Year
2006
Abstract
Implementation of customs installations for the detection of fissile materials (FM) in baggage is related to one of the basic problems of their revealing, when FMs are deliberately shielded. In devices with pulsed neutron sources, it is practically impossible to conceal FM with a lead shield; however, taking into account thermal neutrons used in such devices, FM can be concealed with shields having large thermal neutron absorption cross-section, for example, those of cadmium. The grounds for the feasibility of a customs FM control installation are given by the results of the experiments under ISTC Project No.596 on detection of 235U in a model with graphite neutron moderator and digital discrimination of neutrons and photons in a PSD scintillator. The model represented a graphite parallelepiped of 1300×1300×1200 mm3 in size with an inner chamber of 500×500×1200 mm3. A FM sample being inspected and a scintillation measuring system with a stilbene crystal were placed inside the chamber. A complex of four experiments was carried out, considering the following cases: uranium present in the model, uranium surrounded with a cadmium shield, the cadmium shield without uranium, and absence of all indicated materials. The analysis of the experimental results has shown the detectability of uranium concealed with a cadmium shield owing to the fission of 235U on neutrons with energies above the resonance in the cadmium absorption cross-section. These neutrons are present in the graphite model at ~150 microseconds after a pulse of neutrons from the source. It has been shown that in case of an appropriate choice of the energy threshold of FM response detection, it is possible to obtain practically zero photon background, which determines high sensitivity of the installation. Under these conditions, the discrimination of neutrons and photons in the time dependences is possible under loads on the scintillation channel up to 5·105 particles per second. Analysis of the results of the experiment shows that 5-7 seconds suffice to detect ~10 grams of 235U in a cadmium shield, at a yield of a DD neutron source equal to ~(5·107-108) n/s. Experiments have shown that without a cadmium shield the sensitivity of the device raises more than by an order of magnitude. Presumably, approximately ~16 separate digital scintillation channels will suffice for the real installation with the size of the baggage chamber 550×750×1200 mm. It has also been shown that it is possible to obtain an indirect informative parameter of the cadmium presence in the installation by the time of reaching the maximum in time distributions of photons. Besides, in the photon channel, after the prompt neutrons and photons decay, FM can be detected by the delayed photon radiation. In the passive mode, FM can also be detected by its self-radiation in cases, when radiation is not shielded.