2000 Volume 49 Issue 10 Pages 471-478
In this report are discussed the characteristics of a radiation field generated by a 137Cs γ-ray source and the depth-dose curve in a material placed there. A simpler formulation was achieved to describe fundamental dosimeterc quantities, the kerma and the absorbed dose. The well-known pattern of the depth-dose relation, i.e. rapid increase followed by gradual decrease due to attenuation of primary photons, could be confirmed by a numerical calculation under the condition of an exposure to parallel photon beam in vacuum.
The experiments with CaSO4 TL elements of 15 mg/cm2 thick attached to a plastic substrate of 11 mg/cm2 and aluminum foils, contrary to the theoretical prediction, resulted in a different depth-dependence of about 30% decrease by aluminum of 100 mg/cm2, which corresponds to an attenuation of 26 keV photons. Another experimental trials and a rough estimation of the absorbed dose should lead an inference that the absorbed dose near the surface may be considerably affected by secondary electrons generated in the capsule and/or the air between the source and the detector.
In order to discuss more quantitatively, a Monte Carlo code EGS4 was employed to calculate the energy and the fluence of the secondary electrons with an assumption of a spherical γ-source made of glass in a stainless steel capsule. It was found out that the relative fluence of the secondary electrons was only a few thousandth of the primary photons, but that their contribution to the absorbed dose near the surface amounted to more than 70% owing to a much larger conversion coefficient. A careful consideration to the secondary electrons must be required in the case of γ-ray irradiation to materials or calibration of dosimeters with a γ-ray source.