Calculations for beta dosimetry using Monte Carlo code (OREC) for electron transport in water.

A Monte Carlo computer code (OREC) for calculating the detailed transport and energy deposition for primary electrons and all of their secondaries in liquid water has been investigated for use in beta-ray dosimetry. Some modifications have been made in the original code for its application to tissue and tissue-equivalent materials. The code gives reasonably good agreement with beta spectral data and depth-dose curves measured in tissue-equivalent plastics for several calibrated beta sources. The calculations permit a direct evaluation of the skin dose equivalent, i.e., the dose equivalent, Hs (0.07), at a depth of 0.070 mm in tissue. Calculations are presented for monoenergetic electrons, showing the distributions in the maximum depth of penetration and in the total pathlength traveled. Direct comparisons are made between depth-dose curves calculated for 99Tc and 147Pm plaque sources and measurements made with extrapolation chambers. The energy spectrum of beta particles emerging from a thick 99Tc plaque source also is calculated, and the angular distribution is found to be almost independent of the energy. The pulse-height spectrum in a tissue-equivalent plastic scintillator calculated for this source shows good agreement with the measured spectrum. The calculations also provide the Hs(0.07) dose equivalent for the 99Tc source, which is found to be consistent with that inferred independently from the spectral measurements. A calculated curve for converting spectrometer measurements to Hs(0.07) dose equivalent is in good agreement with a semi-empirical curve that was developed independently. It appears that calculations made with the electron transport code for water can provide useful information for beta dosimetry.