Neutron activation analysis (NAA)

Dr. Sajjad Ullah
Institute of Chemical Sciences, University of Peshawar

Introduction
A non-radioactive sample can sometime become radioactive after
bombardment with particle or gamma radiations.
 Activation Analysis: An analytical technique in which radiation is
induced by bombardment
 Photon Activation Analysis: The method which utilizes gamma
rays for bombardment is PAA.
 Neutron Activation Analysis: The method which utilizes neutron
for bombardment is NAA
-lack of charge and the mass of a neutron allows efficient
penetration and energetic transfer to the nucleus.
Dr. Sajjad Ullah, Institute of Chemical Sciences,
University of Peshawar 2

What is Neutron Activation Analysis (NAA)?
• The analytical technique in which radioactive emissions are
monitored from a sample that has been boarded with
neutrons is NAA
• Neutron Activation Analysis (NAA) is a sensitive multi-
element analytical technique used for both qualitative and
quantitative analysis of major, minor, trace and rare elements
• NAA is a 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.
• NAA was discovered in 1936 by George Charles de Hevesy
(Hungary) and Hilde Levi (Denmark)
G. Hevesy
Hilde Levi

“Neutron Activation Analysis is an ultrasensitive
technique used to determine trace amounts of elements
in samples by bombarding them with a high flux of
neutrons, and measuring the rate of decay of the
radioactive elements formed by this bombardment. Each
radioactive element has a specific and well-known decay
rate (or half-life) and by measuring the decay rate, the
element and its concentration can be accurately found.
The process can be used without destroying the original
sample and, indeed, has been used on works of art and
samples of historical significance.”

Principle of NAA

This analytical technique bases its principals on the capturing of
neutrons by nuclei, thereby inducing radioactive emission from the
excited nucleus.
The sample is bombarded with neutrons, causing the elements to form
radioactive isotopes which emits particles (e.g., β particles) and
radiations (such as gamma rays).
β particles emission is energetically continuous where as gamma ray
emission is discrete (so gamma emission often measured preferentially
though measurement of β emission is more sensitive) )
The radioactive emissions and radioactive decay paths for each
element are well known. Using this information, it is possible to study
spectra of the emissions of the radioactive sample, and determine
which radionuclide is there (qualitative analysis) and what is the
concentrations of the elements within it
Principle…cont’d

• Upon irradiation with neutron (irradiation t = one or serval half-lives) , a thermal
neutron (0.025 eV-0.5 eV) interacts with the target nucleus via a non-elastic
collision, causing neutron capture (so A increase by 1).
• The energy imparted to the product nuclide by the neutron = K.E of neutron + BE
of neutron in the produced nucleus. The imparted E excites the nucleus to a
higher energetic level.
• This excited state is unfavorable and the compound nucleus will almost
instantaneously de-excite (relaxes) into a more stable configuration through the
emission of particles and one or more characteristic prompt gamma (ɤ) photons
(see next slide) .
• In most cases, this more stable configuration yields a radioactive nucleus.
• The newly formed radioactive nucleus now decays by the emission of both
particles (α, β, neutron, proton) and one or more characteristic delayed (see next
slide) gamma photons.
• This decay process is at a much slower rate than the initial de-excitation and is
dependent on the unique half-life of the radioactive nucleus.
• These unique half-lives are dependent upon the particular radioactive species and
can range from fractions of a second to several years.
• Once irradiated, the sample is left for a specific decay period (why?), then placed
into a detector, which will measure the nuclear decay according to either the
emitted particles, or more commonly, the emitted gamma rays
Principle (details)…cont’d

Probability of Nuclear Reactions during Neutron Bombardment
Probability α cross-sectional area of target (σ)
Φ = neutron flux (particles/cm2.s), N = no of nuclei in the sample, N* = the no of radionuclide
formed by the reaction during time t, σ = cross-section area of target (cm2), λ= decay constant (s-1)
The net rate (r) at which a target reacts to produce radioactive atoms while being
bombarded on all sides with neutron is given by
Rate of formation of radionuclide
Rate of decay of
Formed radionuclide
=
The no. of radionuclide
(N*) present at the end of
the irradiation is given by:
The activity (A) of radionuclide
produced when the bombardment
is stopped is given by:
This equation shows that
Activity (A) increased as
bombardment time (t)
increases
Typical bombardment times range from one to six half-lives of the produced radionuclideDr. Sajjad Ullah, Institute of Chemical Sciences,

