In neutron activation analysis (NAA), the sample is exposed to a source of neutrons. The bombardment by neutrons causes 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. NAA is significantly different from other spectroscopic analytical techniques in that it is based not on electronic transitions but on nuclear transitions.
There are several types of neutron sources (reactors, accelerators, and radioisotopic neutron emitters), but nuclear reactors with their high fluxes of neutrons from uranium fission are most commonly used. Different types of reactors and different positions within a reactor can vary considerably with regard to their neutron energy distributions and fluxes. Most neutron energy distributions are quite broad and consist of three principal components: thermal (low-energy), epithermal (moderate energy) and fast (high energy).
A major experimental parameter is whether nuclear decay products (gamma rays or particles) are measured during neutron irradiation (prompt gamma, PGNAA), or at some time after irradiation (delayed gamma, DGNAA).
If NAA is conducted directly on irradiated samples it is termed Instrumental Neutron Activation Analysis (INAA). In some cases irradiated samples are subjected to chemical separation to remove interfering species or to concentrate the radioisotope of interest; this technique is known as Radiochemical Neutron Activation Analysis (RNAA).
Dinescu et al. (2003) compared data obtained using epithermal neutrons (produced by a pulsed reactor) with data obtained using thermal neutrons (produced by a continuous flux reactor).
An additional class of methods is specific to uranium and thorium, and relies on nuclear fission.
Fire assay – neutron activation has been widely used for gold analysis, and can be considered to be a category of INAA/DGNAA.
Dinescu, L.C., Culicov, O.A., Duliu, O.G., Frontasyeva, M.V., Oprea, C.D. (2003) Intercomparison between neutron activation analysis laboratories for trace elements determination in lacustrine sediments. Journal of trace and microprobe techniques v21, no.4, p665-676.
Garrett, R.G., Lynch, J.J. (1976) A comparison of neutron activation delayed neutron counting versus fluorometric analysis in large scale geochemical exploration for uranium. In Exploration for uranium ore deposits. Proceedings of a symposium on exploration of uranium ore deposits, Vienna 1976. International Atomic Energy Agency, Vienna, p321-334.
Category: Analytical techniques
The different kinds of analytical techniques have been classified hierarchically. Examples: “AAS”, “FA-NA”. Taken from the “Analytical_Techniques_SHARED” database table.
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