NEUTRON ACTIVATION
ANALYSIS
Monica Sisti
Università and INFN
MilanoBicocca
The technique
Neutron activation analysis (NAA) is a very sensitive method for
qualitative and quantitative determination of elements based on
the measurement of characteristic radiation from radionuclides
formed directly or indirectly by neutron irradiation of the material.
● Multi-element capability
● Sensitivity for many elements
The principle is very simple:
to be measured
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Brief history
● After the discovery of the neutron by
Chadwick in 1932, neutron activation was
first suggested by G. Hevesy and H. Levi in
1936, using a neutron source (226Ra+Be) to
measure activated Dy atoms.
● In the first decade of activation analysis, many worked on the measurement
of fundamental data of radionuclides, using GM counters and ionization
chambers as major instruments.
● In the 1940s, research reactors became an available source of neutrons
increasing the fluxes at one's disposal of at least six orders of magnitude.
● The availability of scintillation detectors in the 1950s, the development of
semiconductor detectors and multichannel analyzers in the 1960s, and the
advent of computers and relevant software in the 1970s, made the nuclear
technique an important analytical tool for the determination of many
elements at trace level.
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Basic principles of NAA
● A bombarding particle is
absorbed by an atomic nucleus
after a nuclear reaction.
● A compound nucleus is formed
(highly excited, unstable
nucleus).
● The compound nucleus
de-excites, usually by ejecting
a small particle and a product
Most common type of nuclear reaction for NAA nucleus.
Prompt radiation emitted ~10-14 s
after neutron capture. The particle may be an elementary
particle (neutron, electron, proton),
Prompt-gamma Analysis (PGA): an alpha particle or a photon.
The product nucleus may be stable
measurement of gamma-rays during or radioactive.
de-excitation of the compound
nucleus after neutron capture.
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Basic principles of NAA
● A bombarding particle is
absorbed by an atomic nucleus
after a nuclear reaction.
● A compound nucleus is formed
(highly excited, unstable
nucleus).
● The compound nucleus
de-excites, usually by ejecting
a small particle and a product
Most common type of nuclear reaction for NAA nucleus.
Delayed-gamma Neutron Activation Analysis (DGNAA):
measurement of gamma-rays emitted during the decay of
the product nucleus after the capture reaction is stopped.
It is the commonly employed method in NAA.
It is useful for many types of elements that
produce radioactive nuclei. The measuring
time and sensitivity depend on decay half-life.
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Types of NAA
There exist many classifications according to the involved chemistry,
to the energy of incoming neutrons, to the way the irradiation is
performed (e.g. cyclic irradiations).
We consider two broad categories:
Destructive or Radiochemical NAA (RNAA):
A method of NAA in which chemical separations are applied after the
irradiation to separate activities of interest from interfering activities.
Non-destructive or Instrumental NAA (INAA):
The most widely applied method of NAA, in which no chemical
procedures are applied before or after the irradiation. The selectivity
of activities of interest is accomplished by the measurement after
different decay times and by the use of special radiation detectors.
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Neutron sources
Radioisotopic neutron sources:
● Two component neutron source based on (α,n) or (γ,n) reactions,
like 241Am(Be), 124Sb(Be), ...
● Spontaneous fission sources, like 252Cf.
→ different energy spectra and rates depending on the involved
reaction
Neutron generators:
● 2.4 MeV neutrons from D(d,n)3He
● 14 MeV neutrons D(t,n)4He
Spallation neutron sources:
Heavy elements such as W, Pb, U irradiated with high-energy protons
or other particles are spalled into two or more fragments and many
neutrons are released.
Nuclear research reactors:
mostly used
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Nuclear research reactors
Owing to the high neutron flux, nuclear research reactors operating in
the power region of 20 kW -10 MW, with maximum thermal neutron
fluxes of 1011 - 1014 neutrons cm-2 s-1 are the most efficient neutron
sources for high sensitivity activation analysis induced by epithermal
and thermal neutrons.
Neutron energy distribution in a light-water moderated research reactor
φ(E)
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Nuclear research reactors
Owing to the high neutron flux, nuclear research reactors operating in
the power region of 20 kW -10 MW, with maximum thermal neutron
fluxes of 1011 - 1014 neutrons cm-2 s-1 are the most efficient neutron
sources for high sensitivity activation analysis induced by epithermal
and thermal neutrons.
