THE ALBEDO EFFECT ON NEUTRON TRANSMISSION PROBABILITY

The aim of this study is to evaluate the albedo effect on the neutron transmission probability through slab shields. For this reason we have considered an infinite homogeneous slab having a fixed thickness equa l to 20 lambda (lambda is the mean free path of the neutron in the sla b). This slab is characterized by the factor P-s (scattering probabili ty) and contains a vacuum channel which is formed by two horizontal pa rts and an inclined one (David, M. C. (1962) Duc and voids in shields. In Reactor Handbook, Vol. III, Part B, p. 166). The thickness of the vacuum channel is taken equal to 2 lambda. An infinite plane source of neutrons is placed on the first face of the slab (left face) and dete ctors, having windows equal to 2 lambda, are placed on the second face of the slab (right face). Neutron histories are sampled by the Monte Carlo method (Booth, T. E. and Hendricks, J. S. (1994) Nuclear Technol ogy 5) using exponential biasing in order to increase the Monte Carlo calculation efficiency (Levitt, L. B. (1968) Nuclear Science and Engin eering 31, 500-504; Jehouani, A., Ghassoun, J. and Abouker, A. (1994) In Proceedings of the 6th International Symposium on Radiation Physics , Rabat, Morocco) and we have applied the statistical weight method wh ich supposes that the neutron is born at the source with a unit statis tical weight and after each collision this weight is corrected. For di fferent values of the scattering probability and for different slopes of the inclined part of the channel we have calculated the neutron tra nsmission probability for different positions of the detectors versus the albedo at the vacuum channel-medium interface. Some analytical rep resentations are also presented for these transmission probabilities. (C) 1997 Elsevier Science Ltd. All rights reserved.