Neutron activation analysis (NAA) is a nuclear process used for determining
the concentrations of elements in a vast amount
of materials. NAA relies on excitation by neutrons
so that the treated sample emits gamma-rays. It
allows the precise identification and quantification
of the elements, above all of the trace elements
in the sample.
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 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 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.
Neutron Activation Analysis is very sensitive and is
therefore used to analyze for minor elements, which
are present in very low concentrations. The method is especially useful for trace element analysis,
e.g. in high-purity substances, and is therefore
important in semiconductor techniques.
It can also be used to detect trace element in water, biological
material and minerals. In archaeology, NAA can give useful information
about the origin of the findings according to the so-called “fingerprint” of the
individual element composition in their raw materials. It is usually used as an important
reference for other analysis
methods.
NAA can detect up to 74 elements depending on
the experimental procedure, with minimum detection limits ranging from 10-7 to 10-15g/g, depending
on the elements and matrix materials. Some nuclei can capture a number of neutrons and remain
relatively stable, not undergoing transmutation or decay for many months or
even years. Different nuclei have different
cross sections and half lives, and the intensities of the emitted gamma-rays
can also vary – therefore the detection limits are quite variable. Rare
earth elements (REE) have very high thermal neutron
cross sections and NAA is usually the first choice for
the determination of REEs in a trace elements analysis.
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