MONTE-CARLO CALCULATED STEM EFFECT CORRECTIONS FOR NE2561 AND NE2571 CHAMBERS IN MEDIUM-ENERGY X-RAY-BEAMS

This paper presents the stem effect correction factors for the NE2561 and NE2571 ionization chambers calibrated in air and used at a depth i n a phantom irradiated by medium-energy x-ray beams. The in air and in phantom stem effect correction factors were calculated as the ratios of the absorbed dose in the air cavity of an ionization chamber withou t and with a chamber stem. The 'global' stem correction factor was the n calculated as the ratio of the in phantom correction factor to the i n air correction factor. The calculations were carried out using the E GS4 (electron gamma shower version 4) Monte Carlo code system together with the use of a correlated sampling variance reduction technique. T he results show that in general the chamber stern increases the chambe r response in air but decreases it in phantom. For an NE2571 chamber t he 'global' stem effect correction factor varies from 1.014 +/- 0.001 at 70 kV (2.9 mm Al, 0.1 mm Cu) to 1.005 +/- 0.001 at 300 kV (21.5 mm Al, 5.1 mm Cu) while for an NE2561 chamber it varies from 1.039 +/- 0. 002 to 1.010 +/- 0.002 for the same kilovoltage and HVL values. In a C o-60 beam the global stem correction is 1.001 +/- 0.001 for an NE2571 chamber and 1.003 +/- 0.001 for an NE2561 chamber. This is consistent with recent experimental results.