu ae.tsi pnltasH . . vt.-nul re. rioua

a grl Jaeri «lilt i nlau st.wri nee« BVe.i aJnaaieru i. oc Jr i i ii beau

tio Iq 1 ehioost VX erfT , • r ‘ al me orl"

yinleenli eilf to >1:x lc - ■ tu«l e-ii N nqraJ ianlspa ytlenelnl

lo ml s as heir:>Jlnrm oela *1 stiliareqnteJ erit etsi pnlJseri

. . ( r l nwoda ion) ret.roast eJmsqea a emit

87

Prior to cooling and excitation a ll samples were

heated to 400°C to remove adsorbed gas, and after the glow

curve had been recorded, samples were again cooled and

subsequently heated without excitation to determine the level

of background radiation.

Glow curves for enstatite achondrites and synthetic

enstatites are shown in Figs. 37 and 38. The curves were

recorded using either a red f i l t e r (Wratten 25) or a blue f i l t e r

(Wratten 38A) to separate the two emission bands. It was found

that blue thermoluminescence in meteoritic samples was more than

an order of magnitude lower in intensity than red thermoluminescence.

The main characteristics of the thermoluminescence of meteoritic

enstatite are a well-defined peak (red emission) near 0°C and

considerable emission above about 150°C where the peaks are less

well-defined. In many cases defin ite peaks are not discernable

in the high temperature region. The peaks in the glow curve of

Cumberland Falls at 122°C, 190°C, 250°C and about 360°C are in

N 100accord with the results of Liener & Grogler although less

well-defined, whilst the low temperature peaks at -30°C and

-5°C are in agreement with the resu lta of Sun 8. Gonzales10"*.

The glow curve of blue thermoluminescence of Cumberland Falls

is also in good agreement with Sun & Gonzales although these

authors have Incorrectly poaitioned the loweat temperature peak

on account of atarting the glow curve at too high a temperature.

Comparison with synthetic enstatites shows that there

\ ' ‘is a similar trap distribution in synthetic orthoenstatite but

the trap distribution in synthetic clinoenstatites is quite

different (see Fig. 38). Moreover, the glow curve (not shown)

of a sample of the phosphor MgSiOg t Mn (300 ppm) supplied

»an*-'

.V'

pn *v* 8*Ic!i> ti} l l . poifadloxe 'n

■ ■ f ,

bn» i iXooo mV.gs »isw 8eX<j

J»V‘iJ »ri,t eni;.tei ah of nolfhtti'Xv

\ilooo o f toirfl

>v*if*w o f 0°0O* Of b*fs»ri

, i ./

'tifi* fc?*ir>»ri

.

>14 ju m o

ic 1 et-viuo woXC

it t « f l i f t e r »

tit)o i nieu boljiosM

■ :■ ( f (Aff nettai*)

oi;t*r!in\r i-n —* ■! — b■ ■' O'

• •

»uXd * -ic ( if n».t.ts*tV ) is iX H

. w 1

nr.r i »•»<'•( t r t e«vj ;• . i ti i ■ ■ ni t o -o ••..ni' •»fc t ’ ••■•'.♦ :>X- t,..v

oeenlmuioniarii bar r n ' x y i ln o ir i .-.l i«v.t-I : njlnrrf V »»Irs» nr

OÌ f . IP '\1 f*i T f - Ol . . ' ■ ! 11. . " ' * : , 0 ; i . i l ” j : 1 :i . i P f i "

( * ( f ! me •• : - - ‘ • ene

»a**J tr ■ •>; «rii •.).!•)• ' ' i •■• • p p 1 r- .' •< • >

• o • : -

c. ' • ' ' '

■ ; ‘ ■ '•< , , ; •' ioo r M

eael ri . 'lOriiis -lelgoiO A leneli io ttluàs'x «rii riJXw bioooe

1 ■ < -

cor oa. j , ■ r ~

e l i t i bnsXm wuO " eonsot ‘ rin-iiioi “(»rii euXd to eviuo **ig *rfT

s.it il-1-c riir * sei» ut * i i ritiw in ti» ■•XI, hoc, ni oeXu i t»ec, n u i t teqr.ti itmvc.I Hi.- -n< iiisoq yltttm oonl »vsri aioriius

, : , r ' . )«

eierij itr ii eworti* « » i l tarane oiiftriinyt riiiw noeltegmoO

iuri etiistaneorid-io oXiPriinY8 n* nciioriliicib quii i tA l ie l t a «X

•iluf) al »»tiiaienennllo oiiariiny* ni noliuriXiiplh qaii »rii

, . . *'r-*’ in- -

beilqque (nrf) OOC) nM t .QlJJgM **» rfcji*o:irj »di to «Xqricc s to

by Prof. Garlick is different to both the above enatatites.

I t shows a small peak at -70°C and a broad intense peak at

about 210°C. These differences are probably indicative of the

different preparation methods. Orthoenstatite was prepared

using hydrothermal techniques whilst the clinoenstatite

crystallized out from the melt in a reducing environment at

atmospheric pressure. The MgSiOg phosphor was prepared at

atmospheric pressure but without melting. I t may be possible

to use the glow curve technique to distinguish between ortho -

and clinoenstatite, since the production of orthoenstatite usually

requires high pressure.

Excitation of thermoluminescence was attempted with

7 KeV protons since this mode of excitation has been invoked

to predict thermoluminescence e ffects on the Moon's surface.

However, i t was found that the thermoluminescence intensity

after such excitation was about an order of magnitude weaker

than that induced by an equal number of 7 KeV electrons. There

are two possible reasons why this is so. Proton damage effects

may cause trapping of secondary electrons which are not released

during heating. Alternatively, since 7 KeV protons have a very

low penetrating power, the volume o f excitation is extremely

small. In any case, excitation of measurable lunar

thermoluminescence by low energy protons appears to be very

unlikely.

Fig« 37. Thermoluminescence 'glow' curves of enstatite achondrite

meteorites*

1« Red thermoluminescence of Cumberland Falls.

2. Blue thermoluminescence of Cumberland Falls.

3. Red thermoluminescence of Norton County.

Intensity scaling factors and the level of thermal

background are shown.

A ll heating rates are 50 C deg/min*

Exaltation: (a l l aanples) ? .10^ electrons (7 KeV),

F lg. 38. Thermoluminescence 'glow* curves of aynthetic enstatites.

