The NAA Method
The sequence of events occurring during the most common type of nuclear reaction used for NAA, namely the neutron capture or (n,) reaction, is illustrated in Figure 1.
When a neutron interacts with the target nucleus via a non-elastic collision, a compound nucleus forms in an excited state. The excitation energy of the compound nucleus is due
to the binding energy of the neutron within the nucleus. The compound nucleus will almost instantaneously de-excite into a more stable configuration through emission of one or
more characteristic prompt gamma rays. In many cases, this new configuration yields a
radioactive nucleus which also de-excites (or decays) by emission of one or more characteristic delayed gamma rays, but at a much slower rate according to the unique half-life of the radio-active nucleus. Depending upon the particular radioactive species, half-lives can range from fractions of a second to several years.
In principle, therefore, with respect to the time of measurement, NAA falls into two categories: (1) prompt gamma-ray neutron activation analysis (PGNAA), where measurements take place during irradiation, or (2) delayed gamma-ray neutron activation analysis (DGNAA), where the measurements follow radioactive decay. The latter operational mode is more common; thus, when one mentions NAA it is generally assumed that measurement of the delayed gamma rays is intended. About 70% of the elements have properties suitable for measurement by NAA.
