Applications of Liquid Scintillation Counters-6bl

8/11/2019 Applications of Liquid Scintillation Counters-6bl

1/5

IRE

TRANSACTIONS ON

NUCLEAR

SCIENCE

ANTHRACENE

,,..LIGHT

PIPE

RADIOACTIVE

SPECIMEN

-GLASS

MOUNT

Fig.

9-Counting

geometry for

3- y

coincidence

polarized

nuclei.

experiments

with

4C

f>

3u

L2

25

0

o

20

.4

5

0

0.2

3 0.4

015 a6

07

0.8

9 1.0

YC

Fig.

10-Original electron

emission

angle

necessary for

central

detection

in

3-y

coincidence

experiments.

Fig.

11--sy

coincidence

apparatus.

dences

between

electrons

above

a

certain

energy and

the

desired

gamma

rays striking

each

pair

of

gamma

counters

are

recorded,

as well

as all

individual

counting rates.

The

fifth

gamma

counter is used

to

determine the

degree of

nuclear

polarization

as

before.

As can

be

seen

from

the

photograph,

the

geometry

is

rather tight, so

that

correc-

tions

for

angular

resolution must be

made.

Further

experiments

with

this

type

of

apparatus

are

in

progress,

and

some changes in

technique

are

being

planned.

In

particular,

use of

some of

the new

photo-

multipliers

which

can be

operated

at

low

temperatures

should

materially

reduce

heat

leaks

which are

troublesome

with

the

presently

used

light

pipe.

Applications of

Liquid

Scintillation

Counters*

F.

NEWTON

HAYESt

Summary-The liquid

scintillator,

in

just

a

few

years of de-

velopment,

has shown

itself to

be

an extremely

versatile

chemical

system fo r

radiation

detection. Its

evolution has

been

characterized

by

penetration

into

almost

every

phase

of

experimental

science.

A

survey of the

most

notable

applications of liquid

scintillation

counting is

presented.

*

Manuscript

received by

the

PGNS,

February

8, 1958. This

work

was

performed under the

auspices of the

U. S.

Atomic

Energy

Commission

t

Los Alamos

Sci.

Lab.

University

of

California,

Los

Alamos,

N

M

OWARD

the end of

August,

1957,

at

Northwestern

University,

Evanston,

Ill.,

a

three-day

conference on

liquid

scintillation

counting

was held.'

There

were

sessions

on

physics

and

chemistry

of the

liquid

scintillator,

on

instrumentation for

liquid

scintillation

counters,

and

on

applications

of

liquid

scintillation

counting.

Much

of

I

Proceedings of

the Northwestern

University

Conference

on

Liquid

Scintillation

Counting, to

be

published

by

Pergamon

Press Ltd.,

London,

Eng.

351

45

166

December

AlultIXDoM1a1UfIX Ra


2/5

Hayes:

Applications

of Liquid Scintillation

Counters

th e

information

in

this

paper

has been

drawn from

the

Proceedings

of

this

conference.

A

liquid

scintillator2

is

usually

a

three-component

system

composed

of

solvent,

primary

solute,

and

secondary

solute.

An

energy transfer

sequence

proceeds

from

solvent

to

primary

solute,

at which

point

the

process

has

realized

about

3

per

cent

energy

efficiency,

and

energy

states

are

attained

which

allow emission

in

the

350

to

400

m,u

range.

However, photon

emission is

delayed

until

energy

is

transferred

to

the

secondary

solute.

The

quantum effi-

ciency of this

last

step

in

the transfer is about

1,

and

the

photon

emission is

mostly

in the

range

420

to 480

m,u.3

Such

a

spectrum

of

photons

has considerable

mean-free-

path

in

th e

scintillator and has a

high

matching

factor

to

the

sensitivity

spectrum

of

a

photomultiplier

cathode.

Toluene

is

often

an excellent

choice

as

a

solvent4

because

it is

efficient, cheap,

and

commercially

available

in

requisite

purity.

In situations

where

its

fire

hazard

and

toxicity

become

important

considerations,

th e

less

volatile

solvents

triethylbenzene

or

isopropylbiphenyl5

may be

used.