1. Red thermolumineseence of orthoenetatite.

(1000 ppm Mn)

2. Blue thermoluminesoence of orthoenetatite.

(30 ppn Mn)

3. Red thermoluminescence of clinoenstetite.

(150 ppm Mn)

Inteneity scallng factora and thè level of thermal

background are shown.

All heating ratea are 50 C deg/min.

-200 -100 0 . 100 200 300 „^400

88

7. Discussion.

2+7.1 The wavelength of the Mn emission In forsterite and enstatlte.

It is interesting to compare the Mn emissions from

forsterite and enstatite since the environment of the manganese

ion is similar but with specific minor differences. It is known

that in both these structures the Mn ion prefers the larger M2

sites. The symmetry of these sites was shown in F igs. 1 & 3. The

average metal-oxygen distance for these sites is very nearly the

same being marginally larger for the enstatite M2 s ite which is

also more distorted from octahedral symmetry than the forsterite

M2 s ite . However, the M2 site in forsterite is co-ordinated by

six equivalent oxygens each linked to one silicon atom only,

whereas in the enstatite M2 site there are two types of oxygen

ligandst four are attached to one silicon atom only and two attached

to two silicon atoms which on the ionically-bonded model are,

14f Atherefore, 'neutral' • In fact, the value of A estimated foro

2+Fe ions in M2 sites from absorption spectra is greater for the

14cforsterite M2 site than for the corresponding enstatite site .

2+It would, therefore, appear that the Mn emission in

fo rsterite would be expected to be at longer wavelength than that

for enstatita whereas the reverse has been shown to be true.

S trictly speaking, i t i s the wavelength of the corresponding absorption

transitions A1gT„ which should be compared otherwise i t is

"9

assumed that the Stokes shift is the same for these two cases*

However, i t seams unlikely that the difference in Stokes shift for

the two cases would be as large as 40Q& which is about the minimum

which would be required to explain the discrepancy noted above*

Nevertheless, this point cannot be proved without the determination

of absorption spectra which is experimentally very d if f ic u lt

owing to the forbiddenness of the transitions involved and the low

manganese concentration* Absorption measurements on manganese

s ilicates are not applicable since the crystal structure of

magnesium metasilicate (the pyroxene enstatite) is rather different

from manganese metasilicate which is a pyroxenoid*

4The T level of the divalent manganese ion has

electronic configuration (tggJ^Og and since this level is trip ly

degenerate further splitting w ill occur in sites of low symmetry*

I f the octahedron is trigonally distorted as in the forsterite M2

s it * then the T level w ill sp lit into two levels; a doubly

degenerate one and a singly degenerate one. This can be worked

out using Fig* 8. I f , however, the octahedron is distorted so as

to be of monoclinic symmetry, the degeneracy is completely removed.

The magnitude of the splitting w ill depend on the degree of

distortion from octahedral symmetry* Absorption spectra of the

2+Fe ion in the pyroxene site suggest that the splitting of the

14c A3d electron states t_ and e are quite large for this ion • ¿A 2g g o

fo r this site is estimated at 6,800 cm-1 although the splitting

of the e level causes the lower e_ component to be decreased in 9 9

energy by 2,800 cm~\ The energy of the lowest sp lit t2g level is

reduced by about 1,400 cm"\ Since the t2g levels are sp lit

according to a 'centre of gravity' ru le about the octahedrel level,

the net energy of the three electrons occupying these levels w ill

not change* I f , therefore, for the moment, it is assumed that the

sp littin g of the 3d levels of Mn + i s similar to that in Fe^+ , then

the lowest sp lit level of the T ^ level of the manganese ion in the

90,

pyroxene would be about 4,200 cm’ 1 below that for the pure

octahedral case with the same AQ* I f it is further assumed

2+that the Mn emission w ill take place from the lower sp lit level

4of the former level, i t is obvious that this emission is

shifted to longer wavelength* A similar exercise using date, for

2+Fe in the fo rsterite site shows that the lowest sp lit level

2+ 4 - 1of the Mn T4_ level would be about 1,250 cm below that for 1g

the pure octahedral case* This is considerably less than the

reduction in energy of the sp lit level for enstatite* Therefore,

although the assumption that the splitting of the 3d levels of

2+ 2*Mn is similar to that in Fe is probably not quantitatively

valid i t is expected to be qualitatively correct and thus the4

splitting of the lower component of the manganese T,jg level w ill

be considerably greater in enstatite than in forsterite*

In conclusion, therefore, i t is tentatively suggested

that the emission peak in enstatite occurs at longer wavelength

than in forsterite possibly on account of the d iffe ren tia l splitting

of ths 4T1g level although the possib ility of s difference in

Stokes sh ift cannot be ruled out* It is hoped that future attempts

at measuring excitation or absorption spectra may elucidate this

problem* Failing the above explanations) the remaining possibility

would seem to be that the manganese ion is able to cause a larger

local distortion o f the structure in forsterite than in enstatite

thereby making much more room for it s e lf in the former structure*

This would render the comparison of known data for the corresponding

M2 sites Invalid

91

In Fig. 33, the intensity of the Mn + emission in

iron-free synthetic clinoenstatite was shown as a function of

manganese concentration. A point is reached where the addition

of more manganese does not sign ifican tly increase the luminescence

intensity. In fa c t, samples of the phosphor MgSi03 « Mn with

high manganese concentrations (up to 1$0 supplied by Prof. Garlick,

2+show that the intensity of the Mn emission decreases to some

extent with increasing manganese content beyond a certain optimum

concentration (~ 0 .1 $ ) . This behaviour is similar to the

dependence of the thermoluminescence intensity of the 470°K

'glow ' peak upon manganese concentration for manganese-activated

synthetic calcites as found by Medlin * . The explanation

given was as follows. For very low manganese concentrations the

e ffec t of doubling the manganese concentration w ill be to

approximately double the number of isolated emission centres and

hence to give an almost two-fold increase in luminescence intensity.

2+However, at higher manganese concentrations, Mn ions may tend

to cluster and i t has been suggested that the emission may be\ 2+

quenched when another Mn centre is located within a certain radius

which is of the order of a few la ttice units. Thus increasing the

2+manganese concentration, whilst producing more Mn centres, w ill

2+also increase the probability of Mn centres being located close

together in the lattice . When, eventually, more quenching clusters

are produced than new isolated centres the addition of more

manganese w ill decrease the luminescence efficiency.

