Evaluation of microshield build-up factors and their limits of applicability

Abstract

MicroShield is a point-kernel computer code used for gamma-ray shielding and dose rate assessment. It allows the modeling of simple source geometries and simple shielding layouts which lead to very accurate results with low computer time consumption, compared with more advanced methods such as Monte Carlo transport code (e.g., MCNP), among others. The short computer time in finding solutions is due to a deterministic transport using analytic solutions for the direct source contribution (unscattered radiation) to the detector, and then corrected by a build-up factor appropriate for the geometry used in the calculation. On the other hand, Monte Carlo transport codes, such as MCNP, solve the detailed physics of the transport in the real geometry (calculating a build-up factor as a by-product); however, the set up of the geometry and the implementation of variance reduction techniques (always necessary in shielding problems) is complex and time consuming. Although the MicroShield software give good results for many 7-ray shielding and dose rate calculations, there are still many unanswered questions about the behavior of MicroShield build-up factors and their limit of applicability when the line connecting source and detector is not parallel to the normal of the shielding, creating an angle a (offset from symmetry axis). The objective of this work is to compare similar shielding layouts using MicroShield and MCNP for typical shielding materials and typical energies of interest but explicitly assessing the incidence of the offset ; in the results, and therefore determining a limit of applicability for MicroShield The build-UD factors used in the current model of MicroShield have shown to yield reasonable results in terms of the effective dose rates when compared with MCNP calculations. Using the maximum percentage error of 30 percent as recommended from MS manual, the limit of applicability of MS was determined at various materials thickness of lead, iron, and concrete.