The

solvent

combination

of

p-dioxane

and

naphthalene

is

very

useful6

when water must

be

dissolved

in

the

liquid

scintillator.

The most

popular

primary

solutes7 are

p-terphenyl,

2,5-diphenyloxazole

(often

referred to

as

PPO)

and

2-(4-biphenylyl)-5-phenyl-1, 3,

4-oxadiazole

(sometimes

called

PBD).

Today s

most

useful

secondary

solutess

are

2,5-p-phenyl-

ene-bis(5-phenyloxazole) (almost

always

referred

to

as

POPOP and

9,10-diphenylanthracene.

For

the

last five

years

the

most

popular

liquid

scintillator

has

been

composed

of

toluene,

p-terphenyl,

and a

secondary

solute.

There

ar e

no

unequivocal

optima

in

solute

con-

centrations9

where

economy

is

the

natural

antagonist

of

performance.

A

solution of 5

g/1

p-terphenyl

and 0. 5

g/ 1

POPOP

in

toluene

is

now

considered

to be

a

superior liquid

scintillator

over

the million-fold

range

in

volume

found

in

liquid

scintillation detectors.

This

solution

yields

about

8

photons,

air-saturated,

or

10

photons,

inert-gas

saturated,

per

kev

deposited

by

an

exciting particle

at

minimum

2

F.

N.

Hayes,

Liquid scintillators:

attributes and

applications,

Intern.

J.

Appl.

Radiation

and

Isotopes,

vol.

1,

pp . 46-56;

July,

1956.

3

D.

G.

Ott,

F.

N.

Hayes,

E.

Hansbury,

and

V.

N.

Kerr, Liquid

scintillators.

V.

Adsorption and

fluorescence

spectra

of

2,5-diaryloxa-

zoles and

related

compounds,

Am.

Chem.

Soc.,

vol.

79, pp .

5448-

5454;

October,

1957.

4

F. N.

Hayes, B. S.

Rogers,

and P.

C.

Sanders,

Importance

of

solvent in

liquid

scintillators,

Nucleonics,

vol.

13, pp .

46-48;

January,

1955.

5

W.

L.

Buck

and R. K.

Swank,

Use

of

isopropylbiphenyl as

solvent in

liquid

scintillators,

Rev.

Sci.

Instr., vol.

29, p.

252;

March,

1958.

6

W. H.

Langham, W. J.

Eversole,

F.

N.

Hayes,

and

T.

T.

Trujillo,

Assay

of

tritium

activity

in

body fluids

with use

of

a

liquid

scintillation

system,

J.

Lab.

Clin.

Med., vol.

47,

pp .

819-825;

May,

1956.-

7F.

N.

Hayes,

D.

G.

Ott, V.

N.

Kerr,

and

B.

S.

Rogers,

Pulse

height

comparison of

primary

solutes,

Nucleonics,

vol. 13, pp .

38-41;

December,

1955.

8

F.

N.

Hayes,

D.

G.

Ott,

and V. N.

Kerr,

Pulse-height

com-

parison

of

secondary

solutes,

Nuicleonics,

vol.

14,

pp . 42-45;

January, 1956.

D.

G. Ott,

see

footnote 1.

ionization. A

liquid

scintillator

responds

to

th e

densely

ionizing

5-mev

alpha

particle

as

though

it

were

a

0.5-mev

electron.

The

liquid

scintillator is a

member of

th e

organic

scintillator

family

which

also

includes

crystals, plastics

and

gels.

It

has

certain

useful

properties

which

clearly

differentiate

it

from its

organic relatives,

as

well as

from

inorganic-crystal

and

noble-gas

scintillators.

These

are:

low

cost,

ease

of

preparation,

high transparency,

short

decay

time,

absence

of

self-dictated size

limitation,

and

ease

of

alteration.

