The dissertation is devoted to studies of the mechanisms of interaction of fast neutrons with the material of single-crystal and composite oxide scintillators.
The creation of compact high-sensitivity detectors for neutron and gamma-neutron radiation control systems to combat the illegal transportation of radioactive materials is an urgent task. The most common are inspection systems based on 3He-neutron counters and have a low registration efficiency of ~ 10%, due to the need to moderate fast neutrons to thermal energy. The scarcity and high cost of manufacturing 3He stimulates the search for new detectors and new principles for recording fast neutrons. Previous studies have shown that the mechanism of inelastic scattering (n, n′ γ)in can be used to detect fast neutrons with detectors based on heavy oxide scintillators. The recording efficiency was ~ 0.5 for detectors of small size (~ 10 mm3). In this case, the signals were registered, the duration of which was formed (constant of the integration time) was in the microsecond range. This was due to the need to reduce the effect of secondary cascade gamma quanta arising in the scintillator substance. The registration of response pulses in the microsecond range allowed to register only high-energy (> 20-30 keV) gamma quanta from the inelastic scattering reaction of fast neutrons (n, n′ γ) arising from the discharge of excited single-particle and collective states of medium and heavy scintillator nuclei. Therefore, we proposed to use the gamma-ray cascades generated not only in the inelastic scattering reaction, but also in the resonant and radiation capture reactions to increase the sensitivity of the fast neutron detector. Fast neutrons 239Pu-Be sources with maximum energy E ≤ 10 MeV in the process of scattering and deceleration in the substance of the oxide scintillator with linear dimensions of ~ 40-50 mm and more in the reactions of inelastic and resonant scattering, radiation capture pass three energy regions: the region of inelastic scattering in the reaction (n, n′ γ)in (~ 10 MeV - 100 keV), the region resonant capture (n, n′ γ)res (100 keV - 100 eV) and the radiation capture region (100 eV - 0.025 keV). In these reactions, the states of compound nuclei (A + 1) are excited with a lifetime of ~ 10-14 s - 10-12 s, and the states of finite nuclei (A) with lifetimes from picoseconds to tens of microseconds, and delayed gamma can be born -quants of γdel caused by wanderings of secondary neutrons from reactions (n, n′ γ)in and (n, n′ γ)res in the scintillator substance. Thus, the response of the detector to one input particle (ie fast neutron) is a mixture of gamma quanta and intermediate neutrons, so the number of registered secondary particles (ie gamma quanta) by the detector may significantly exceed 1. Since the nuclei that are part of the oxide scintillators (W, Gd, Zn) have significant values of the cross-section of the interaction in the resonant region, ~ 50 - 500 bar, while the cross-sectional values in the inelastic region are units of bar (~ 2 – 3 bar) , the registration of gamma quanta associated with these processes can significantly increase the statistics of events per input neutron and, as a consequence, increase the efficiency of neutron registration. Such processes have a small discharge energy in the range from eV units to hundreds of keV and lifetimes in the range τ ~ 10 14 s - 10-5 s.