This model seems to explain adequately the dependence

2+of luminescence intensity of the Mn emission upon manganese

concentration in enststite.

7.2 Other f a c t o r s a f f e c t i n g the luminescence em iss ion in e n s t a t i l e .

92

The introduction of Fe into metal cation positions

in clinoenstatite causes d ifferen tia l quenching of the blue and

red emissions. I f there is no energy transfer between ’b lue'

2+centres and Mn centres, then at least some of the quenching

2+of the Mn' emission must occur by energy transfer (probablyo, 9.

resonance transfer) from to to Fe since direct competition

for the excitation energy between the luminescence centres and

2+Fe alone would lead to equal quenching rates for the blue and

red emissions. On the other hand, i f energy transfer does occur

between 'b lue ' and 'red* centres, although there is no evidence

that it does, then the kinetics are complex and it is d iffic u lt

to generalise about quenching mechanisms.

7.3 Comparative efficiencies of electron and proton-excited luminescence.

In Chapter I , Section 3.4, evidence from the literature

was presented to show that one might expect that the luminescence

efficiency of sulphides and silicates under proton excitation

would approach that for electron excitation of similar incident

energy. Results presented in Section 5 of this chapter show that

previous workers who have suggested that comparative efficiencies

for these two types of excitation d iffe r by orders of magnitude

have not corrected for the rapid in it ia l deterioration of

luminescence efficiency under proton excitation when large proton

fluxes are used. However, possible differences in efficiency are

s t i l l apparent at low energies after correction for radiation

damage effects. There are two possible causes. At low energies

a considerable fraction of the incident proton energy is dissipated

in collisions with lattice ions, or the ionisation density is such

as to drastically increase non-radiative recombinations (ionisation

2+

quenching). Theory suggests that for proton-energies above 1 KeV

the cross-section fo r nuclear co llis ions is an order of magnitude

smaller than the cross-section for electronic excitation . Thus

we are le f t with the poss ib ility of ionisation quenching. I f

ionisation quenching increases at lower energies and thereby causes

a reduction in luminescence e ffic ien cy then the luminescence

intensity w il l not be a linear function of incident proton energy.

However, usually a linear relationship is observed experimentally

suggesting that the luminescence effic ien cy is independent o f proton

energy in the KeV range. I f the absolute e ffic iency measurements 41

of Hanle & Rau are reasonably correct then these suggest that

25 KeV hydrogen ions are as e ff ic ie n t as 20 KeV electrons in

producing luminescence in ZnS j Ag and Zn^SiO : Mn. According to

43 120the results o f Van Wijngaarden et al and Eve 8. Duckworth ,

5 KeV protons should be almost as e ff ic ie n t as 25 KeV protons in

producing luminescence in these phosphors. Thus one would expect

very l i t t l e d ifference in luminescence e ffic ien c ies for 5 KeV proton

and electron excitation . Therefore, either the absolute e ffic iency

measurements o f Hanle & Rau are too high by a factor of ^ 5 or

there are appreciable errors in the determination o f the particle

flux densities in the preliminary comparative measurements presented

here.

I f 60 KeV protons were less e ff ic ien t than 60 KeV electrons

in producing luminescence by a similar factor then the effic ien cy

estimates quoted for various samples would have to be reduced by

this factor

94 .

Dust and rock samples returned by the Apollo 11 and 12

missions have been investigated. Proton-excited luminescence

spectra of such samples have been measured in the v is ib le and

near infra-red region o f the spectrum.

1.1 Luminescence spectra of lunar fin es.

Fresh samples o f Apollo 11 lunar fines (10084-6) were

found to luminesce very weakly under 60 KeV proton excitation.

The luminescence e ffic ien cy was estimated to be not more than

10 i . e . about an order of magnitude lower than most meteorite

dust samples. On account of the extremely low luminescence

e ffic ien cy o f these samples, the spectrometer s l i t s were removed

en tire ly in order to improve the signal-to-noise ra tio and,

therefore, the static bandwidth was increased to about 200A.

The luminescence spectrum of a typical fines sample is shown

in F ig . 39 . The emission is nearly white but the spectrum

shows two discernable peaks at about 4500& and 5600A, the la tte r

being s ligh tly more prominent.

For comparison, the proton-excited luminescence spectrum

of a typical terrestria l basalt powder (U.S. geological survey

121standard BCR 1 ) was determined. The emission from this sample

showed the same two peaks but with the blue peak slightly more

prominent (see Fig. 41 )• Towards the end of the scan there

was evidence of another more intense emission band which subsequent

spectral scans in the near infra-red region showed to have a

peak around 7300A. The luminescence efficiency of this basalt

is considerably higher than that of lunar fines and is estimated

to be about 4 * 10”4.

1. 'Hie lum inescence o f lunar s u r fa c e m a t e r i a l .

Although lunar fines do show some emission in the

in fra-red no prominently discernable emission peak has been

94

Dust and rock samples returned by the Apollo 11 and 12

missions have been investigated* Proton-excited luminescence

spectra of such samples have been measured in the v is ib le and

near infra-red region of the spectrum.

1.1 Luminescence spectra of lunar fin es .

Fresh samples o f Apollo 11 lunar fines (10084-6) were

found to luminesce very weakly under 60 KeV proton excitation .

The luminescence e ffic ien cy was estimated to be not more than

10 i . e . about an order of magnitude lower than most meteorite

dust samples. On account of the extremely low luminescence

e ffic ien cy of these samples, the spectrometer s l it s were removed

en tirely in order to improve the signal-to-nolse ra tio and,

therefore, the static bandwidth was increased to about 200A.

The luminescence spectrum of a typ ica l fines sample is shown

in Fig. 39 . The emission is nearly white but the spectrum

shows two discernable peaks at about 4500& and 5600A, the la tter

being s ligh tly more prominent.

For comparison, the proton-excited luminescence spectrum

of a typical terrestria l basalt powder (U.S. geological survey

standard BCR 1 ) was determined. The emission from th is sample

showed the same two peaks but with the blue peak sligh tly more

prominent (see Fig. 41 ) . Towards the end of the scan there

was evidence of another more intense emission band which subsequent

spectral scans in the near in fra-red region showed to have a

peak around 7300A. The luminescence efficiency of this basalt

is considerably higher than that of lunar fines and is estimated

to be about 4 . 10“4.

