Year
2001
Abstract
This work is the first stage of the ISTC project ¹ 596 on customs control of fissile materials (FM) in airports. On the first project year the physical method of the FM detection in a model with two hydrogen-containing neutron moderators and a pulse neutron source was experimentally substantiated. The neutron moderators have various thermal neutrons decay constants, and one of them is closed by a cadmium screen. Calculated and experimental optimization studies were carried out with the purpose of peak effic iency of the model. The FM response as a function of the sizes of moderators, cavity in one of them, leaden neutron multiplier and location of the neutron source was investigated in experiments. Besides, maximum probability of the FM detection was investigated as dependence on the time after the neutron pulse. MCNP-4a code was used in calculations. The purpose of the ISTC project ¹ 596 is the development of the FM detection technology in the customs offices (for example, airports), based on the use of pulse neutron generators. The difficulty in the application of an external neutron source is connected with the fact that sometimes it is impossible to accomplish the FM control on their self-radiation in short time (~ 5 seconds), and at presence of leaden or other absorbing screens. The developed technology assumes the use of several independent physical techniques, so that shielding of FM would be useless. The plan of research includes a complex of calculated and experimental studies on radiation safety of the facility and consequences of possible accidents, up to the destruction of the neutron source. Four various types of one moderator in combination with five types of the other moderator have been investigated. The results show that the probability of FM detection strongly depends on the combination of moderators of various sizes and location of the moderator in the detecting system of helium counters. The probability of FM detection substantially depends not only on sizes of the moderators, but also on the code separation of thermal neutron decay constants in them. The designed model of installation detects ~8 grams of 235U in 5 seconds, the rate of neutrons from a DT source being at a level of ~108 n/s.