The 5

g/1 p-terphenyl

and

0.5

g/1

POPOP

solution

in

toluene

has

a

materials

cost of

about

2.80/1.,

and

it s

preparation

time is

the

short

time

required

through

heating

and/or stirring

to

dissolve

the

solutes in

th e

solvent. This

solution

has

an

average mean-free-path

for

its

emitted

light

of th e

order

of 10

meters,

and

it s

decay

time is


3/5

IRE

TRANSACTIONS

ON NUCLEAR SCIENCE

SMALL-VOLUME

INTERNAL-SAMPLE

COUNTING

Since

1951,

small-volume liquid scintillation

counters

have

been used for beta-counting

of

isotopically

labeled

organic compounds

found

especially

in biological,

chemical,

and

industrial

research.

Historically,

the

simplest case

in

the

chemistry

of

this

detector

was developed

first.

Gaseous, 2

3

liquid

and

solid

compounds

soluble in

toluene

were

dissolved

directly in

the

liquid

scintillator

and,

for C' 4

and higher energy

beta

emitters, counting

effi-

ciencies

from 50

to

100 per

cent

were

obtained. This

represented

improvement

over

Geiger

and ionization

chamber

measurements

in that

sample

processing

was

kept

to a

minimum,

thereby

allowing

considerable

saving

in

the

over-all

assay time.

Such

considerations

as correc-

tions

for

self-absorption

and geometry

disappeared since

the

dissolution

of

the sample

makes

it

infinitely

thin

throughout

the

detector and

gives

it a

47r

geometry.

Especially

for liquid and

solid

samples, the

liquid

scin-

tillation

method

offers

a

considerable

chance for high

sensitivity.

Guinn has made

comparisons

with

thin-

window

Geiger

counting

of

minimum specific

activity

detectable

to I10

per cent

standard deviation

in

30-

minute sample

counting

time and

30-minute background

counting time.

He

finds

that the liquid

scintillation

counter

may yield

higher

concentration sensitivity by

the

following

factors:

7,800

fo r

H3,

1,000

fo r

C'4,

870

for

S33,

156

fo r

Cl36, and

60

fo r p32.

Guinn

quotes

counting

ef-

ficiencies

of 25

per

cent

for

H3,

75

per

cent

fo r

C14,

and

100 per

cent fo r

p32.

Many

workers

have

reported

successful

routine

analyses

of

binary

and

ternary

mixtures

of

beta-emitting isotopes.

This

takes advantage

of

the

energy

dependence

of

light

output

from

a

liquid

scintillator.

Some

examples

are

H3-C14,

H3-Na22,

and H3_C'4_p32.

For

compounds

insoluble

in

toluene,

two variants

in

the

scintillator system

have been

developed.

The

first

is

effecting

a

solution

of the

sample

without

seriously

impairing

the scintillating

action

of the

solution,

an d

the

second

is

dispersal

of

the insoluble

sample

in

a

finely

divided

state

throughout

the

scintillator.

The

first

method

can

be

accomplished

by

use of

the

water-miscible

solvent

systems,

p-dioxane

and

naphthalene

or

toluene

and

ethanol,

or

by

chemical treatment

of the

sample

to

make

it

soluble

in

toluene.14

5

The

second method

was

first

tried

with

no

suspending

agent'6

and

then with

agents

11

M.

S.

Raben

an d

N.

Bloembergen,

' Determination

of

radio-

activity

by

solution

in

a

liquid scintillator,

Science,

vol.

114, pp.

363-364; October,

1951.

12

B. N.

Audric

an d

J.

V. P.

Long,

Use of

dissolved

acetylene

in

liquid

scintillation

counters for the measurement

of

carbon-14

of

low specific

activity, Nature,

vol.

173,

pp.

992-993;

May,

1954.

13

G.

W.

Barendsen,

Radiocarbon

dating

with

liquid

CO2

as

diluent

in

a

scintillation

solution,

Rev. Sci.

Instr.,

vol.

28,

pp.

430-432;

June,

1957.

N.

S.

Radin,

see footnote

1.

J.

M.

Passman,

N. S.

Radin,

and J.

A.

D.

Cooper,

Liquid

scintillation

technique

for

measuring

carbon-14-dioxide

activity,

Anal.

Chem.,

vol.

28, pp.

484-486; April,

1956.

