CALCULATION OF ABSORBED DOSE RATIOS USING CORRELATED MONTE-CARLO SAMPLING

A correlated sampling variance reduction technique has been implemente d in the EGS4 (Electron Gamma Shower version 4) Monte Carlo code syste m in order to calculate correction factors for radiation dosimeters. T hese correction factors are calculated as the ratios of absorbed doses in various geometries irradiated by photon and electron beams. In thi s paper the efficiency and accuracy of the calculation of the absorbed dose ratios with correlated Monte Carlo sampling is discussed. Theore tical analysis shows that the calculation efficiency and accuracy incr eases with the degree of correlation between the calculated doses in t he individual geometries. Calculations of the absorbed doses in thin s lab geometries and in air-filled ionization chambers have been perform ed. The results show that the gain in computing efficiency is up to a thousand fold for the calculations of the effect of the central electr ode material and size on the response of cylindrical ionization chambe rs. The calculation accuracy (systematic uncertainty) is significantly improved when good correlation exists between the calculated doses in similar thin-slab geometries.