Neutron Activation Analysis

References:

Alfassi, Z.B., 1994, Determination of Trace Elements,(Rehovot: Balaban Publ.)Alfassi, Z.B., 1994b, Chemical Analysis by Nuclear Methods, (Chichester: Wiley)

Alfassi, Z.B., 1990, Activation Analysis, (Boca Raton: CRC Press), p. 161. Balla, M., Keömley G., Molnár Zs., 1998, Neutron Activation Analysis in Vértes, A., Nagy

S., Süvegh K., Nuclear Methods in Mineralogy and Geology (New York: Plenum), chapter 2, pp.115-143.

Handbook of Nuclear Chemistry

Contents

• Principle of activation analysis (AA)• Different types of AA • Neutron Activation Analysis (NAA)

- different types of NAA (prompt - delayed, instrumental-radiochemical)- neutron sources used for NAA- measuring equipment used for NAA- quantification of NAA (absolute, relative and comparator techniques)- properties of INAA (sensitivities, and non-destructive multielement character)- radiochemical separations in NAA

• INAA - application examples• RNAA – application examples

Principle of activation analysis

INCIDENT NEUTRON

TARGET NUCLIDE

COMPOUND NUCLIDE

RADIOACTIVE NUCLEUS

STABLE NUCLEUS

PROMPT GAMMA RAYS

DECAY GAMMA RAYS

X

CHARGED PARTICLE

PHOTON

Elemental – multi-elemental – trace elemental analysis

Steps of analysis:sample preparation (homogenization, weighing)

optional: pre-irradiation chemistry

irradiation

cooling (different times)

optional: post-irradiation chemistry

measurement by gamma spectrometry

evaluation

History

• Hevesy and Levi 1936: principle of NAA

• Neutron sources became available in the fifties

• Low resolution detectors (proportional counters, NaI scintillator)

• High resolution semiconductor detectors

• Alternative non-nuclear methods (AAS, ICP-OES, ICP-MS)

Various types of AA

1. Charged particle activation analysis

2. Photon activation analysis

3. Neutron activation analysis (NAA)1. Thermal neutron activation analysis

2. Epihermal neutron activation analysis (ENAA)

3. Fast neutron activation analysis (FNAA)

4. Neutron capture prompt gamma activation analysis (PGAA)

Neutron sources used for NAA

-emitter

Half life

Neutrons s-1 Ci-1 emitted

average neutron

energy [MeV] 227Ac 22 y 1.5x107 4 226Ra 1620 y 1.3x107 3.6 239Pu 2.4x104 y 1.4x107 4.5 210Po 138 d 2.5x106 4.3

Isotopic neutron sources:

Spontaneous fission of actinides:

252Cf (half life 2.6y): 3.76 neutrons of 1.5 MeV per event

1mg 252Cf emits 2.28x109 neutrons/secNeutron generators:

deuterons accelerated by 200 kV: 3H(d,n)4He

monoenergetic neutrons: 14 MeV (fast n reactions: (n,p), (n,α), (n,2n))

neutron yields: 1011 neutrons/s/ mA, neutron flux: 109 neutrons/cm2/s

Research reactors

Research reactors as neutron sources:thermal power: 100 kW-10 MWthermal neutron flux: 1012-1014 neutrons cm-2 s-1thermal epithermal +resonance fast neutrons <0.05 eV 0.1eV<E<1 eV 0.5 Mev<E

1eV<E<1 keVmean:0.04 eV

2200 m/s (n,γ) (n,γ) (n,p),(n,α),(n,2n)

cold neutron beam

Measuring systems used for NAA

Gamma spectrometers:scintillation detectorGe(Li) detectorHP Ge detector

Quantification

N: number of interacting isotopes

(E): cross-section [cm2] at neutron energy of E [eV]

(E): neutron flux per unit of energy interval [1/cm2/s/eV]

R: reaction rate

In reactors the integral is replaced:

0

)()( dEEENR

0IR epithth th: conventional thermal neutron flux [in cm2]th : effective thermal neutron cross-section [in cm2]e: conventional epithermal neutron flux [in cm-2 s-1 eV-1]Io: resonance integral cross section (in epithermal region), for 1/E epithermal spectrum [in cm2]

A∞=N

A = ( Iethth o) DSA

Nfm

rel

Avi

Activity of the nuclide at the time of measurement:

NAv: Avogadro number fi : isotopic abundancem : the mass of the irradiated elementArel : atomic mass of target elementti : time of irradiation; td : time of decay; : decay constantS: saturation factor: S=1-e-ti

D: decay factor: D=e-td

Activity is determined from measurements:N A f tP m

Np: net peak area, f: gamma abundance,

: detection efficiency, tm: measuring time

Combining the last 2 equations, the mass of the unknown element can be calculated:

meththiAv

p

SDtIffN

MNm

)( 0

Concentration c is determined from measured „m” and the volume/mass (V/G) of the sample:

c=m/V and c=m/G,

respectively.

Standardization

• Absolute methodBased on the expression of „m”

Parameters to be measured: Np, tm, ti, td

Parameters to be determined by calibration:

ε, φth, φe

Parameters derived from tables (nuclear+additional):

σth, I0, fγ, fi, λ, NAv, M

spspmspsp

spsp

m

p

sp

mtDS

NI

SDt

NI

I

Im

,

• Relative methodEach element is measured against an „element standard” irradiated together with the sample

Parameters not used:ε, φth, φe, σth, I0, fγ, fi, NAv, M

• Comparator method: „k” methodAll elements are measured related to a single element, the comparator.

