The Monte Carlo simulation approach to verification of the attenuation factors k for isotropic gamma-ray sources commonly used in industry, medicine and science

An important problem, which is currently overlooked in radiological protection, is the omission of the contribution from low-energy photons when estimating the dose received by persons working in conditions of exposure to ionizing radiation. As shown in this study, this contribution cannot be omitted because it determines the dose in the workplace when an isotropic source of gamma rays (uncollimated beams) and thick shields are used. The omission of the contribution from low-energy photons causes that the gamma-ray attenuation factors, often marked in radiological protection with the letter k, as well as the transmission curves for isotropic gamma-ray beams resemble those for collimated beams without the typical features of uncollimated beams. This causes the estimated doses to be significantly underestimated. In this work the gamma-ray attenuation factors k were determined by Monte Carlo simulations for basic shielding materials, for commonly used isotropic gamma-ray sources. These factors are the basic parameters used to calculate the gamma-ray dose that a person who performs professional activities with gamma rays can receive. Accurate calculation of this dose translates into work safety. Justification for this research is the necessity to apply the exact k-factor values in radiological protection in Poland where doses at workplaces are calculated according to the procedure given in Gostkowska's book based on the old Polish norm (1987). The ground of this norm are transmission curves from NCRP Report 49 (1976) based on data obtained in the 1930's, 40's and 50's, unsuitable for isotropic sources. Although the obtained results refer to the Polish standard, the world's norms based on the NCRP Report 49 also need innovation, because the subsequent versions of this report published in 1991, 1994 and 1998 as well as the NCRP Report No. 151 considered as the new face of the NCRP Report No. 49 do not make the required correction. This work meets these requirements by using the Monte Carlo simulation method based on the GEANT4 code.