Dose distribution from a degraded 60Co gamma field.

Dose distribution in two cylindrical phantoms, a small phantom (12.8 cm in diameter and 39.4 cm in length) and a large phantom (25.2 cm and 35.1 cm in length), exposed to a degraded 60Co gamma field was studied. The angular distribution and energy spectrum of the 60Co radiation was changed by ground and air scattering. The dose was measured using thermoluminescent dosimeters (LiF and CaF2). The energy-dependent responses of the dosimeters were corrected using a method developed by Momeni et al. which is based on differential responses of the two types of dosimeters. The dose distribution was calculated from a three-dimensional interpolation-extrapolation of the measured doses. Analysis of the data suggests that the half-value layer concept for a determination of depth dose is not applicable to exposures where the angular distributions and the energy spectra are field variables. Application of thermoluminescent dosimeters for personnel dosimetry in compliance with federal regulations requires correction for the energy dependence of the dosimeters. In exposure cases where the radiation field is nonuniform, use of multiple badges should be considered.