1. The luminescence o f lun a r s u r fa c e m a t e r i a l «

Although lunar fines do show some emission in the

infra-red no prominently discernable emission peak has been feund

95

in this spectral region.

Since plagioclase is the only major, re lative ly iron-

free, mineral phase common to both, i t was probable that this

mineral was the major contributor to the luminescence, as it

has been shown to be so for the majority of stony meteorites.

Both lunar fines and basalt BCR 1 contain appreciable

amounts o f plagioclase near anorthite in composition (10064-6,

~ 15# j BCR 1, ~ 7 5 # ). In both cases, the other major mineral

component is an iron-rich pyroxene (probably augite) which is

not expected to show any appreciable luminescence. The main

difference in mineralogy is that of the presence of the opaque

mineral ilmenite ( ~ 9 # ) , a ferrous titanate, in the lunar fines.

This mineral contributes to the dark colour of lunar fines.

However, both the intricate microstructure of the fines as

revealed by scanning electron microscope photographs11 and the

possib ility of an opaque coating of the grains as suggested by

122Hapke et a l may also be contributory factors. The albedo of

fresh lunar fines (10084-6) was found to be about 7# of that of

a fresh MgO screen when a tungsten lamp operating at a colour

temperature of 2850°K was used as a source and a tr ia lk a li

photomultiplier (EMI 9558B) used as detector. The angles of

incidence end observation were both 45° but in mutually perpendicular

planes. The albedo of basalt BCR 1 was about 4Q# under these

conditions*

Although the proportion of plagioclase in basalt BCR 1

is some f iv e times higher than that in lunar fines 10084-6, the

luminescence efficiency of the former is considerably more than

five times that of the latter. This discrepancy may be accounted

for by the difference in a lbedos-

96.

i . e . a considerable degree of self-absorption of the luminescence

probably occurs in lunar fines. However, differences in the

concentration of ’ k i l le r ' centres in the plagioclase may also

exist.

Garlick and Lamb^’ ^ have found that heating lunar

fines to a temperature of 760°C in argon caused a change in

colour from dark grey to brown. Dr. Steigmann has ascribed the

change to a chemical dissociation of ilmenite into haematite and

ru t ile on the evidence of X-ray powder photographs. However, such

a reaction requires a supply of oxygen and it is not yet certain

whether this occurs as an impurity in the argon or is supplied

by the s ilica te environment.

The luminescence spectrum of such a heat treated sample

is shown in Fig* 39 • The luminescence efficiency is more than

doubled and the 5600l emission peak is shifted to longer wavelength.

It is noteworthy that the albedo of th is sample is also nearly

doubled with respect to the virgin sample. Therefore, both the

increase in efficiency and the spectral shift may simply be due to

changes in the self-absorption and the absorption spectrum of the

material* In fact, the changes in luminescence properties are

consistent with the change in the d iffuse reflectiv ity spectrum as

determined by Garlick and Lamb. Heating the altered sample to

higher temperatures (960°C) causes the colour to become darker again*

Garlick and Lamb noted that the diffuse re flectiv ity spectrum tended

to become more nearly like that of the in it ia l virgin sample and

that it s overall re flectiv ity or albedo was also reduced* Again

we have a correlation with the luminescence efficiency and spectrum

since the efficiency of a sample heated to 960°C is intermediate

97

between that of the virgin sample and the 760°C heated sample

and the 560C)X emission peak is shifted back towards its o rig ina l

position (see Fig. 39 )• Thus i t does not appear necessary to

invoke a change in the luminescence properties of the plagioclase.

Two sets of fines from the core tube 10004 were

availablet each set consisting of 5 x 25 mg samples from d ifferen t

depths (top to 13 cm below the surface). It was thought that

the luminescence efficiency may increase with depth i f the layers

nearer the surface had been radiation damaged to a greater degree

than those at deeper levels. However, no consistent variation was

found, and the slight differences between various samples were

probably due to variation in the plagioclase content particu larly

in view of the small sample size. Thus, i t is concluded that

radiation damage, i f present, is roughly the same at a l l depths

down to about 13 cm. The decrease of luminescence efficiency

with time of irradiation for lunar fines was, in general, s ligh tly

less rapid than for basalt BCR 1 samples and although i t is tempting

to use this fact as evidence for previous radiation damage of lunar

fines, it may equally well simply reflect the fact that the

plagioclases are somewhat different.

Lunar fines obtained by the Apollo 12 mission show a

greater variety than those returned by the previous mission.

However, the proton-excited luminescence spectra are remarkably

similar (see F ig . 42 ) . Sample 12070 is only marginally lighter

in colour than 10064-6 samples and it s luminescence efficiency is

very similar* Nevertheless, samplss 12032 and 12033 are considerably

lighter in colour than Apollo 11 fines and although these show a

similar luminescence spectrum to 10064-6 samples, the luminescence

efficiency is considerably greater. Once more, a correlation ex ists

98.

between luminescence efficiency and sample albedo. Sample

12033 which has the highest albedo also has the highest

luminescence efficiency which is an order of magnitude higher

than 10084-6 samples. It is , of course, true that the reason

for the higher albedo is an increase in the proportion of

plagioclase present ( ^ 7 5 % for 12033) and, possibly a reduction

in the proportion of ilmenite. However, whilst the plagioclase

content of 12033 is higher by a factor of five as compared with

Apollo 11 fines the luminescence efficiency is higher by a factor

of ten. No prominent infra-red emission peak has been found for

any of the Apollo 12 fines.

1.2 Luminescence spectra of lunar rocks and breccias.

The proton-excited emission spectra of two rock chips

(10058-37 and 10057-54) have been determined and are shown in

Fig. 40 • The former sample was a coarse-grained gabbro-like

rock with large chunks of plagioclase in a matrix of pyroxene

and ilmenite. The 5600X peak was more prominent fo r this sample

than for the fines and the luminescence efficiency was considerably

higher than for 10064 fines. Sample 10067-54 showed appreciable

emission only from it s small white inclusions. The 5600& peak

was less pronounced fo r this sample and the luminescence efficiency

and spectrum as a whole was more nearly like that of the 10064

fines. Some breccias contained small inclusions of cristobalite

(a form of s i l ic a ) which also gave a bluish white luminescence.