18

F.

N.

Hayes,

B. S.

Rogers,

and

W. H.

Langham,

Counting

suspensions

in

liquid

scintillators,

Nucleonics,

vol.

14,

pp.

48-51;

March,

1956.

which

yield

rigid

gels17 or

thixotropic

solutions. 8 9

A

very

satisfactory

new

suspension

procedure

20

is

the use

of

a 25 per

cent solution

of

polystyrene in

a

toluene-

solvent liquid

scintillator.

This gives

a transparent

solution

viscous

enough

to

support suspended

matter

during

reasonable

counting

times

but

fluid

enough

to

allow

further

addition after

the original

counting

period of a

substance

which can

serve

as

an

internal

standard

fo r

efficiency

calibration. The

specific

application

of this

procedure

has involved

samples

of 10 g of ashed

and

powdered

bone in

15

ml

of the

viscous

scintillator

from

which

bone sample

assays

as

low as 10 Sr90

d/min/g

can

be determined.

Quenching2

of

th e

scintillation

process and

absorption

of th e

emitted light

may be corrected

for,

in

homogeneous

systems,

by

use of

soluble internal

standards.2 22

Electronics

for

small-volume

internal-sample

liquid

scintillation

counting

has been

distinguished

by

having

either one photomultiplier

in

a single

channel

circuit2326

or

two

photomultipliers

in coincidence.27 28

The com-

plexity

of

th e

coincidence-type

circuitry

is

required,

with

the

present

state

of

photomultiplier

development, for

counting

applications

with

low-energy

beta

emitters

wherein great

sensitivity

is

required. That

Pringle29 30

has

accomplished

single channel

counting of

natural-C'4

for

dating

of archaeological

specimens

as

old as 38,500

years

with

i9 per

cent

precision,

is a

tribute

to his

complete

coordination

of

optimization

in effectiveness

of

shielding,

elimination

of electrical

noise,

selection

of

a

photomultiplier

with

a

low

noise

to

gain

ratio, and

use of

a

scintillator

system

with

high

photon

yield.

Nir' has

reported

on

liquid

scintillation

counting of

natural tritium

and

its

applications

to

hydrology

and

B. L.

Funt and

A.

Hetherington,

Suspension counting

of

carbon-14

in

scintillating gels,

Science,

vol.

125,

pp. 986-987;

May,

1957.

18

S. Helf,

see footnote

1.

C.

C.

White

and

S.

Helf,

Suspension

counting

in

scintillating

gels,

Nucleonics,

vol.

14, pp .

46-49;

October, 1956.

20

H. Foreman, se e

footnote

1.

21

V.

N.

Kerr,

F.

N.

Hayes,

and D.

G.

Ott,

Liquid

scintillators.

III.

Th e

quenching

of

liquid-scintillator

solutions

by organic

com-

pounds,

Intern.

J. Appl.

Radiation

and ISotopes, vol.

1, pp .

284-

288; January,

1957.

22

D. L. Williams,

F.

N.

Hayes,

R. J.

Kandel,

and

W.

H.

Rogers,

Preparation

of C14

standard

for

liquid

scintillation

counters,

Nucleonics,

vol.

14,

pp .

62-64;

January,

1956.

23

B.

Gordon

and

T. S.

Hodgson,

se e

footnote

1.

24

E.

C.

Farmer

and

I.

A.

Berstein,

Determination

of

specific

activities of

C'4-labeled

organic

compounds

with

a

water-soluble

liquid

scintillator, Science,

vol.

1 15, p p.

460-461;

April,

1952.

25

D. J. Rosenthal

and

H. 0.

Anger,

Liquid

scintillation

count-

in g

of

tritium

and C 4-labeled

compounds,

Rev. Sci.

Instr.,

vol.

25,

pp .

670-674;

July,

1954.

26

J.

C.

Roucayrol

and

E.

Oberhausen,

Measurement

of

activity

of

compounds

traced

with

low-energy

beta

emitters,

Science,

vol.

122,

pp .

201-202;

July,

1955.

27

R.

D. Hiebert

and

R.

J.

