Transcript Neutron Activation Analysis

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, instrumentalradiochemical)
- 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

PROMPT
GAMMA RAYS
COMPOUND
NUCLIDE

DECAY
GAMMA RAYS
RADIOACTIVE
NUCLE US
INCIDENT
NEUTRON
CHARGED
PARTICLE
PHOTON
X
TARGET
NUCLIDE


STAB LE
NUCLE US
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.
2.
3.
4.
Thermal neutron activation analysis
Epihermal neutron activation analysis (ENAA)
Fast neutron activation analysis (FNAA)
Neutron capture prompt gamma activation
analysis (PGAA)
Neutron sources used for NAA
Isotopic neutron sources:
 -em itter
H alf life
N eu tron s
average
s -1 C i -1
n eu tron
em itted
en ergy [M eV ]
227 A c
22 y
1.5x10 7
4
226 R a
1620 y
1.3x10 7
3.6
239 P u
2.4x10 4 y
1.4x10 7
4.5
210 P o
138 d
2.5x10 6
4.3
Spontaneous fission of actinides:
252Cf
(half life 2.6y): 3.76 neutrons of 1.5 MeV per event
1mg 252Cf emits 2.28x109 neutrons/sec
Neutron 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 MW
thermal neutron flux: 1012-1014 neutrons cm-2 s-1
thermal
epithermal +resonance
<0.05 eV
0.1eV<E<1 eV
1eV<E<1 keV
mean:0.04 eV
2200 m/s
(n,γ)
(n,γ)
cold neutron beam
fast neutrons
0.5 Mev<E
(n,p),(n,α),(n,2n)
Measuring systems used for NAA
Gamma spectrometers:
scintillation detector
Ge(Li) detector
HP Ge detector
Quantification

A∞=N R  N  s ( E )   ( E ) dE
0
N: number of interacting isotopes
s (E): cross-section [cm2] at neutron energy of E [eV]
f (E): neutron flux per unit of energy interval [1/cm2/s/eV]
R: reaction rate
In reactors the integral is replaced:
R   th s th   epi I 0
sth: conventional thermal neutron flux [in cm2]
fth : effective thermal neutron cross-section [in cm2]
fe: 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]
Activity of the nuclide at the time of measurement:

A = (  th  s th   e  I o )
NAv:
fi :
m:
Arel :
ti :
td :
l:
S:
D:
m  f i  N Av
S D
Arel
Avogadro number
isotopic abundance
the mass of the irradiated element
atomic mass of target element
time of irradiation;
time of decay;
decay constant
saturation factor: S=1-e-lti
decay factor: D=e-ltd
Activity is determined from measurements:
N P  A  f     tm
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:
m 
N pM
N Av f i f    (  th s th   e I 0 ) SDt
m
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 method
Based 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
• Relative method
Each element is measured
against an „element standard”
irradiated together with the
sample
I
m 
I sp
Np
I 
SDt
I sp 
m
N sp
S sp D sp t m , sp m sp
Parameters not used:
ε, φth, φe, σth, I0, fγ, fi, NAv, M
• Comparator method: „k” method
All 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
N p'
k 
I sp
I
*
sp

S ' D 't m ' m '
N p*
S * D *t m * m *
Np
m 
I
**
I sp k

SDt
m
N p ** k
S ** D ** t m ** m **
k factors are constant under constant irradiation and
measurement conditions
(including the same geometry):
k 
M * f  f i  ( th s th   e I 0 )
Mf  * f i * * ( th s th *   e I 0 * )

M * f  f i s th ( th /  e  I 0 / s th )
Mf  * f i * *s th * ( th /  e  I 0 * / s th * )
• the k0 method
A pure nuclear constant k0 was derived from k factor by deCorte,
that is independent of measuring and irradiation conditions:
k0 
M * f  f is
M f  * f i *s *
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:
k0 
I sp  * ( th /  e  I 0 * / s th * )
I sp * 
( th /  e  I 0 / s th )
Properties of INAA
Advantages
• Sensitive, trace elements are determined
• Multi-element method
• Matrix dependence is often small
• Non-destructive
Disadvantages
• Neutron source and gamma spectrometer are
needed
Expensive and „nuclear” method
Application of INAA
Geological 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
condrites
volcanic activities
Biological samples
- Analysis of Na, K, Al, Se in brain samples to study deseases e.g.
Alzheimer
Archaeological samples
Provenance studies on Roman ceramics
Provenance studies on the jars storing the Dead Sea Scrolls
Gold in fibres of the royal gown
See the home page for details!
Types of radiochemical NAA
• Post-irradiation chemistry (RNAA)
no contamination hazard
addition of carriers – no radiocolloids
yield determination
shielded, remote-controled devices
separations:
matrix separation
group separation
single element separation
• Pre-irradiation chemistry (PC NAA): pre-concentration
contamination hazard
• Pre- and post-irradiation chemistry (PC RNAA)
extremely high sensitivites e.g. 129I determination in environment
• Chemical AA (Ch NAA): separation for speciation purposes
pre-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 alloys
separation of Ni by DMG
• Analysis of Mo/W coumpounds
Mo 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,
removal:
HAP
K,
TiO2
P,
Al2O3
Br, Cl
volatilization
• Single element separations:
- Se:toxic/micro-nutrient
separation by extraction or
precipitation of elemental Se with ascorbic acid
- I: essentiel element
separation: I2 extraction + AgI
- Hg: toxic element
separation by volatilization or
extraction + precipitation as HgI2 or HgS or Hg
- Sr: major interest as natural carrier for 90Sr
separation by co-precipitation with Ca
- Th and U: radioactive elements
Th:separation of 233Pa by co-precipitation with MnO2 and BaSO4
Th:PC RNAA of Th: pre-conc of Th by ion exchange
separation of 233Pa by ion exchange or extraction
U, Th: separation of 239Np and 233Pa by TBP
• Single group separations
Pt group elements:Ru, Rh, Pd, Os, Ir, Pt.
history of rocks
environmental 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 significance
Pietra method: 50 elements in biological materials
separation by all types of analytical methods
ion exchange, sorption…)
(volatilization,
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 osteomalacia
separation by cation exchange
Iodine speciation in sea water
separation by anion exchange
Nem nukleáris elemanalitikai módszerek
Tömegspektrometria:
Röntgenfluoreszcencia:
PIXE
EPMA
Optikai módszerek:
SS MS
TI MS
ICP MS
GD MS
WD XRF
ED XRF
TR XRF
SR XRF
abszorpciós:
emissziós:
Elektrokémiai módszerek: Voltammetria
Coulombmetria
Polarográfia
AAS (láng, lámpa)
OES (=AES)
ICP OES
Elemanalitikai
módszerek
érzékenysége
összehasonlítás