Photon backscatter measurements of the polymethylmethacrylate phantom used in the US personnel dosimeter accreditation programs (NVLAP, DOELAP)

A study was conducted to determine the photon backscatter contribution to the response of thermoluminescent dosimeters (TLDs) that are irradiated on polymethylmethacrylate (PMMA) phantoms. Because the dose equivalent for photons is computed from the product of the exposure, the exposure to dose conversion factor, the backscatter factor, and the quality factor, it is necessary to know the backscatter factor to evaluate the dose equivalent. The backscatter factor was determined for the photon beams used in proficiency test irradiations. X-ray beams with effective energies ranging from approximately 15 to 120 keV were used along with Cs-137 and Co-60 gamma-rays. Thin LiF dosimeters were exposed in air and on the PMMA phantom, and the ratio of their responses per unit exposure were taken as a measure of the backscatter factor. These data were then compared with a previous set of Monte Carlo computer calculations, and were used to compute exposure to dose equivalent (Cx) factors that were compared with the Cx factors used for the DOELAP and NVLAP programs. The results of these measurements will be described in this presentation. 21 refs., 4 figs