The most interesting sample was a separated light

fraction ( 9306 plagioclase) from rock 10044-43 which was kindly

loaned to us by Prof. Zussman. The proton-excited luminescence

spectrum of this sample showed a strong 5600A peak and the

99

efficiency was estimated to be of the order of 10 . Infra-red

scans of this sample revealed that although there was no strong

I.R» emission peak, a weak I.R . emission band was apparent with a

discernable maximum intensity at about 7700 A (see Fig. 43 ) ,

Sippel and Spencer have fa iled to detect an I.R . emission peak in

lunar plagioclases although they found that a set of 20 te rrestria l

plagioclases a ll exhibited such an emission band110.

In conclusion, therefore, it appears that although lunar

plagioclases do not exhibit the strong infra-red emission band

characteristic of most terrestria l samples, I.R . emission is present

but much less intense than the blue and green emissions.

2. Luminescence spectra of terrestria l plagioclases.

For comparison with lunar plagioclase, the luminescence

spectra o f a number of terrestria l samples of pure plagioclase

were determined. These samples were kindly supplied by

Prof. G .F.J. Garlick of Hull University and Dr. J . Esson of the

Geology Department of Manchester University. The emission spectra

were determined both in the visib le and near infra-red region of

the spectrum and are shown in Figs. 4 l , 4 3 * 4 4 •

OA ll samples show a prominent peak at around 5600 A besides

showing some emission in the blue region. Most samples exhibit an

infra-red emission band which is often the most intense of the

three emission bands. However, the absence of a prominent I.R .

emission peak is not peculiar to lunar plagioclases as was suggested

by Sippel and Spencer110. A sample of labrsdorite/bytownite©

(F ig . 43 ) does not show a discernable I.R . peak although the 5600 A

peak has a shoulder on the long wavelength side and is considerably

broadened. It is also noteworthy that the luminescence spectra of

•3

100,

meteorites in which plagioclase is the major luminescent

component ( i . e . bronzite and hypersthene chondrites and pyroxene-

plagioclase achondrites) do not show a strong I.R . emission peak.

The spectrum of Juvinas (a pyroxene-plagioclase achondrite) shows

no discernable I.R . emission peak (see Fig. 43) although the 5600 A

emission peak is again prominent. Occasionally, as in the

te rres tr ia l o ligoclase sample examined, the I.R . emission band and

the 5600 A band are of similar in tensities (see F ig . 41),

X-ray fluorescence analysis of a number o f plagiocleses

carried out by Dr. J. Esson in the Geology Department showed that

a ll plagioclases examined contained more than 100 ppm Mn. The

separated plagioclase from lunar rock 10044-43 had an estimated

Mn content of about 200 ppm which was about the average fo r the

te rres tr ia l samples investigated. The actual manganese contents

of particular samples where determined are given in the captions to

Figs. 41 and 43. Whilst there was some evidence both experimental

and theoretica l (as w il l be discussed la ter) which suggested that

the 5600 A emission peak might be due to Mn substituting in a

metal cation s ite , i t was not conclusive. In order to test this

p oss ib ility , a natural sample of a typical plagioclase ( labradorit.e)

was, therefore, doped with manganese in Prof, G arlick 's laboratory

by heating the sample to 1050°C fo r 30 mins in argon with 0.1#

hydrated manganese sulphate. A sim ilar undoped sample was given

the same heat treatment in order to chetk whether the luminescence

spectrum was affected by heat treatment alonf. The proton-

excited luminescence spectra of these treated samples are shown

in F ig. 44. I t is evident that the addition of manganese enhances

the green-yellow emission band considerably whilst at the same

time reducing the intensity of the blue and infra-red emission

Fig» 39 Proton-excited luminescence spectra of lunar fines

10004-6 before and after heat treatment»

1• Virgin sample»

2» After heating to 760°C in argon»

3. After heating to 960°C in argon.

The intensity scale is the same for a ll three curves*

Fig» 40 Luminescence spectra of lunar rocks» 10058.37

(coarse-grained with large plagioclase crystals)»

and 10057.54 (fine-grained with only occasional

plagioclase crystals).

The intensity scale for 10058.37 is about 50 x

that for 10057.54.

Fig. 41 Luminescence spectra of a pure sample of oligoclaaa

(Mn content 140 ppm)» and of a typical sample of

basalt BCR.1*

The intensity scale for oligoclaae is about 3 x

that for the basalt.

rela

tive

in

tens

ity

Fig« 42 Comparison of proton-excited luminescence spectra

of lunar fines from Apollo 11 and 12 missions*

Fig* 43 Luminescence spectra of a separated plagioclase

fraction from lunar rock 10044-43 (Mn content 200 ppm),

of a terrestria l labradorite bytownite (Mn content

500 ppm) and of the meteorite Juvines (a plagioclase-

pyroxene achondrite).

Fig* 44 The luminescence spectrum of a labradorite!

1* Natural sample*

2* After doping with 0.1# Mn.re

lati

ve

inte

nsi

ty

4 9

101.

bands. Heating the labradorite to 1050°C without added manganese

does not a ffect the emission spectrum appreciably, although such

heating probably causes a structural change from the low temperature

anorthite form to the high temperature form. In the la tter form

'•U- 4+the structure becomes less ordered and the A1 and Si ions no

longer occur in alternate tetrahedra as in the low temperature form

described in Ch. 1 Section 1.3.

I t might be expected that the intensity of the green-

yellow emission band would give a measure of manganese content in

plagioclases. However, the emission spectra of the natural plagioclases

examined do not show a simple correlation between the intensity of

2+the Mn' emission and manganese content as measured by X-ray

fluorescence analysis. However, this type of analysis measures total

manganese and not the amount of manganese in metal cation sites which

is what is important. Moreover, d ifferen t plagioclases contain

d ifferin g amounts o f "k il le r s " such as iron which probably a ffec t

2+the intensity of the Mn emission. In this context i t is worth

2+ 2+recalling the e f fe c t of small amounts of ^e on the Mn emission

in enstatites and fo rs te r ites (Ch. 3 Section 3.3 ).

2+The peak of the Mn emission band varies a l i t t le in

position from sample to sample but usually lie s in the range 5550 -

5650 A. The band has a half-width of about 1000 A but is not

symmetrical! the intensity fa l ls less steeply on the long wavelength

side.of the peak (c f Zn2Si04 t Mn).