Activation via (n, γ) reactions
σ(E)
Neutron capture cross section vs Energy for major actinides
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Activation rate
The number of radioisotopes that each second are created by
neutron-induced reactions (the activation rate R) is related to the
amount (N) of the original, stable isotope in the sample:
Usually one defines an effective cross section,
which gives the mean value of the cross section
weighted for the neutron energy distribution: it
is different in the various irradiation facilities.
The activation rate is then simply related to the integral neutron flux:
Monte Carlo simulations like MCNP may be used for the numerical calculation of the
effective cross section [e.g. D.Chiesa et al., Ann. Nucl. Energy 85 (2015) 925]
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Typical applications of NAA
● Archeology: amber, bone, ceramics, coins, glasses, jewellery, metal
artefacts and sculptures, mortars, paintings, pigments, pottery, raw
materials, soils and clays, stone artefacts and sculptures, …
● Biomedicine: animal and human tissues activable tracers, bile, blood and
blood components, bone, brain cell components and other tissues, breast
tissue, cancerous tissues, ...
● Environmental: aerosols, atmospheric particulates (size fractionated),
dust, fossil fuels and their ashes, soils, sediments, tobacco and tobacco
smoke, surface and ground waters, volcanic gases, …
● Forensics: bomb debris, bullet lead, explosives detection, glass
fragments, paint, hair, …
● Geology and geochemistry: asbestos, crude oils, kerosene, petroleum,
rocks, sediments, soils, …
● Industrial products
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Application to low background experiments
NAA can achieve substantially greater sensitivity than direct
γ-ray counting. It can be applied to measure natural contaminant
(232Th, 238U, 40K ) concentrations in detector materials with no
long-lived neutron activation products.
● For natural decay chains (232Th, 238U) it is complementary to γ-ray
counting, since it measures only parent nuclide concentrations
(as ICP-MS) → no infos on secular equilibrium breaks.
● For 40K, NAA reaches far greater sensitivities than all other
techniques (ICP-MS has low sensitivity to 40K because of
interferences, mainly 40Ar).
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Key ingredients for NAA
X+n
A A+1
X
cascade
β-
Three key ingredients:
● High neutron flux
● High enough neutron capture cross section
● “Convenient” daughter nucleus (γ emission, half-life time)
Sensitivity depends on:
● type pf material (short-lived activation products)
● neutron exposure time
● interferences in the matrix
● background in the region of the gamma emission
● care in the sample preparation
is extremely important!
● the radiopurity of the sample
container is also of concern!
tirr twait tmeas
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NAA for 40K, 232Th, 238U
● 41
K isotopic abundance is 6.7%
β-
● 40
K isotopic abundance is 0.01%
n+ K 41 42
K 42
Ca
12.36 h 40
K contamination is calculated from 41K one
n + 238U 239
U
The material of the sample
β -
23.5 m container should not form
β- long-lived radioisotopes during
239
Np 239
Pu neutron irradiation: too long
2.36 d cooling times after the irradiation
may prevent measuring shorter
living nuclides, like 42K.
n + 232Th 233
Th
β- 22.3 m
β-
233
Pa 233
U
27.0 d
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NAA for 40K
● 41
K isotopic abundance is 6.7%
β-
● 40
K isotopic abundance is 0.01%
n+ K 41 42
K 42
Ca
12.36 h 40
K contamination is calculated from 41K one
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NAA for 238U
n + 238U 239
U
β- 23.5 m
β-
239
Np 239
Pu
2.36 d
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NAA for 232Th
n + 232Th 233
Th
β- 22.3 m
β-
233
Pa 233
U
27.0 d
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Concentration of trace elements evaluation
During the irradiation, the time evolution of the production of the
activated isotope (with decay constant λ) in the irradiated sample is:
At the end of the irradiation, the number of activated nuclei is:
The amount (N) of the original, stable isotope in the sample is
then calculated via the counts measured with HPGe detectors in
the gamma peaks following the decays of the activated isotope :
Two HPGe detectors at the
Radioactivity Laboratories
of INFN Milano-Bicocca
Monica Sisti - LRT 2019 GeGEM εrel30% BeGE detectorεrel50% 18
The relative method – irradiation standards
To calculate the amount (N) of the original, stable isotope in the sample
we should know precisely ΦTOT and σeff in every position of the reactor
and for every irradiation campaign:
with
To avoid this, one usually uses irradiation standards, containing the
same elements to be traced in the sample with a known amount.
