Prediction of effective atomic number (Z) for laminated shielding material

Abstract

Dose and energy absorption buildup factors and effective atomic number (Z[]) were calculated for various combinations of double-layered and triple-layered shields containing low atomic member (Z) material followed by high Z material and high Z material followed by low Z material using the Monte Carlo code, MCNP. In this study, the shielding materials water, iron, and lead were used in various combinations as multi-layer shielding for buildup factor calculation. For the multi-layered shields, buildup factors were calculated for different mean free paths (mfp) for point isotropic sources emitting 0.5, 1, and 3 Mev photons. Some calculations were performed for double layered shields and some for three layers using spherical geometry. For two layers, the buildup factors were calculated for lead followed by water, varying in thickness from 1 to 6 mfp, and water shields followed by lead were also investigated. Likewise, iron-water and water-iron shields were studied as well as lead followed by iron and water and other combinations. From all of these studies, the dose buildup factors calculated by the MCNP simulations were found to be very close to those from the EGS4 calculations (within 5%) and compared well with results from other calculations for similar multi-layer shielding problems. Using the energy absorption buildup factors, Z[] was calculated for each combination of laminated shields for 0.5, 1.0, and 3.0 MeV monoenergenetic photons. Then, these calculated Z[] values were used to formulate two general mathematical expressions that predict these factors for use in general shielding calculations.