Dose Buildup Factors of Multi-Layer Slabs for a Point Isotropic Source

Abstract

Dose buildup factors for multi-layer slabs composed of lead and graphite were obtained experimentally for a point isotropic source of 60Co and 137Cs. The experimental results are compared with those calculated by the Monte Carlo method, and show a good agreement. The buildup factors for the second material behind the first increase — or decrease — very markedly for heavy-light — and light-heavy — layers. From these experimental results, it would be possible to assume that the dose buildup factor for the second material of a double layer slab is composed of two parts, namely asymptotic and transient. The transient part is that which increases or decreases very rapidly within about 1 mean free path from the interface of the first and second layers. The asymptotic part increases proportionally with the buildup factor for the second material. The rise or fall of the buildup factor in the second material can be explained from the change of the spectra in the second material. From the experimental results, the dose buildup factors for multi-layer slabs may be expressed as follows: The experimental results are also compared with those calculated by Kalos' formulas and that of Broder. Broder's buildup factors are very different from the experimental results.