Watts,

Fast-coincidence

circuit

for

H3

and

C'4 measurements,

Nucleonics,

vol.

11,

pp . 38-41;

Decem-

ber,

1953.

28

F.

N.

Hayes

and

D.

G.

Ott,

Los Alamos

Doc.,

LA-2095; 1957.

29

R.

W.

Pringle,

W.

Turchinetz,

and

B. L.

Funt,

Liquid

scintil-

lation techniques

for

radiocarbon dating,

Rev.

Sci.

Instr.,

vol.

26,

pp.

859-865-

September,

1955.

B

Fn adS

Dnlk

0

R.

W.

Pringle,

W.

Turchinetz,

B. L. Funt,

and S.

S.

Danyluk,

Radiocarbon

age

estimates

obtained

by

an

improved

liquid

scintil-

lation

technique, Science,

vol.

125,

pp .

69-70;

January,

1957.

168

December



4/5

Hayes: Applications

of

Liquid Scintillation

Counters

meteorology.

He

has

enriched the

natural tritium

in

water

from

rain,

ground

water,

and

lakes,

and

has

ex -

changed

the resultant water with

toluene under

acid

catalysis. After this

treatment,

25

per

cent

of

the

original

tritium resided in the toluene.

With this

toluene as

the

solvent

of

a

liquid

scintillation

counter,

operating

at

-70C in

coincidence,

a

limiting

sensitivity

of

one

tritium

atom

in 1018

hydrogen

atoms

was

realized.

The

counting

efficiency

for

tritium

was

45

per

cent.

Bell

has

used

liquid

scintillation

counting

of

tritium

in

tritium-labeled water

because

of its

ready

applicability,

both

to

great

numbers

of

samples

and

to

realization

of

high

sensitivity.

He

has carried

out

experiments

on

model

as

well as actual

rivers,

assaying

tritium

activity

vs

time

and position

relative

to the

point

of

entry

of

the

original

labeled

water

sample.

Guinn'

has

quoted

some industrial

applications

of

small-

volume,

internal-sample

liquid

scintillation

counting

in

industrial research.

Examples

are:

tritium

counting

of

exhaust water after

use of tritium-labeled

lubricating

oi l

to

follow

the

rate

of

combustion

of

such

oils

in

operating

engines,

C' 4

counting

to

determine the

stability

of

a

fuel

oil

additive

in

long-term

storage,

S35

counting

to

estimate

the

exchange

of

hydrogen

sulfide

with

sulfur

atoms

in

metal

sulfide

catalysts,

and

p3 2

counting

to

study

entrain-

ment

in

aqueous

large-scale

distillation

columns

with

P32-labeled

phosphoric

acid

as a

nonvolatile

tracer.

Langham

has reported

on

th e

small-volume,

internal-

sample liquid

scintillation

counting

of

H3,

C 4,

Na22,

S35,

Ca45

5r90 j131

CS134,

CS137,

U233,

U235,

and Pu239,

i

various chemical

forms,

as

a

part

of a

biological

and

medical research

program.

As an

example

of

part

of

th e

tritium applications,

counting

of

tritium

water

which

appears

in

the

blood

and

urine

of a

person

after

ingestion

has

led

to such

physiological

data

for

that

person

as

gastric

hold-up

time, equilibrium time

between

gas-

trointestinal

tract and

blood,

equilibrium time

between

blood and total

body fluids,

total

body

water,

rate of

turn-over

of

body

water, per

cent lean

body

mass,

and

per

cent

gross

body

fat. Another

similar

and

quite

practical

applications of

tritium

water

counting

is

th e

on-the-hoof

determination

of

per

cent

body

fat

in

a

steer.

Roucayroll

in

France

has taken

paper

chromatograms

of

C'4,

1131,

and

P32-labeled

compounds, has

wet

th e papers

with

a

liquid

scintillator,

and

has

then

scanned them

with

a

single

photomultiplier.

Special

techniques

have

been

developed fo r

handling

toluene-soluble,

as

well

as

toluene-

insoluble,

compounds.