9

3. Discussion.

2+3.1 The Mn ' emission.

Almost a l l plagioclases exhibit a green-yellow emission band

110although Sippel and Spencer have noted that pure albitee do not

102

appear to show such an emission peak. These authors, therefore,

conclude that this emission peak is caused by a divalent activator

substituting for Ca in the la tt ic e . Results presented in the

previous section seem consistent with Mn being the divalent

activator responsible for the green-yellow emission band. On account

of the charge compensation and ionic size i t might be expected that

Mn2+ would replace Ca2+ in the la ttic e as i t does in CaF2 t Mn123,

124 125 25CaO t Mn , CaSiO^ i Mn , CaCO J Mn' and the manganese-activated

calcium halophosphates"'2^. The colour of the Mn2+ emission in

these phosphors varies from green to orange-red.

In the low temperature form of anorthite ('low* anorthite)

there are four possible calcium sites of s ligh tly d ifferin g symmetry

and average metal-oxygen distances as mentioned in Ch. 1 Section 1,3.

A ll four are seven-fold co-ordinated i f metal-oxygen distances up to 0

3.1 A are counted although one approximates to six-fold co-ordination.

127More recently, Megaw et al have determined the structure of

bytownite and found that the calcium environment is changed very l i t t l e

by the introduction of a certain amount of sodium into the structure.

2+The environment of Ca in the anorthite structure w ill , therefore,

be discussed as being applicable to calcium-rich plagioclases in

21general. Bond lengths are given by Kempster et al who describe

the site symmetries as distorted cubes with one corner missing (o r

two corners missing for the approximately s ix -fo ld co-ordinated s ite )i

four of the bonds approximate closely to cube-corner directions

although the sites are, in general, of low symmetry. The range of

Ca - 0 bond lengths in A for the four sites are as followai-

C1 l 2.28 - 3.09 t average 2.54

(s ix -fo ld approx. 2.28 - 2.62 i average 2.45)

C2 l 2.32 - 2.81 i average 2.54

Cg t 2.35 - 2.72 s average 2.50

i 2.34 - 2.82 i average 2.50

2+I t is not clear whether Mn w ill have a preference fo r any

2+particular s ite since Mn has zero CFSE and a ll s ites are more

than big enough to accommodate th is ion. Since the magnitude o f A has not been determined fo r any transition metal substituting in a

calcium s ite , i t is d if f ic u lt to predict the wavelength of emission

of Mn2+ in these s ites. A might be expected to be somewhat

smaller than fo r a cubic co-ordination with a sim ilar average

metal-oxygen distance and, therefore, possibly 20 - 3<$ smaller than

for the corresponding octahedral case.

In it ia l ly , a comparison o f the wavelength of emission

and average metal-oxygen distance w il l be made with CaO t Mn and

CaSiOg i Mn. In CaO t Mn the wavelength o f the emission peak is

about 5900 X, and its structure is o f the NaCl type ( i . e simple face-

2+centred cubic). The Ca ion is , therefore, in octahedral co-ordination

and the Ca - 0 bond length is 2.40 X128. It might, therefore, ba

expected that the wavelength of emission in anorthite would be

considerably less than 5900 A. In the pyroxenoid ^l-CaSiOg t Mn

( |2>-wollastonite) the wavelength of the emission peak is about 6200 A

*129and the average metal-oxygen distance is 2.39 A • There are three

slightly d ifferent calcium sites of distorted octahedral symmetry13<'>l131 •

In * -CaSi03 i Mn (pseudo-or-oC - wollastonite) the wavelength of

emission is about 5600 A, which suggests a much reduced crystal f ie ld ,

but unfortunately a detailed structure with bond lengths has not yet4

been determined. In (b -wollastonite a splitting of the T level

would be expected owing to the distortion from octahedral symmetry

2+and this factor may be the reason why the Mn emission occurs at

longer wavelength than in CaO i Mn (c f . Mn24 in forsterite and

103.

104

enstatite Ch. 3 Section 7.1) although differences in Stokes shift

may also occur.

2 *Since the possible Mn sites in anorthite are of low

symmetry, considerable sp litting of the 4T1g level is again likely .

Thus we have the factors of large average metal-oxygen distance and

a crystal fie ld of lower intensity than for cubic co-ordination,

tending to suggest a low value of A and, therefore, short

wavelength (possibly green) emission whereas the splitting of the4

level is tending to sh ift the emission wavelength towards the

red. In view of these factors it seems reasonable to expect the

emission somewhere in the region 5500 - 6000 A.

Apart from possible differences in Stokes shift between

the emissions in the phosphors compared, differences in the nature

of the ligands have not been considered. Whilst a l l ligands are

oxygens they are not a ll equivalent and this factor can appreciably

affect the value of A as noted earlier (Ch. 3. Section 7 .1 ).

The study of the absorption and emission bands due to

2+Mn in various silicates warrant» further attention particularly

now that many refined structural analyses are available giving

detailed descriptions of the metal cation environments. An interesting

possibility for further detailed study is the monoclinic pyroxene,

132 2+diopside, CaMgSijOg. According to E.S.R. measurements , Mn ions

substitute in both calcium and magnesium sites with a slight

preference for the calcium site , thus two d ifferent emission bands

2+due to Mn ions should be evident.

105

3.2 The blue and infra-red emission bands.

A ll plagioclases show at least some blue emission as

do iron-free enstatites, forsterites and many other s ilicates .

The emission band is broad and somewhat variable in the position

of the peak. I t is likely that such emission bands have a

similar origin in most relative ly iron-free silicates and, as

indicated ea r lie r , probably arise from a particular type of lattice

defect. Sippel and Spencer have found that some heavily shocked

plagioclases and s ilica mineral phases show an enhanced blue

emission, which again suggests that lattice defects may be

responsible for th is emission. However, the precise nature of the

centre cannot be ascertained without further detailed investigation.

Most plagioclases show a prominent emission band in the

near infra-red although lunar and meteoritic plagioclases do not

show such a pronounced band. Indeed, in some plagioclases this

emission band is either very weak or absent altogether (see Fig. 43).

