As mentioned previously, during the activation by neutrons, a (n, Y) reaction takes place. This reaction produces radionuclides that can also emit gamma photons. This means that two gamma photons are formed as expressed by the following scheme:

Target nuclide (n, Y) radioactive nuclide! radiation of gamma photon.

NAA (see Section 10.2.2.1) provides analytical information using the gamma photons irradiated by the radioactive nuclide. The gamma photons formed in the nuclear reaction itself are called “prompt gamma photons” because they are formed within 1014 s after neutron capture. PGAA detects these gamma photons and pro­vides analytical information independent of whether the product nuclide is stable or radioactive. Similar to NAA, the energy of gamma photons gives qualitative infor­mation, while the activity or intensity provides quantitative information. The kinet­ics of activation is also the same.

The prompt gamma photons can be detected only during irradiation, i. e., the excited nucleus formed by neutron capture, called a “compound nucleus” (see Section 6.1) emits gamma photons. The excitation energy is in the range of
the binding energy of neutrons (7—8 MeV). The de-excitation (i. e., the emission of gamma photons) can happen in one step or in several steps, emitting one gamma photon with high energy or a cascade of gamma photons with low energies. For this reason, the prompt gamma spectra are rather complicated.

PGAA has the same advantageous properties as NAA. In addition, PGAA can detect all elements because prompt gamma photons as emitted photons are pro­duced in every (n, Y) reaction. (In NAA, only radioactive product nuclides with suitable half-lives can be measured after irradiation.) Thus, PGAA is especially important in the analysis of light elements (such as H, B, and N; see Table 10.4). Because of its high sensitivity, it is a very useful method to use in the analysis of elements in tracer quantities (Cd, Hg, etc.). The disadvantage of this method, how­ever, is that the gamma photons have to be measured directly in the neutron beam. This results in high background radiation. In addition, to detect the prompt gamma photons, the neutron beam has to exit the nuclear reactor, which significantly decreases the neutron flux. Since gamma photons can be produced as cascades of photons with low energy, the gamma spectra are usually very complicated and require special evaluation procedures. At the same time, PGAA is an expensive method, which restricts its widespread application.

As seen in Figure 6.4, the cross section of the nuclear reaction with neutrons is inversely proportional to the neutron energy. Thus, by decreasing the energy of the neutrons (i. e., using cold neutrons (see Section 5.5.3)), the cross section can increase by as much as two orders of magnitude. This increases the number of nuclear reactions, improving the sensitivity of PGAA. The application of the cold neutrons in PGAA is the most important supplement to the traditional NAA. When irradiating with cold neutrons, the background intensity is significantly smaller, providing a possibility for in vivo applications.

The detection limit of this method is in the range of 10_5—10_9 g, depending on the cross section of the (n, Y) reaction of the isotopes of the elements. It is applied in the analysis of the following:

• Light elements (H, B) for which NAA cannot be used.

• Main (Si, Al, H, C), trace — (Cu, Cd, Hg, Pb) and indicator (B, Rb, Sm, Gd) elements in geological formations.

• Toxic elements (Cd, Hg)—macro — (H, C, O, Ca) and microelements (Cu, Zn, Fe) in bio­logical, medical samples.

In Figure 10.8, a prompt gamma activation spectrum of a standard cement sample taken using a high-purity germanium detector (HPGE) with Compton suppression is shown.