Content area
Full Text
Abstract
Exposure buildup factors have been calculated by Monte Carlo method for point isotropic gamma ray sources, penetrating a two-layer spherical shield of water surrounded by lead, and the effect of bremsstrahlung radiation on buildup factors has, in particular, been investigated, the results of which are in good agreement with previous works. The buildup factors were then calculated for various combinations of water and lead layers at some gamma ray energy points in the range from 0.04 MeV to 10 MeV and for shield thicknesses from 1 to 10 mean free paths (mfp). From the results obtained, one can select the proper (or optimum) water and lead combination which results in minimum value of buildup factor for a two-layer water-lead shield of a given thickness (in mfp) at each energy point. Here the optimization analysis has been performed for a shield of 10 mfp thick at gamma ray energies from 0.04 MeV to 10 MeV.
Keywords: Stratified shields; Monte Carlo simulation; buildup factors; bremsstrahlung radiation
(ProQuest: ... denotes formulae omitted.)
1. Introduction
The photon build up factor is defined as the ratio of the total photon beam response (e.g., flux, dose or exposure) to the response of the uncollided photon beam fraction. Buildup factors which account for multiple scattering of photons and secondary sources of annihilation, fluorescence and bremsstrahlung radiation are basic data for point kernel methods that are used in gamma-ray shielding and dosimetry calculations. The buildup factor can be obtained, in principle by experiment, but since the attenuation coefficients and scattering cross sections are known with reasonable accuracy, buildup factors are customarily obtained either by solution of the photon transport equation or by Monte Carlo method. A detailed historical review on build up factor calculation and use is given by Harima [1]. Although buildup factors have been widely calculated and used for mono-layer shields [2-8], they have also been considered for multilayer shields [9-14].
In this work, a Monte-Carlo program was written to simulate the transport of gamma rays through matter and to calculate the exposure buildup factors for a point mono-energetic isotropic gamma ray source located at the center of a stratified spherical shield of water (as a low Z-value material) surrounded by lead (as a high Z-value material). We began...