THE VACUUM GEOMETRY EFFECT ON NEUTRON TRANSMISSION AND SPATIAL-RESOLUTION OF NEUTRON TRANSMISSION

Frequently, shields used against radiation contain some vacuum channel s. We have therefore considered an infinite slab with a fixed thicknes s (thickness 20 lambda, with lambda the mean free path of the neutron in the slab) and an infinite plane source of neutrons which arrived on the left side of the slab; transmitted neutrons through the slab to i ts right side are detected by finite detectors having windows equal to 2 lambda. This slab contains a vacuum channel. This channel has many legs with several horizontal parts. We used the Monte Carlo method for sampling the neutron history in the slab with a spatial biasing techn ique in order to accelerate the calculation convergence (Levitt, L. B. (1968) Nuclear Science and Engineering 31, 500-504; Jehouani, A., Gha ssoun, J. and Aboubker, A. (1994) In Proceedings of 6th International Symposium on Radiation Physics, Rabat, Morocco). We studied the effect s of the angle position and the number of horizontal parts of the chan nel on the neutron transmission. We have studied the effect of the vac uum channel opening (Artigas, R. and Hungerford, H. E. (1969) Nuclear Science and Engineering 36, 295-303) on the neutron transmission; for several values of this opening we have calculated the neutron transmis sion probability for each detector position. This study allowed us to determine the optimal conditions of vacuum geometries to improve prote ction against neutrons. In the second part we considered a shield whic h consists of a slab and a two-legged vacuum channel with two horizont al parts. The spatial distribution of neutrons transmitted through the protection screen was determined. This distribution shows two peaks. The study was made for different distances between the two horizontal parts. We have determined the smallest distance between the two horizo ntal parts for which the two peaks can be resolved. (C) 1997 Elsevier Science Ltd. All rights reserved.