A METHOD TO DECREASE THE LOAD ON SCINTILLATION CHANNELS IN FISSILE MATERIAL DETECTION AND CONTROL DEVICES WITH PULSED NEUTRON SOURCES

Implementation of fissile material (FM) detection devices that use various neutron energy groups is feasible with application of a graphite moderator and a pulsed neutron source. This is necessary to detect FM deliberately concealed with shields having large thermal neutron absorption cross-section. In order to increase the efficiency of detection of fission neutrons, PSD scintillators can be used. Experiments show that in such devices, it is practically impossible to use conventional spectrometers due to high variable loads on scintillation channels reaching up to a level of 106 particles per second. This work (ISTC project #2978) uses the digital technology of discrimination of neutrons and photons, but even this technology cannot provide processing of the complete experimental data array under that high loads. Therefore, a method is proposed to decrease the load on the scintillation channels with the help of composite scintillators, those being cylinders containing cylindrical shells of a thermal neutron absorbing material. The composite scintillator consists of two zones, the sizes of which allow implementing equal constants of thermal neutron decay, the values of which considerably exceed the decay constant of a homogeneous detector. This major factor reduces the load after a pulse of source neutrons. The materials considered for this purpose are cadmium and lithium carbonate with various content of 6Li. Lithium carbonate contains no hydrogen, thus producing no additional background photons during the thermal neutron capture. Lithium absorbs thermal neutrons well, therefore practically no photons occur. The efficiency of this type of detector remains practically constant in relation to fission neutrons, but the number of detected photons occurring in the reaction of radiation capture on the hydrogen of the scintillator sharply decreases due to the increase of the thermal neutron decay constant of the composite detector. In view of this fact, this work suggests the use of a composite two-zone scintillator with an inner wall of lithium carbonate, the use of which decreases the load on the scintillation channel at various times after a pulse of source neutrons by a factor of 2 to 11.