Neutron Activation Analysis and Thermal Neutron Analysis

Neutron activation analysis (NAA), a nondestructive method of quantitative chemical analysis, is based on equation (13-27). The unknown number of target atoms of an element of interest is NA in Chapter 13 (Section 13.4.2, footnote 19). When a sample is irradiated with slow neutrons, the stable nuclei of some of the constituent elements will become radioactive, usually by the (n, y) reaction. Ideally, the quantity of a radionuclide produced can be determined by quantitative measurement of its characteristic radiation, by y-ray spectroscopy, without the need for chemical separation from radioisotopes of other elements.

NAA has been used for the determination of about 60 chemical elements in concentrations ranging from that of a bulk component to that of a trace component at the parts-per-million to parts-per-billion level. The sensitivity for a trace element depends on the neutron activation cross section of the target nuclide and on the properties of the product radionuclide (i.e., the halflife, and type, energy, and intensity of the radiation emitted).

Examples of determinations of trace concentrations in environmental samples are Cu and Mn in both raw and treated water supplies, Cl in pesticide residues, rare earth elements contained in cracking catalysts used in oil refineries and released into the atmosphere in particulate form, and a system of several elements (e.g., As, Sb, Se, Zn, In, Mn, V) present in atmospheric pollution aerosols and having the relative concentrations characteristic of their sources.

A variation of NAA is based on measurement of the characteristic energies of the prompt y rays emitted by the target nuclei as the neutron capture reactions occur. It is advantgeous for the analysis of H, B, C, N, and Si. One application, known as thermal neutron analysis (TNA), was developed to detect chemical explosives in luggage at airports. Most chemical explosives have a relatively high nitrogen content. When nitrogen is irradiated with slow neutrons, the reaction 14N(n, y)15N releases a 10.8-MeV y ray—an easily detected signature for the reaction. The intensity of the y ray is proportional to the nitrogen concentration. Several TNA detectors were built (using 252 Cf as the neutron source) and were tested with limited success (excessive false alarms). Changes in the design of devices using neutrons may eventually give them a significant security role.

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