The position of the emission peak is variable ranging from about

7300 l to 7700 A and is usually quite broad, being generally much

2+broader than the Mn emission band. The I.R. band is often the

dominant one as in the labradorite examined, and it does not appear

to be associated with manganese since the addition of manganese

reduces its intensity. I f this emission is due to an impurity

activator then it should be possible by selective doping with

suspected activators to determine the nature of the impurity. The

activator should be present naturally in amounts not less than

about 100 ppm. Since most plagioclases contain iron as an impurity

in quantities up to 1# and since Fe often gives rise to absorption

bands in the near infra-red in s ilicates , this ion might be thought

to be a possible activator, although Fe more usually constitutes

a ' k i l l e r * c e n tr e when p re s e n t in s i l i c a t e s

Preliminary investigation of the spectrum of

labradorite, after heating to 1050°C in argon with 0.1% hydrated

ferrous sulfílate, did not reveal any enhancement of the infra-red

emission band re lative to the natural sample. In fact, there

appeared to be a slight reduction in intensity of this emission.

There remains, therefore, much more work to be done on selective

doping and comparison of analyses of different plagioclases before

any conclusions can be reached concerning the nature of the

infra-red emission centre.

107

1. The Electrostatic Getter-Ion ("Orbitron") Pump»

1.1 Operating principles and design.

Sputter-Ion pumps of various types have been commercially

available (AEI, Mullard, Ferranti, etc .) fo r several years and

fu ll descriptions of the structure and action of these pumps can

be found in recent books on UHV (e .g . Redhead et a l87 and Power88) .

The advantages of such pumps over conventional diffusion pumps

are many. Pressures of less than 10”^ torr are easily attainable

without trapping and pressures of less than 10" ^ torr are

achievable in good UHV systems. Backing pumps are not required

once the ion pump is operative and the pumping system does not

employ either o i l or mercury and, therefore, the vacuum system is

free from contamination by such materials.

However, sputter-ion pumps do have certain disadvantages.

The confinement of the ions is usually by a magnetic fie ld and thus

fo r large pumps large magnets are required and hence such pumps

may not be suitable for use on particle accelerators, electron

microscopes or any application where stray magnetic fie ld s are

undesirable. Pumping speeds fo r inert gases are also low compared

with speeds for active gases. In order to achieve high pumping

speeds, m ulti-cellu lar devices must be used and the overall size

and weight of the pump becomes large. Sputter-ion pumps also suffer

from "memory” effects on account of the fact that pumping occurs on

the same surface on trtiieh sputtering occurs.

Many o f these disadvantages are overcome by the

89 90electrostatic getter-ion pump ' • The structure is basically

very simple (see Fig. 45 ) and by using electrostatic instead of

magnetic confinement comparatively largo pumping speeds can be

obtained with compact size - e .g . an "orbitron" pump of 2” diameter

108.

has a pumping speed fo r nitrogen of the order of 100 1/sec*

The pump operates by the injection of electrons from

a tungsten filament into the electrostatic fie ld between two

co-axial cylinders. The outer cylinder is the wall of the tube;

the inner is a ^16 " diameter tungsten rod which is the anode.

Anode potential is usually a few kilovolts positive with respect

to the pump wall for pumps up to about 4" diameter. The electrons

91go into orbits known as Kingdon orbits about the central anode

producing ionisation of the residual gas. Eventually the electrons

bombard a cylindrical titanium slug of about diameter which is

carried on the anode. This causes the titanium to become white

hot and sublime onto the walls of the pump thereby "gettering"

the active gases. Inert gases are pumped by ionisation and

physical burial of the inert gas ions on the wall of the pump by

titanium. The inert gas pumping speed is determined by the amount

of ionisation caused by orbiting electrons and by the "sticking

probability" of the inert gas ions with respect to the wall of the

92pump. Kornelsen has shown that the sticking probability of inert

gas ions on tungsten is in the range 0.2 - 0.6 for incident energies

of around 1 KeV. However, sticking probabilities fa l l drastically

at lower energies - e .g . argon ions have a sticking probability on

-3tungsten of about 10 at 100 eV. Navertheless, it appears that

the sticking probability of inert gee ions on stainless steel

approach unity at energies sligh tly less than 1 KeV. Now in the

"orbitron" pump, inert gas ions which are not produced cloae to

the anode w ill have re lative ly low energy on reaching the wall of

the pump and are unlikely to stick to i t . Therefore, in order

that a ll inert gas ions reach the wall with energies not less than

about 1 KeV, Croas^3 has suggested that a grid at a potential of

109

about 1 KeV be placed close to the w all* This simple modification

was found to increase the argon pumping speed considerably.

94B ills has suggested a m ulti-ce llu lar design of

electrostatic getter-ion pump which separates the ionisation

function of the electrons from the heating of the titaniun. The

latter function is replaced by resistive heating leaving the

electrons merely to provide ionisation. This means that the anode

does not carry a titanium slug and, therefore, the electrons w ill

traverse a longer path before reaching the anode thus increasing

ionisation. Four "orbitron" ce lls of anode, grid and filament

are arranged around a central evaporator of resistive ly heated

titanium. The cylindrical grids around each anode are maintained

at a potential of a few hundred volts with respect to the pump wall

(cathode) so that the sticking probability of inert gas ions on

the wall is near unity. Improved performance is , therefore,

obtained at the expense of simplicity in design. An additional

advantage of the B ills pump is that when pumping at low pressures

i t is possible to decrease the titanium sublimation rate without

decreasing the ionisation rate upon which the inert gas pumping

speed depends. Thus i t is more economical on titanium.

1»2 Testing of a prototype.

89Using the experience of Herb et a l , a 2" diameter

orbitron pump was designed with the help o f Dr. J. Cross, formerly

of this Department, and made by a local engineering firm (P .S .I . L td .).

The simplest possible design was used in order to be tested as a

prototype. A right angle bend was incorporated in the pump body

between the pump its e lf and i t s flange in order to prevent titanium

reaching the system being pumped and also to reduce the amount of

110.

light entering the system from the pump. O riginally, the pump was

air-cooled using a fan but later the pumping efficiency was found

to be increased by using water cooling.

The optimum filament position has been found experimentally

89by Herb et al • Herb suggested that the filament support be

placed between the anode and the filament in order to reduce the

number of electrons travelling d irectly to the anode. Straight

filaments of .006" diameter and about in length were used

paralle l to the anode and positioned about £ " from the anode.