Calibration phase:k factors are determined for each element compared to the comparator element(irradiation together)* refers to comparator element‘ refers to analyte in calibration procedure

Measurement phase:Sample is irradiated together with the single comparator, sample and comparator are measured** refers to comparator element during measurement phase

****

*

''''

'

*

mtDS

NmtDS

N

I

Ik

m

p

m

p

sp

sp

********

****

mtDS

kNSDt

N

kI

Im

m

p

m

p

sp

k factors are constant under constant irradiation and measurement conditions

(including the same geometry):

)//(

)//(

)(

)(

**0****

0*

*0****

0*

theththi

theththi

eththi

eththi

IfMf

IffM

IfMf

IffMk

• the k0 method

A pure nuclear constant k0 was derived from k factor by deCorte,

that is independent of measuring and irradiation conditions:

***

*0

i

i

ffM

ffMk This value was measured and checked.

Knowing k0 the standardization procure is simplified or omitted. „k” can be calculated.

K0 values were experimentally determined/checked according to the following equation:

)//(

)//(

0

**0*

*0

theth

theth

sp

sp

I

I

I

Ik

Properties of INAA

Advantages• Sensitive, trace elements are determined• Multi-element method• Matrix dependence is often small• Non-destructiveDisadvantages• Neutron source and gamma spectrometer are

neededExpensive and „nuclear” method

Application of INAAGeological samples:• NAA at INT-TU Budapest

- 1 minute irradiation,15 min cooling times: (28Al decays) Ti, V, (Cu), Mn, Cl, Dy and Ca are determined.- 8 hour irradiation in a thermal channel of the reactor measurment twice: one week, one month- usually 25-30 elements can be determined

• Epithermal NAA- gross activity due to 24Na, 56Mn, 46Sc, 28Al: low Io/σth - analytes (Rb, Sr, Ba, Ga, As, Mo, Ag, In, Sn, Sb, Sm, Tb, Ho, Ta, W, Au, Th, U): high Io/σth - epithermal AA in Cd wrapping → high sensitivity.

• Disturbing nuclear reactions- The same radionuclide is produced from two different elements: e.g. 28Al : 27Al(n,γ)28Al 28Si(n,p)28Al.

thermal n fast n- samples can be activated twice, with and without cadmium filter, in order to determine both Al and Si

• Studies on lanthanides to derive concentrations relative to standard condritesvolcanic activities

Biological samples- Analysis of Na, K, Al, Se in brain samples to study deseases e.g. Alzheimer

Archaeological samplesProvenance studies on Roman ceramics Provenance studies on the jars storing the Dead Sea ScrollsGold in fibres of the royal gown

See the home page for details!

Types of radiochemical NAA• Post-irradiation chemistry (RNAA)

no contamination hazardaddition of carriers – no radiocolloidsyield determinationshielded, remote-controled devicesseparations:

matrix separationgroup separationsingle element separation

• Pre-irradiation chemistry (PC NAA): pre-concentrationcontamination hazard

• Pre- and post-irradiation chemistry (PC RNAA)extremely high sensitivites e.g. 129I determination in environment

• Chemical AA (Ch NAA): separation for speciation purposespre-irradiation (irradiation may change the chemical conditions)

Chemistry is typically simple!

Application of RNAA in material sciences

Separation of the matrix• Analysis of impurities in high purity Al

27Al(n,α)24Na Na separation by HAP

• Analysis of Ni based alloysseparation of Ni by DMG

• Analysis of Mo/W coumpoundsMo and W separation by anion exchangers

Separation of single elements• Si analysis in Mo: 32Si is short-lived, can be counted by beta

detector, Si is separated• P analysis in semiconductors: 32P is pure beta emitter

separation with AMP

Application of RNAA for the analysis of biological, environmental and geological samples

• Matrix removal:matrices are: Na, K, P, Br, Clremoval: HAP TiO2 Al2O3 volatilization

• Single element separations:- Se:toxic/micro-nutrient

separation by extraction orprecipitation of elemental Se with ascorbic acid

- I: essentiel elementseparation: I2 extraction + AgI

- Hg: toxic elementseparation by volatilization orextraction + precipitation as HgI2 or HgS or Hg

- Sr: major interest as natural carrier for 90Srseparation by co-precipitation with Ca

- Th and U: radioactive elementsTh:separation of 233Pa by co-precipitation with MnO2 and BaSO4Th:PC RNAA of Th: pre-conc of Th by ion exchange

separation of 233Pa by ion exchange or extractionU, Th: separation of 239Np and 233Pa by TBP

• Single group separationsPt group elements:Ru, Rh, Pd, Os, Ir, Pt.

history of rocksenvironmental concern: catalysts in cars, medicines- RNAA: OsO4 + RuO4 /CCl4 extraction,

anion exchange of chlorides- PC NAA: fire assay/NiS preconcentration

Rare earth elements (REE): - co-precipitation with ferric hydroxide- others: ion exchange, extraction

• Several elements and various groups separations„historic significancePietra method: 50 elements in biological materials

separation by all types of analytical methods (volatilization, ion exchange, sorption…)

Application of RNAA for the determination of radionuclides• 129I

PC RNAA: volatilization, extraction of I2, precipitation of PdI2

• 237Np• 99Tc

Application of NAA for speciation studies ChNAA• Non-protein bound Al or protein bound Al in urine:

role in osteomalaciaseparation by cation exchange

• Iodine speciation in sea waterseparation by anion exchange

Nem nukleáris elemanalitikai módszerek

Tömegspektrometria: SS MSTI MSICP MSGD MS

Röntgenfluoreszcencia: WD XRFED XRFTR XRFSR XRF

PIXEEPMA

Optikai módszerek: abszorpciós: AAS (láng, lámpa)emissziós: OES (=AES)

ICP OESElektrokémiai módszerek: Voltammetria

CoulombmetriaPolarográfia

Elem-analitikai

módszerekérzékenysége

össze-hasonlítás