Neutron Activation Analysis

Neutron activation analysis is a physical tool by which various elements may be quantitatively determined, often with an extreme degree of sensitivity. Hitherto, factors of cost and distance involved in securing a neutron source have apparently precluded extensive biologic usage but, with the more recent availability of smaller nuclear reactors, more extensive biologic investigation may be predicted. The basic principles of neutron activation analysis might be briefly elucidated (1). A specimen, when placed in a reactor, is subjected to bombardment by thermal neutrons. The following scheme illustrates the resultant radiative capture (n, gamma) reaction: As an example, , an unstable isotope, then decays according to its usual scheme and, by virtue of its disintegrations, may be detected and quantitated. Thus the method is a twofold procedure; first, one induces radioactivity in a specimen by exposure to a neutron flux, and then the induced radioactivity is analyzed by the application of counting procedures to the radioactive emissions. The amount of produced radioactivity may be calculated and is proportional to various physical characteristics of the involved element. There is a direct proportionality to the neutron flux of the source, the neutron capture cross section of the target nuclide, the mass of the isotope precursor irradiated, and the duration of irradiation. An absolute calculation for the produced activity may be inferred from the following formula: where A is activity in microcuries; W is weight of the particular element, in grams; n is the neutron flux; σAct is the neutron capture cross section in barns; f is the fractional precursor abundance; 1 − e−λt is the saturation factor for the produced nuclide; and At is the atomic weight of the element. By algebraic manipulation of this equation, the weight of the unknown element can be calculated on an absolute basis, assuming the activity is known. Exact determination of neutron flux proves difficult, however, and in practice it is more practical to irradiate a weighed standard at the same time, thus obviating several problems. No longer need the exact flux be known, nor the activity calibrated in microcuries; instead, a formula for relative counting is applied, and standard and unknown are counted in sequence, under the same conditions. Decay corrections are also circumvented. An obvious shortcoming of this technic may be immediately noted; neither hydrogen, carbon, nitrogen, oxygen, sulfur, nor iron can be analyzed, because of the failure to produce countable quantities of radioisotopes from these elements. Material suitable for such determinations is nonetheless wide, and is, in most instances quite sensitive (see page 857). The calculations are based upon a neutron flux of 2.5 × 108 (as obtainable in the AGN-201 nuclear reactor) and are expressed in grams.