Prompt-gamma neutron activation analysis (PGAA): nuclear decay products (gamma rays
or particles) are measured during neutron irradiation
• PGNAA is characterized by short irradiation times and short decay times, often in the order
of seconds and minutes.
• PGNAA is generally applied to:
elements with extremely high neutron capture cross-sections (B, Cd, Sm, Gd);
elements which decay too rapidly to be measured by DGNAA;
elements that produce only stable isotopes
or elements with weak decay gamma ray intensities.
Delayed gamma neutron activation analysis (DGNAA): nuclear decay products (gamma
rays or particles) are measured at some time after irradiation
• DGNAA is characterized by long irradiation times and long decay times (hours, weeks or
longer)
• DG analyses are often performed over days, weeks or even months.
• DGNAA is applicable to the vast majority of elements that form artificial radioisotopes. This
improves sensitivity for long-lived radionuclides as it allows short-lived radionuclide to
decay, effectively eliminating interference.
• Thus high selectivity.

Forms of NAA
 Destructive ( radiochemical )
- The sample is chemically manipulated after bombardment but before counting
- If chemical separations are done to samples after irradiation to remove interferences or to
concentrate the radioisotope of interest, the technique is called radiochemical neutron
activation analysis (RNAA)
- the resulting radioactive sample may be chemically decomposed, and the elements are
chemically separated (chromatography, ion-exchange, extraction, electrochemical
separation)
 Nondestructive ( instrumental )
- the resulting radioactive sample is kept intact
- It is generally possible to simultaneously measure more than thirty elements in most
sample types without chemical processing.
- The application of purely instrumental procedures is commonly called instrumental
neutron activation analysis (INAA) and is one of NAA's most important advantages over
other analytical techniques.

 Sampling
NAA can perform non-destructive analyses on solids, liquids, suspensions,
slurries, and gases with no or minimal preparation. Due to the penetrating
nature of incident neutrons and resultant gamma rays, the technique
provides a true bulk analysis
 Pre-irradiation sample treatment (such as cleaning, drying or aching,
pre-concentration of elements of interest or elimination of interfering
elements, sub-sampling and packing)
 Irradiation (and prompt gamma-ray counting in PGNAA)
 Radiochemical separation (only in RNAA)
 Radioactivity measurement
 Elemental concentration calculation
NAA procedure

There are several types of neutron sources reactors, accelerators, and
radioisotopic neutron emitters.
Nuclear reactors with their high fluxes of neutrons offer the highest available
sensitivities for most elements.
Most neutron energy distributions are quite broad and consist of three principal
components: Thermal, epithermal, and fast (see later)
Irradiation

Neutron sources
 An An actinide such as californium (252Cf) which emits neutrons through spontaneous
fission
 Neutron generators
 Nuclear reactors
 An alpha source such as radium or americium, mixed with beryllium; this generates
neutrons by a (α,12C+n) reaction
4
9Be + 2
4He  6
12C + 0
1n + Q
 An gamma source mixed with beryllium or deuterium; the gamma radiation induces
neutron emission from Be or Deuterium
4
9Be + ɤ  4
8Be + 0
1n + (-Q)
1
2H + ɤ  1
1H + 0
1n + (-Q)
Q= E emitted for each nucleus of the target during rxn
Exoergic reaction: E is produced (Q= +ve)
Endoergic reaction: E is absorbed (Q= -ve)

Neutrons
 Thermal : 0.025 eV-0.5 eV (NAA): Most often used in NAA
 Epithermal: 0.5 eV-100 keV used in Epithermal NAA (ENAA)
 Fast: 0.5-25 MeV used Fast NAA (FNAA)
 The fast neutron component of the neutron spectrum (energies above 0.5 MeV) consists of the
primary fission neutrons which still have much of their original energy following fission.

Detection
 Irradiated samples are analyzed by gamma-spectrometry

Gamma-ray spectrometer
e
e
Q
Voltage
supply
Germanium
crystal
eh
h
h
e
+
—
display
Multichannel pulse-
height analyzer
gamma
ray
energy
counts

Gamma-ray Spectroscopy
- Detect the gamma-rays (prompt and delayed) with gas detector, scintillators,
semiconductors
- Gamma spectrum is characteristic of the nuclides in the source (or elements
that are activated in NAA)
 Equipment:
 Detector (NaI, HPGe) - voltage pulse
 Amplifiers or multi-channel analyzers -
shape the pulse
 ADCs - collects data, produces spectrum