The pump has two such filaments although only one is used during

( operation^ the other is a standby in case the filament should break

or burn out unexpectedly during pumping. A hairpin filament support

wire of stainless steel was used. The tungsten anode was about 4"

in length (measured from the filament position) and carried a

diameter, £" long titanium slug positioned about half-way along its

length.

In order to start the pump for the f ir s t time the pressure

-3had to be reduced to less than 10 to rr by the roughing pumps and

1 the emission current of electrons from the filament kept low. Using

a large emission current (above about 5 mA) in it ia lly caused the

anode to be rapidly heated causing rapid out-gassing and thereby

raising the pressure in the system. A balance had to be achieved

within the pump so that eventually the rate of pumping exceeded the

/ ofrate'the out-gassing. This, in adverse circumstances, took a few

hours to achieve. Once the pump had been in fu ll operation and

( thoroughly outgassed by increasing the emission current in stages

it proved advisable to keep the pump under a good vacuum when not

in operation. This was achieved by an isolation valve of bore equal

to the pump throat diameter between the pump and the system.

111.

Allowing the pump to be let us to atmospheric pressure along

with the system meant that at least some outgassing of the anode

would again be necessary before the pump became fu lly operative.

Accidental flooding of the pump with gas whilst in operation also

had a very deleterious effect on its performance and several hours

of careful outgassing would then be necessary to nurse the pump

back to maximum performance. However, i f the pump had been kept

at high vacuum when not in operation, i t could be switched on whilst

s t i l l isolated from the system and then introduced when the pressure

-3of the system reached about 5 . 10 to rr. The orbitron pump

would then immediately pump the system and the roughing pumps could

be isolated.

It was also noticed that the orbitron pump would continue

to pump for some time after it s power supplies had been switched o ff .

This effect is presumably due to adsorption of residual gas by the*

anode on cooling, particularly by the titanium.

89 90From the work of Herb et a l and Douglas et al the

pumping speed for active gases varies approximately as the square

of the pump diameter ( i . e . it is proportional to throat conductance),

-5and varies l i t t le with pressure below 10 torr. Therefore, for a

2" pump a pumping speed for nitrogen of around 100 1/sec would be

expected. From tests carried out the present pump appears to have

a pumping speed of this order. Tests were also carried out on a

2^" diameter triode pump in which the grid was mounted from

the pump wall. This pump was designed by Dr. Croat for Applied

Research & Engineering Ltd. using the experience gained by Dr. Croat

and the author in testing prototypes. It haa a considerably

improved argon pumping speed compared with the diode pump but

otherwise its characteristics are similar.

*

Fig» 45 Cross-sectional diagram of the electrostatic

getter-ion ('o rb itron ') pump tested and used in this

work»

Fig* 46 Power supply circuit for the above pump with adjustable

latching relay trip and voltage bias fa c ilit ie s for

filament and grid*

112.

1.3 Materials and Maintenance.

The l i fe of a tungsten filament is rather unpredictable

but depends on such factors as the temperature at which i t is run.

Filaments almost invariably break in the centre where the possibility

of heat dissipation to the support i s a minimum and often blow as

the current is switched on. Mechanical shock may also cause rupture

since tungsten becomes very b rittle a fte r being heated to white

heat. Filament l i fe can be improved by using tungsten-rhenium

wire instead of pure tungsten. The filaments are spot-welded to

their supports and are easily replaced on removal of the pump head

although this means that the pump has to be opened to the a ir .

The l i fe of the anode obviously depends on how quickly

the titanium is evaporated and, therefore, on the level of emission

current used. Strictly speaking, i t depends on the power absorbed

by the titanium. I f the pump was used continuously to keep a

reasonably leak-free system under high vacuum, a fa ir ly low emission

current would suffice and the anode would probably last for several

weeks. I f , however, the pump is used fo r frequent re-evacuation

as is usual its pumping l i fe w ill be considerably less. For the

application fo r which the pump was used here, i .e . frequent

re-evacuation plus pumping on a hydrogen leak for long periods, its

pumping l i fe may be no more than about a hundred hours.

The tungsten anode, like the filaments, becomes very

b ritt le and i f the filaments should need replacement before the

anode, it would almost certainly shatter on removal of the pump

head. This problem can be overcome by using a tungsten-molybdenum

alloy anode which does not become so b r it t le on heating. Pure

molybdenum anodes were also tried but were unsatisfactory owing to

113.

violent spasmodic outgassing during the outgassing procedure which

caused flash-over between the anode and the filament supports. An

anode rod cannot normally be re-used since remnants of the titanium

slug remain welded to i t .

A fter two or three anode slugs have been evaporated onto

the pump wall a layer of titanium has built up on the wall which may

begin to flake causing a noticeable deterioration in performance.

The most successful way of removing this layer is by submerging the

pump body in a bath of 5# hydrofluoric acid in dilute n itric acid

and then thoroughly washing in water. Great care is necessary in

the use of hydrofluoric acid which should never come into contact

with the skin. The operation should be carried out in a fume chamber.

1.4 Power Supplies.

The basic requirement for a 2" or 2^" diameter pump is a

D.C. supply of about 3 to 5 KV capable of giving a current of up to

40 mA. Good smoothing and stabilization are not essential and|

therefore, the c ircu it can be simple. A voltage doubling circuit

was used as shown in Fig. 46 . An auxiliary A.C. supply is

necessary for the filament which requires a current of up to 4 amps.

This is provided by a 0 - 6V variable supply (unstabilized ). The

total ion current is controlled by the emission from the filament

which of course depends on its temperature. The temperature is in

turn a function of the current passing through i t and is , therefore,

controlled by the voltage across the filament.

I f the pressure in the pump accidentally rises to greater

than 10 '2 torr a gas plasma may strike and the ion current would

then rise alarmingly. A current trip in the H.T. supply is , therefore,

essential. A thermal trip can be used but operation is rather slow

and, therefore, a latching relay trip is preferred.

114

A voltage bias fo r the grid of a triode pump is

obtained by allowing the anode current to pass through a high

wattage resistor to earth. A smaller adjustable voltage bias

( 100V) for the filament and termination plate is obtained in a

similar fashion. The position on the titanium slug or anode

which becomes hottest due to electron bombardment can be adjusted

to some extent by putting such a voltage bias on the filament.

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