N is thus obtained by comparing ndec for standards and sample
The irradiation standards
are irradiated together with
the sample in the same
irradiation channels.
When multi-element searches are performed, e.g.
in environmental samples, the k0-comparator
method (non-relative method) is used to reduce
the number of irradiation standards.
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HPGe measurement efficiency
To evaluate ndec from gamma-ray spectroscopy with HPGe
detectors, the detection efficiency must be known. This is best
achieved through MonteCarlo simulations of each experimental
configuration (sample-HPGe):
where Cmeas and Csim are the
gamma-ray peaks' counts for the
measured and simulated spectra
with nsim simulated decays for
each isotope of interest.
Example of a reconstructed experimental
configuration with a GEANT4 MonteCarlo
simulation.
Monica Sisti - LRT 2019 A.Borio di Tigliole et al. Prog. Nucl. Energy 70 (2014) 249 20
NAA procedure
A neutron activation campaign may involve some or all of the following steps:
● Sample preparation
→ cut to fit in irradiation container, cleaning, packing
(eventual pre-treatment)
in ultra-trace measurements, extreme care is
needed to avoid adding unwanted contaminants
during this step
● Irradiation / Activation at the nuclear reactor
● Radiochemical separation (only in RNAA) Clean room preparation of
samples at Milano-Bicocca
● Activity measurements by HPGe detectors
● Elemental concentration calculation
Radiolysis during neutron irradiation
must be taken into account!
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Examples of achievable sensitivities with NAA
Neutron irradiation:
TRIGA Mark II
research reactor
(250 kW) - Pavia, Italy
Sample preparation
and
HPGe measurements
2x GMX detector
at ●
Coaxial detector (n-type)
Milano-Bicocca ●
Relative efficiency: 100%
GMX2019 GMX2200 ●
Ultra Low Background
configuration
●
Low Threshold ( 20 keV)
●
Muon veto
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Examples of achievable sensitivities with NAA
Neutron irradiation:
TRIGA Mark II
research reactor
(250 kW) - Pavia, Italy
Sample preparation
and
HPGe measurements
at Sensitivity achieved on Acrylic @ INFN Milano-Bicocca
PRELIMINARY RESULT
Milano-Bicocca
@ 90% C.L.
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Examples of achievable sensitivities with NAA
Neutron irradiation:
TRIGA Mark II
research reactor
(250 kW) - Pavia, Italy
Sample preparation
and RNAA: in this case 233Pa was chemically
HPGe measurements separated using an Actinide Resin
at
Milano-Bicocca Sensitivity achieved on Copper @ INFN Milano-Bicocca
using RNAA:
@ 90% C.L.
M.Clemenza et al., LRT 2010, AIP Conf. Proc. 1338 (2011) 37
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Examples of achievable sensitivities with NAA
[1] J. Boger et al., Nucl. Instr. and Meth. A 449 (2000) 172
[2] R.v. Hentig et al., Nucl. Phys. B (Proc. Suppl.) 78 (1999) 115
and many other [3] Z. Djurcic et al., Nucl. Instr. and Meth. A 507 (2003) 680
[4] D.S. Leonard et al., Nucl. Instr. and Meth. A 591 (2008) 490
materials in [5] D.S. Leonard et al., Nucl. Instr. and Meth. A 871 (2017) 169
these papers [6] N. Abgrall et al., Nucl. Instr. and Meth. A 828 (2016) 22
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How to improve sensitivities?
HPGe background and, more severely, interfering isotopes in the
matrix that are activated during the neutron irradiation (examples
are 24Na and 82Br) may spoil the achievable sensitivity because of
the Compton tails of the main peaks.
One possibility to reduce that is to profit from decay coincidences.
coincidences
X+n
A A+1
X
cascade
β-
β-ϒ coincidence detector
See M. Nastasi's talk later today
INFN Milano-Bicocca
Radioactivity Laboratory
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Conclusions
NAA is a very powerful analytical technique complementary to
other assay methods for material selection and screening in
low background experiments.
Trace elements analysis requires careful preparation of the
irradiation campaign and of the test samples in order to reach
sub-ppt sensitivities.
Chemical treatments and/or coincidence spectroscopy may
help increasing the achievable sensitivities.
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