Qualitative and Quantitative Analysis by NAA
Modern instruments generally monitor the gamma emission from the sample.
Stage 1: Bombardment for a fixed t
Stage 2: Decay for a certain time
Stage 3: Recording the spectrum (with a Ge(Li) detector which detects the
ɤ photons and resolve them into spectral peaks
Qualitative Analysis:
• The E of emitted ɤ-rays is characteristic of the change in nuclear energetic
levels for a particular nuclide.
• Emission occurs at discrete energies which can be used to identify the
emitting nuclide.
• Peak Energy is thus used for qualitative analysis
• In other words, compare ɤ-rays spectrum of sample with those of known
nuclides for qualitative analysis
Quantitative Analysis: Peak Areas is used for qualitative analysis

Basically there are two standardizations methods used in NAA:
 The relative method
 The non-relative method
The Relative Method:
• The Sample and element standard(s) are simultaneously bombarded with neutron and later
measured under the same counting conditions (sample-to-detector distance, sample size,
composition (if possible)).
• Normally a single standard is used for quantitative analysis.
• The peak areas of the sample and the standard are used to calculate the number of nuclei of
a particular type in the sample.
• A direct proportionality can be assumed between the peak area and no. of nuclei when equal
irradiation times and neutron fluxes are used (when sample and standard are simultaneously
bombarded)
• The relative method promises the highest accuracy when the standard and sample match
each other well in composition, irradiation and counting conditions.
Quantitative Analysis: Peak Areas is used for qualitative analysis

Characteristics of INAA
 Non-destructive analysis
 The chemical form and physical state of the elements do not
influence the activation and decay process as the vast majority of
samples are transparent to both the probe (neutron) and the
analytical signal (the gamma ray).
 Multi-element analytical technique
 H, C, O, N, P, and Si (matrix –forming elements) hardly form any
radioactive isotopes (so less matrix effect in INAA)
 Suitable even for determination of masses in the order of 10–6–10–9
g and less
 In certain fields, NAA is irreplaceable (e.g., analysis of solid material
which are difficult to dissolve or when samples have low trace
element concentrations, analysis where high degree of accuracy is
required). Dr. Sajjad Ullah, Institute of Chemical Sciences,

Detection Limits: The detection limit (DL) represents the ability of an analytical
method to determine the minimum amounts of an element reliably (with a
specified precision and reproducibility). In other words, the DL is the lowest
detectable level of analyte distinguishable from zero
Detection limits of INAA
The detection limit depends on:
(1) The amount of material to be irradiated and to be counted
(2) The neutron fluxes
(3) The duration of the irradiation time.
(4) The total induced radioactivity
(5)The duration of the counting time
(6) The detector size, counting geometry
and background shielding
In NAA, the detection limit relates to the ability of detecting a gamma-ray
peak in the presence of interference from natural radioactivity and other
radioactivities induced by neutron activation. A peak is detected when it is
distinguishable from the uncertainty in this noise level. This signal-to-noise
situation makes that the DL is related to the square root of the noise or
background level.

http://archaeometry.missouri.edu/naa_overview.html
Detection limits of INAA
The sensitivity obtained by NAA is a function of nuclear parameters of the
element in question (neutron cross section, isotopic abundance, half-life,
gamma-rays abundance), available neutron flux, Length of irradiation,
detector efficiency, matrix compositions, and the total sample size

• Small sample sizes (0.1 mL or 0.001 gm)
• Non-destructive
• Can analyze multiple element samples
• Often doesn’t need chemical treatment (sample digestion or dissolution)
• High sensitivity, High accuracy (5%), high precision (0.1%) and low
detection limits in the sub-ppm range
• Very low detection limits for 30-40 elements
• Lowest Detection limits can be achieved by optimizing irradiation
parameters (Energy, Fluence rate of neutrons, times of irradiation, counting)
• Significant matrix independence as most sample are transparent to both
neutron and gamma rays
• The possibility of non-destructive analysis (instrumental NAA or INAA),
• The use of radiochemical separation to overcome interference in complex
gamma-ray spectra (radiochemical NAA or RNAA),
• An inherent capability for high levels of accuracy compared to other trace
element analysis techniques.
• It is self-validating: Two or more analytical gamma lines may be used for the
determination of the same elements, allowing a cross-check f the process.
Advantages of NAA

• It is not a quick method and is often time-demanding and obtaining the results
may take up to 2-4 weeks, depending on the half lives of the elements (it may
be longer or shorter).
• Also it cannot preform analysis on certain elements such as oxygen and
carbon.
• NAA only gives information on the total concentration of an element. No
information of compound chemical structure and physical state can be
immediately obtained.
• Interferences may arise if different elements in the sample emit gamma rays of
nearly the same energy (solution: choose alternate gamma rays for these
element or wait for the shorter-lived nuclide to decay before counting is done)
• Less common than other analytical techniques due to necessity of having
access to a nuclear reactor or neutron generator.
Limitations of NAA

https://repository.tudelft.nl/islandora/object/uuid:438b9110-fb94-4015-b277-1c5fba96ac71?collection=research

