Calibration of plane-parallel chambers and determination of P (wall) for the NACP and Roos chambers for 60Co gamma-ray beams

Procedures for the calibration and use of plane-parallel ionization chambers in high-energy electron and photon beams have been given in the international code of practice IAEA TRS-381. In the present work, plane-parallel ionization chambers of the type PTW-34001 Roos and Scanditronix NACP02 have been calibrated using two NK -based procedures. For the NACP chamber the difference between the N D ,air chamber factors determined in an electron beam and in a 60 Co -ray beam, respectively, is of the same magnitude as the experimental uncertainty. Results for the PTW Roos chambers, however, do not agree, in accordance with recent findings of other authors. The value determined in a 60 Co -ray beam is questioned and the reason for the discrepancy assigned to the correction factor for the perturbation due to the chamber wall, p wall . New values of p wall have been experimentally determined by comparing absorbed dose measurements based on air-kerma and absorbed dose to water calibration procedures. A new p wall factor for the Roos chamber in 60 Co -ray beams in water (1.009±0.6%) was derived as the weighted average of the different determinations. The value is not significantly higher than the p wall factor given in TRS-381 (1.003±1.5%), but the combined standard uncertainty is reduced. The chamber to chamber variation for six commercial PTW Roos chambers and a Roos prototype was found to be very small.