Arnold

3

has

been able to

effect

very

sensitive counting

of

natural

C'4

in

a

100-ml

scintillator

solution,

whose sol-

vent

is a

mixture of

hexane

and octane

equivalent

to

47

g

of

carbon.

The

solvent was

prepared

in the

sequence:

sample,

carbon

dioxide,

acetylene,

the

6-carbon

and

8-carbon

polymers of

acetylene,

and

hexane

and octane.

Baranov,

Goldanskii

and

Roganov32

in

Moscow

have

made

a

high-energy

neutron

dosimeter

from

a 70-ml

volume of

a

liquid

scintillator

which

serves both as

a

source

of

carbon

and

as

a

detector of C

disintegration

positrons.

The C

results

from the

C'2(n,

2n)C reaction

which has

a

20.6-mev

threshold.

The useful

range of the

dosimeter

is

from 40 to

400

mev.

LARGE-VOLUME

EXTERNAL-SAMPLE COUNTING

The

large liquid

scintillation

counter is

usually

more

notable

for the

large

number of

photomultipliers used

than

for

th e

large

volume

of

liquid,

since

many

2-inch

cathodes

are

required

in

a

large

counter to

allow even

a

small fraction

of

the

inner surface

to

be

photosensitive.

It is common

practice

to

furnish

the remainder

of

the

inner

surface

with a

diffuse

reflector

whose index

of

refraction is

higher than that

of the scintillator

which

bathes

it.

An interesting

difference

between

the

small and

large

scintillation

counters involves

the lifetime

of a

solution

used

in

filling

the

counter.

This time

is

usually

quite

short

with

the

small

counters,

since the solution

is

asso-

ciated with

only

one

counting

sample and must be

replaced

by

a

new

solution when

th e next

assay

is

run.

The labora-

tory

in which 100

analyses a

day ar e

run,

using

25

cm3

of scintillator

for each

sample,

will

use up 2.5

liters

in

such

a

day.

In 120

such

days, the total

volume used

will

equal

the 0.3

m3

volume

of

the oft-termed

giant

liquid

scintillation

detector

used

in the 1953 search

fo r

th e

free

neutrino.

Whereas

the small

detector

is

continuously

being

refilled, th e

large detector

filling

tends to

stay put.

This

introduces

an

important

question

for

large

detectors

as

to aging

of a

liquid

scintillator.

A

specific quotable

case

involves a

Los

Alamos

counter

whose

p-terphenyl-aNPO-

toluene

filling after 3'

years

still

had 0.86 of

its

original

pulse height. A loss

in

efficiency

on

standing

in a

detector

may arise

from

leaching of

trace

materials

from the

reflector, the

container,

cement,

or

0-rings,

or

from

chemical

action

of

dissolved

oxygen

on

th e

scintillator

components.

It

seems to

be

important

in the use

of

triethylbenzene

in

a

large counter

to

replace the

dissolved

and free air

in the

counter

with an

inert gas

such as

argon.

A

fascinating

property

of

large-volume

liquid

scintilla-

tion

detectors

which

have

good light

collection

efficiency

is

their

ability to

give

sharp energy

resolution

for

energetic

gamma

rays.

The

dimensions

of

the

detector allow

an

entering

gamma

ray

to

impart

essentially

al l of

its energy

to

the

scintillator by

multiple

Compton

scattering.

A

0.06 m3

square

cylindrical

detector8 with

about 4 per

cent

of

it s

internal

surface

photosensitive

and

with an

internal

titanium

dioxide

reflector gave

about

13.5 per

cent

resolu-

tion

(full

width at half

maximum)

for

4.43-mev

gammas

from

excited C'2

and,

with

decreasing gamma

energy,

J.

R.

Arnold,

Scintillation

counting of

natural

radiocarbon: 32

P. S.

Baranov, V.

I.

Goldanskii,

and V. S.

Roganov,

Dos-

I.

The

counting

method,

Science,

vol. 119,

pp.

155-157;

January,

imeter

for

high-energy

neutrons,

Rev. Sci.

Instr., vol.

28,

pp . 1-29-

1954.

1032;

December, 1957.