Applications of INAA
 Environmental studies (migration of pollutants in
ecosystems, air pollution studies using moss-biomonitors)
 Biotechnology for medicine (development of new
pharmaceuticals and sorbents)
 Material Science (high purity materials, nanoparticles
and objects of national heritage)
moss species [Hypnum cupressiforme Hedw. and Scelopodium touretii (Brid.) L. Kock]
Moss

 - Determine the chemical composition of a
sample
 - artifacts, forensics
 - Can identify up to 74 different elements in
gases, liquids, solids, and mixtures
 - Can also determine the concentration of
the elements of interest:

Some elements of interest
 Arsenic
 Chromium
 Selenium
 Chlorine
 Mercury
 Magnesium

Used to Find:
Impurities in industrial products and foods
Poisons in human hair
Hazardous material at dumps
Trace elements in archaeological remains
Testing for elements in air filters

Dependence of counts on distance
between detector and gamma-source
4 5 6 7 8 9 10 11 12 13 14 15 16
3000
4500
6000
7500
9000
10500
12000
13500
15000
16500
18000
19500
21000
22500
24000
152
Eu 121,8keV
countsinpeak(time7min)
distance (cm) 4 5 6 7 8 9 10 11 12 13 14 15 16
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
152
Eu 1408keV
countsinpeak(time7min)
distance (cm)

Neutron Activation Analysis
The following slides illustrate the process of NAA
with a plant sample to determine the amount of a
particular metal its tissues contain (e.g., arsenic).
(An example)

1. Obtain plant sample.
Wash the sample to remove possible contaminants.

2. Dehydrate the sample
 Dehydration methods use
heat or freeze-drying.
 A lyophilizer is frequently
used for the freeze-dry
method.
 A plant sample is then
placed in the chamber
atop the lyophilizer to the
right.

3. The dehydrated sample is then prepared for testing.
A small amount of dehydrated plant material is removed
from this bag.

4. A portion of the dried plant material is ground into a
powder using a mortar and pestle. Sterile technique is
required to avoid cross contamination.

5. Bag the sample.
Some of the ground plant powder is bagged into a small
plastic envelope. The sample is double-bagged and labeled.

6. Select a standard for comparison.
 When looking for arsenic in
plant material, you would
need to prepare a sample of a
standard containing arsenic.
 The “standard” contains a
known quantity of the
element you are looking for.
 Containers of certified
standards are pictured.

7. Place packages of both the prepared sample and
standard sample in a capsule.

8. Take sample to the rabbit system apparatus.
 The rabbit system works much
like the system used by banks at
drive-through windows. A
canister carries items back and
forth between the customer and
teller.
 The sample is sent through the
wall in a mini canister into the
nuclear reactor located behind
the wall.
 Once inside the reactor, the
sample is irradiated with
neutrons.

9. After irradiation of the sample in the capsule, and before removing
it from the reactor site, it must be determined if the capsule is safe
for transfer. A Geiger counter is used to assess whether the
radioactive decay has reached low enough levels to be safe.

10. The prepared sample and standard sample are
placed in a “detector” one at a time.
 The detector system
counts and records
gamma radiation
emissions for a period of
time.
 Time varies, but is usually
in the range of 5 minutes
to an hour.

11. Counts recorded by the detector system is
sent to a computer.

12. Specialized software analyzes radiation peaks.
Peak data is correlated to specific elements for
identification and quantification.

13. Computer data is compared to a nuclide chart
to evaluate the results.

Recommended article
Article Title: Some applications of neutron activation
analysis: A review
Authors: E. Witkowska, K. Szezepaniak, M. Biziuk
Reference Source: Journal of Radioanalytical and Nuclear
Chemistry, volume 265, no. 1, page 141-151

References
 http://archaeometry.missouri.edu/naa_overview.html
 Introduction to Instrumental Analysis by Robert D. Braun
 E. Witkowska, K. Szezepaniak, M. Biziuk, Some applications of
neutron activation analysis: A review, Journal of Radioanalytical and
Nuclear Chemistry, volume 265, no. 1, page 141-151
 P. Bode: Instrumental and organizational aspects of a neutron
activation analysis laboratoty
 http://reactor.engr.wisc.edu/naa/
 http://tin.er.usgs.gov/geochem/doc/inaa.htm
 http://reactor.engr.wisc.edu/naa/UWNRNAA.htm
 http://www.reak.bme.hu/Wigner_Course/WignerManuals/Budapest/N
EUTRON_ACTIVATION_ANALYSIS.htm

Neutron activation analysis (NAA)

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