2. What is NAA?
Neutron activation analysis (NAA) is a nuclear process used for
determining the concentrations of elements in a vast amount of
materials. NAA allows individual sampling of elements as it
disregards the chemical form of a sample, and focuses exclusively on
its nucleus. The method is based on neutron activation and therefore
requires a source of neutrons. The sample is bombarded with
neutrons, causing the elements to form radioactive isotopes. The
radioactive emission 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 the
concentrations of the elements within it. A particular advantage of
this technique is that it does not destroy the sample, and thus has
been used for analysis of works of art and historical artifacts. NAA
can also be used to determine the activity of a radioactive sample.
3. Radioactive isotope
Radioactive isotope, also called radioisotope, any
of several species of the samechemical element with
different masses whose nuclei are unstable and
dissipate excess energy by spontaneously
emitting radiation in the form of alpha, beta, and
gamma rays.
4. Overview
Neutron activation analysis is a sensitive multi-element
analytical technique used for both qualitative and quantitative
analysis of major, minor, trace and rare elements. NAA was
discovered in 1936 by Hevesy and Levi, who found that
samples containing certain rare earth elements became highly
radioactive after exposure to a source of neutrons. This
observation led to the use of induced radioactivity for the
identification of elements. NAA is significantly different from
other spectroscopic analytical techniques in that it is based not
on electronic transitions but on nuclear transitions.
5. Basic theory and principle
To carry out an NAA analysis the specimen is placed into a
suitable irradiation facility and bombarded with neutrons, this
creates artificial radioisotopes of the elements present.
Following irradiation the artificial radioisotopes decay via the
emission of particles or more importantly gamma rays, which
are characteristic of the element from which they were emitted.
6. Basic procedure
First the sample must be selected carefully. More commonly a small sample
must be taken. About 50 mg is a sufficient sample.
The sample is then encapsulated in a vial made of either high purity linear
polyethylene or quartz.
The sample and a standard are then packaged and irradiated in a suitable
reactor at a constant, known neutron flux
A typical reactor used for activation uses uranium fission, providing a high
neutron flux and the highest available sensitivities for most elements. The
neutron flux from such a reactor is in the order of 1012 neutrons cm−2 s−1.
The type of neutrons generated are of relatively low kinetic energy(KE),
typically less than 0.5 eV. These neutrons are termed thermal neutrons.
Upon irradiation a thermal neutron interacts with the target nucleus via a
non-elastic collision, causing neutron capture.
7. Contd.
This collision forms a compound nucleus which is in an excited state.
This excited state is unfavourable and the compound nucleus will
almost instantaneously de-excite (transmutate) into a more stable
configuration through the emission of a prompt particle and one or
more characteristic prompt gamma photons.
The newly formed radioactive nucleus now decays by the emission of
both particles and one or more characteristic delayed gamma photons.
This decay process is at a much slower rate than the initial de-
excitation and is dependent on 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.
8. contd.
Once irradiated the sample is left for a specific decay period then placed
into a detector, which will measure the nuclear decay according to either the
emitted particles, or more commonly the emitted gamma rays .
9.
10.
11. Neutron sources
A range of different sources can be used:
A nuclear reactor
An actinoid such as californium which emits neutrons
through spontaneous fission
An alpha source such as radium or americium, mixed with
beryllium; this generates neutrons by a (α,12C+n) reaction
A D-T fusion reaction in a gas discharge tube.
12. Nuclear reactor
Some reactors are used for the neutron irradiation of
samples for radioisotope production for a range of
purposes. The sample can be placed in an irradiation
container which is then placed in the reactor.
13. fusors
A relatively simple Farnsworth-Hirsch fusor can be
used to generate neutrons for NAA experiments. The
advantages of this kind of apparatus is that it is
compact, often bench top-sized, and that it can simply
be turned off and on. A disadvantage is that this type
of source will not produce the neutron flux that can be
obtained using a reactor.
14. Isotope sources
For many workers in the field a reactor is an item
which is too expensive, instead it is common to use a
neutron source which uses a combination of an alpha
emitter and berylium. These sources tend to be much
weaker than reactors.
15. Gas discharge tube
These can be used to create pulses of neutrons, they
have been used for some activation work where the
decay of the target isotope is very rapid. For instance in
oil wells.
16. Detector
There are a number of detector types and configurations used
in NAA. Most are designed to detect the emitted gamma
radiation. The most common types of gamma detectors
encountered in NAA are the gas-ionisation type, scintillation
type and the semiconductor type.
Of these the scintillation and semiconductor type are the most
widely employed.
There are two detector based on their physical structure , they
are the planar detector, used for PGNAA ( Prompt Gamma ray
NAA) and the well detector, used for DGNAA ( Delayed
Gamma ray NAA).
17. The planar detector has a flat, large collection surface
area and can be placed close to the sample.
The well detector ‘surrounds’ the sample with a large
collection surface area.
18. Scintillation-type detectors
use a radiation-sensitive crystal, most commonly
thallium-doped sodium iodide (NaI(Tl)), which emits
light when struck by gamma photons.
These detectors have excellent sensitivity and stability,
and a reasonable resolution.
19. Semiconductor detectors
Utilise the semiconducting element germanium. The
germanium is processed to form a p-i-n (positive-intrinsic-
negative) diode, and when cooled to ~77 k by liquid nitrogen to
reduce dark current and detector noise, produces a signal
which is proportional to the photon energy of the incoming
radiation.
There are two types of germanium detector, the lithium-drifted
germanium or Ge(Li) (pronounced ‘jelly’), and the high-purity
germanium or HPGe.
20. Particle detectors
Can also be used to detect the emission of alpha (α) and beta
(β) particles which often accompany the emission of a gamma
photon but are less favorable, as these particles are only
emitted from the surface of the sample and are often absorbed
or attenuated by atmospheric gases requiring expensive vaccum
conditions to be effectively detected. Whereas gamma rays are
not absorbed or attenuated by atmospheric gases, and can also
escape from deep within the sample with minimal absorption.
21. Analytical capabilities
NAA can detect up to 74 elements depending upon the
experimental procedure. With minimum detection limits
ranging from 0.1 to 1x106 ng g−1 depending on element
under investigation. Heavier elements have larger nuclei,
therefore they have a larger neutron capture cross-section
and are more likely to be activated. Some nuclei can
capture a number of neutrons and remain relatively stable,
not undergoing transmutation or decay for many months
or even years. Other nuclei decay instantaneously or form
only stable isotopes and can only be identified by PGNAA.
22. Conclusion
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. As different
radioisotopes have different half-lives, counting can be delayed to allow
interfering species to decay eliminating interference. Until the
introduction of ICP-AES and PIXE NAA was the standard analytical
method for performing multi-element analyses with minimum
detection limits in the sub-ppm range. Accuracy of NAA is in the
region of 5%, and relative precision is often better than 0.1% .There are
two noteworthy drawbacks to the use of NAA; even though the
technique is essentially non-destructive the irradiated sample will
remain radioactive for many years after the initial analysis, requiring
handling and disposal protocols for low-level to medium-level
radioactive material; also the number of suitable activation nuclear
reactors is declining, with a lack of irradiation facilities the technique
has declined in popularity and become more expensive.