1958

169



5/5

IRE

TRANSACTIONS

ON NUCLEAR SCIENCE

responded according to the relationship:

Per

cent

R

=

kE

1/2

The earliest large-volume counters

were about 0.03

m3

in

volume

and were used for studies

of cosmic

rays33 and

of total absorption

of

gamma rays.34 It was indeed a

spectacular

increase

when

Cowan

and Reines35 jumped

to 0. 3 m3

for th e Hanford

phase of

the

search fo r the free

neutrino. Hydrogen,

for the

most

part

in

th e toluene

solvent,

was allowed

to

be transmuted

by

neutrinos

to

give, in

each

interaction,

a

positron

and a n eu tr on . The

positron gave

a

prompt

scintillation

and then

after a short

slowing

down

time,

the neutron was

captured in cadmium

to give

about

9.1

mev

of

gammas,

which

contributed a

delayed

scintillation. The

cadmium was in

th e

form of

cadmium propionate,

which was

dissolved in

th e

scintilla-

to r

with

th e

help

of methanol.

A similar detector

has

been used

to set a

1022

years lower

limit

on the lifetime

of

a nucleon36

and

to

measure

th e

distribution

of fission neutron

multiplicities.37'38

In th e

latter application

an

81

per

cent

efficiency

for

couniting

neutrons

of

a

few

mev

was

realized.

By

this

time,

cadmium

2-ethylhexanoate

was

found to be

a

better

homogeneous

source

of cadmium

than cadmium

propionate

and

metha-

nol.

This

same 0.3

m3

detector

system

has been

used in

a

velocity

as

well

as

time

distribution

study

of

prompt-

neutron

emission

from

spontaneous

fission

modes

of

Cf232.39

Both boron-loaded40 and cadmium-loaded4 liquid

scintillation counters

have

been used

for neutron detection.

A 0.57

m3

detector

has been

used

in an

attempt

to

identify

double

beta

decays.42

Most

of

th e volume

served

as

a cosmic

ray

and

gamma ray

anticoincidence

shield.

A

small

detector

in th e

center,

which

used

th e

same

liquid

as

th e

outer

detector,

was a

two-section coincidence

detector

for betas.

Harrison'

has

reported

that

he is

now

studying

the

strength

of th e

interaction

between

cosmic

ray ,u-mesons

33

F.

B. Harrison,

Large-area

liquid

scintillation

counters,

Nucleonics,

vol.

10,

pp.

40-45;

June,

1952.

34

M.

R.

Cleland

and H.

W.

Koch, High-energy

gamma-ray

spectrometer,

Nucleonics,

vol.

10 , p.

41; March,

1952.

35

C.

L.

Cowan, Jr.,

F.

Reines,

F. B.

Harrison,

E. C.

Anderson,

and F.

N.

Hayes,

Large liquid

scintillation

detectors,

Phys.

Rev.,

vol.

90,

p.

493;

May,

1953.

36

F. Reines,

C.

L.

Cowan,

and M.

Goldhaber,

Conservation

of th e

number

of

nucleons,

Phys. Rev.,

vol.

96, pp .

1157-1158;

November,

1954.

37

B. C.

Diven,

H. C.

Martin,

R. F.

Taschek,

and J.

Terrell,

Multiplicities

of

fission

neutrons,

Phys.

Rev.,

vol.

101,

pp .

1012-

1015;

February,

1956.

38

D.

A.

Hicks,

J.

Ise,

Jr.,

and R.

V.

Pyle,

Probabilities

of

prompt-neutron

emission

from

spontaneous fission,

Phys.

Rev.,

vol.

101,

pp.

1016-1020;

February,

1956.

3

W.

E.

Stein

and S.

L.

Whetstone,

Jr.,

submitted to

Phys.

Rev.

40

L. M.

Bollinger

and

G.

E.

Thomas,

Boron-loaded

liquid

scintillation

neutron

detectors,

Rev. Sci.

Instr.,

vol.

28 , pp . 489-496;

July,

1957.

41

F.

Reines,

C.

L.

Cowan,

Jr.,

F.

B.

