The Investigation of Mechanisms of Fast Neutron Registration in Oxide Scintillators

Abstract

The contributions of gamma rays from the inelastic scattering reaction and the resonant reaction in the process of slowing down fast neutrons inside the oxide detector volume are investigated. For this the countable efficiency of the ZnWO₄, CdWO₄, Bi₄Ge₃O₁₂ oxide scintillators in terms of pulses/neutron during the registration of the fast neutrons from a ²³⁹Pu-Be source was measured. It is assumed that the response of detectors during neutron moderation in oxide scintillators with an effective thickness of about 40—50 mm is formed by instantaneous gamma rays from inelastic, resonant inelastic scattering reactions, as well as delayed gamma rays from the capture reaction of resonance neutrons. The parameters of the nuclei, which determine the detector response — the density of the nuclear levels of the compound nuclei being formed, the widths of the resonance regions, the lifetimes of the excited nucleus state were considered. It was found that the registration of a cascade of gamma rays from the discharge of excited levels leads to a significant increase in the countable efficiency of the detector and, as a consequence, an increase in the sensitivity of the detector to fast neutrons. The measured response in terms of pulses/neutron for the ZWO detector — 64, for CWO — 36, for BGO — 2.5. The response of the detectors was recorded by the broadband tract with a time feedback of τ~```0.7 ns. The measured values of the efficiency are explained by the fact that, in our case, the reaction of inelastic scattering is the starting process, which starts the process of discharging nuclear long-lived (~``` 1–1000 ns) states excited in both inelastic scattering and in resonance capture reaction. The registration of the gamma-quanta from discharge leads to an increase the countable efficiency of the detector. The observed increase of the countable efficiency of the secondary gamma quanta is realized when neutrons are moderated inside the oxide detectors with a thickness of 40—50 mm or more. The measurement error of the registration efficiency was about 7%.