An experimental setup for Prompt Gamma Neutron Activation Analysis

Prompt Gamma Neutron Activation Analysis (PGNAA) became a very powerful method complementary to other nuclear methods, PIXE, NAA, XRF. One of the most spectacular applications of this method was connected with the fast analysis of materials in bulk. This nuclear method is very suited for 'on-line' analysis of coal, cement, ores or slurries. The method was gradually improved and now many installations are commercially available. A PGNAA experimental setup was developed at Horia Hulubei Institute of Physics and Nuclear Engineering. It consists of an Am-Be neutron source (107 n/s) placed in a cylindrical container and a semiconductor Ge-Li detector. The experimental arrangement is shown. The prompt gamma spectrum is recorded and analysed using an integrated computerized system. The calibration in energy was carried out using the known prominent gamma lines in the different elements. The neutron spectra depend on the moisture content and are also influenced by the density of the sample. Monte Carlo simulations were carried out in order to evaluate the neutron energy spectra in the different parts of the sample and an approximate formula was deduced to correlate the peak areas with the moisture and sample composition. The prompt gamma spectra generally consist of two types of lines: inelastic scattering lines produced from the fast neutron reactions and capture lines produced by thermal and epithermal neutrons. In case of the Am-Be source, the primary spectrum is a fast one, with the average energy of about 4.6 MeV. The energy distribution of the neutrons changes dramatically in the sample, so that the gamma source for a certain line varies throughout the sample. Although the intensity of the prompt gamma lines can be directly correlated with the elemental concentrations, the details of the neutron interactions must be well known especially for the elements that are good moderators. We have carried out measurements on different types of coals, sands, ores, and aqueous solutions. The main peaks arise either from inelastic scattering reactions produced by fast neutrons (C, O, Al, Si) or from thermal neutron captures (H, Si, S, Cl, Fe). (authors)