Harrison,

and

D. L.

Carter,

Detection

of neutrons

with

large liquid

scintillation

counter,

Rev.

Sci.

Instr.,

vol.

25,

pp .

1061-1070;

November,

1954.

42

C.

L.

Cowan, Jr.,

F. B.

Harrison,

L. M.

Langer,

and F.

Reines,

A test of

neutrino-antineutrino

identity,

Nuovo

cimento,

vol.

3,

pp . 649-651;

March,

1956.

and carbon nuclei which ar e part

of

a large-volume

detector. The

1.4 m3 of liquid is contained in a polyethylene

container to minimize background above 5

mev

by remov-

in g the Fe(n, -y) source of background which

occurs

with

an iron

container.

In case

al l

this mention of

volume in

m3 is difficult to

visualize,

a

corollary

consideration in

weight

units is

interesting. One m3 of a liquid scintillator

weighs

about

one

ton.

The

most giant,

large-volume, external sample detector

reported to

date43

is

th e 1.8

m3

volume, three

of

which

were

used

at th e same time, for

the

Savannah

River

phase

of

th e

search

for the free neutrino. In this case, the

identifiable

neutrino

interactions

took place in two water

targets containing

cadmium

chloride

oriented in a

double-

decker

sandwich-array

with the

three

scintillation counters.

Delayed gamma

coincidences from

positron

annihilationl

followed

by neutron-cadmium

reaction were d et ec te d in

th e liquid

counters. Each

counter had 110 5-inch photo-

multipliers,

half

of which were

at

each end

of

a slab

configuration.

The

solution

which

was

used

was

3

g/1

p-terphenyl

and 0. 2

g/1 POPOP in

triethylbeuizenie.

Biological and

medical

employment of large-volume,

external-sample liquid scintillation detectors44 has made

use

of

an

enveloping,

4r-approximating geometry around

th e sample

which

is

easily

introduced

into

th e

countinlg

position

in

an

axially-oriented cylinder inside

an

annular

ring

of

liquid.45

Two versions

which

exist

at

Los Alamos

ar e the small-animal

counter

whose scintillator volume

is

0.036

m3,

and the

Human

Counter46

(0.42 m3). Both

counters

are used

in

studying

gross as well as

specific

metabolism

of

radioisotopes

in

live experimental animals

and

human

beings.

Reines,

1 while

speculating

on th e

limits in

size

on

a li(uid

scintillation

detector,

conjured

up

a vision of

a

10

m

X 10

m

cylinder holding about 700

m3

of

liquid.

It has a few

hundred 5-inch

photomultipliers

and

an

efficient diffuse

reflector,

and

gives

about

50

per

cent

resolution

for

a

1-mev

event.

The time

response

of this

detector

is limited

by

the

velocity

of

light,

it

has as

much

mass

pe r

uinit

area

as the entire

atmosphere,

and

it

represents

a total

absorber

for

charged particles

up

to 1.5

bev,

considering

ionization

as the

only process contributing

to

energy

loss.

Such

a

detector,

which

is

almost

400

times

larger

than the

largest

in use

today,

may

not be

made for

many years,

but

as

its

size

is

approached,

today's

large-volume

counters

will

have to be

renamed.

43

C. L.

Cowan, Jr.,

F.

Reines,

F.

B.

Harrison,

H.

W.

Kruse,

and

A. D.

McGuire,

Detection

of the free neutrino:

a confirma-

tion, Science,

vol.

1 24 , p p. 103-104;

July,

1956.

44

W.

H.

Langham,

se e footnore 1.

45

M.

A.

Va n

Dilla,

R.

L.

Schuch,

and

E. C.

Anderson,

A

large

47r

gamma-ray detector,

Nitcleonics,

vol.

12, pp . 22-27;

September,

1954.

46

E.

C.

Anderson,

R.

L.

Schuch,

J. D.

Perrings,

and W.

H.

Langham,

The Los

Alamos

human

counter,

Nucleonics,

vol.

14,

pp . 26-29; January,

1956.

170

December

Documents

Applications of Liquid Scintillation Counters-6bl