Research of the Single Crystal and Multilayer Composite Detectors Response Under Irradiation by Fast Neutrons

The object of this work was to study the response of the detectors based on the oxide scintillators under irradiation by a flux of fast neutrons from 239Pu-Be source by counting photomultiplier tube pulses.  In the process of the research the counting efficiency of the detectors was measured in units of (pulse×s-1×cm-2)/(neutron×s-1×cm-2) for single-crystal and multilayer composite detectors ZWO (ZnWO4), CWO (CdWO4), BGO (Bi4Ge3O12, composite). The measured response for ZWO detector was ~ 64 pulse/neutron, for CWO ~ 36 pulse/neutron, for BGO ~ 0.44 pulse/neutron. The detectors response was registered by a fast preamplifier with the operation speed of up to 500 MHz, based on high-speed operational amplifiers with voltage feedback. The statistical error of measurement for the neutron registration efficiency by the broadband channel made 7% for the detectors with the effective thickness of ~ 40-50 mm, which is due to the spherical geometry of the experiment. The formation of the detector response is affected by the following parameters of neutron reactions: cross section of inelastic and resonant scattering of scintillator nuclei, density of composite nuclei levels, resonance region width, lifetimes of long-lived states and their number. The measured values of the counting efficiency of fast neutrons registration are accounted for the fact that the inelastic scattering reaction for some nuclei is the starting point that triggers the cascade process of the nuclear states discharge. The registration of the cascade of the discharge gamma-quanta, ranging from nanoseconds to a few microseconds, causes an increase in the detector counting efficiency and, as a consequence, an increase in the detector sensitivity to neutron detection. The observed increase in the counting efficiency of secondary gamma quanta is realized when the neutrons are slowed down in the detectors having sufficiently noticeable thickness and appropriate isotopic composition.