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C M Ma

Publications and source records attributed to C M Ma.

77 records · Page 5Linked to original sources
PubMedDate not supplied

Wall-correction and absorbed-dose conversion factors for Fricke dosimetry: Monte Carlo calculations and measurements.

For megavoltage radiotherapy photon beams, EGS4 Monte Carlo calculations show, and experimental measurements confirm with an accuracy of 0.2%, that glass or quartz-walled vials used in Fricke dosimetry increase the dose in the Fricke solution. This is mainly caused by increased electron scattering from the glass which increases the dose to the Fricke solution. The dose perturbation is shown to vary from nothing in a 60Co beam up to 2% in a 24-MV beam. For plastic vials of similar shapes, calculations demonstrate that the effect is in the opposite direction and even at high energies it is much less (0.2% to 0.5%).

Ferrous Compounds↗
PubMedDate not supplied

Calculation of absorbed dose ratios using correlated Monte Carlo sampling.

A correlated sampling variance reduction technique has been implemented in the EGS4 (Electron Gamma Shower version 4) Monte Carlo code system in order to calculate correction factors for radiation dosimeters. These correction factors are calculated as the ratios of absorbed doses in various geometries irradiated by photon and electron beams. In this paper the efficiency and accuracy of the calculation of the absorbed dose ratios with correlated Monte Carlo sampling is discussed. Theoretical analysis shows that the calculation efficiency and accuracy increases with the degree of correlation between the calculated doses in the individual geometries. Calculations of the absorbed doses in thin slab geometries and in air-filled ionization chambers have been performed. The results show that the gain in computing efficiency is up to a thousand fold for the calculations of the effect of the central electrode material and size on the response of cylindrical ionization chambers. The calculation accuracy (systematic uncertainty) is significantly improved when good correlation exists between the calculated doses in similar thin-slab geometries.

Biophysical Phenomena↗