Performance analysis and determination of the pwall correction factor for 60Co γ-ray beams for Wellhöfer Roos-type plane-parallel chambers

Abstract

The wall perturbation correction factor pwall in 60Co for Wellhöfer Roos-type plane-parallel ionization chambers is determined experimentally and compared with the results of a previous study using PTW–Roos chambers (Palm et al 2000 Phys. Med. Biol. 45 971–81). Five ionization chambers of the type Wellhöfer PPC-35 (or its equivalent PPC-40) are used for the analysis. Wall perturbation correction factors are obtained by assuming ND,air chamber factors determined by cross-calibration in a high-energy electron and in a 60Co γ-ray beam to be equal, and by assigning any differences to the wall perturbation factor. The procedure yields a pwall value of 1.018 (uc = 0.010), which is slightly higher than the value 1.014 (uc = 0.010) formerly obtained for the PTW–Roos chambers using the ND,air method. The chamber-to-chamber variation in pwall for the Wellhöfer–Roos chambers is found to be very small, with a maximum difference of 0.3%. The effect of using new pcav values for graphite-walled Farmer-type chambers used in water in electron beams is to decrease pwall by approximately 0.5%. The long- and short-term stability of the Roos-type chambers manufactured by Wellhöfer is investigated by measurements at the IAEA Dosimetry Laboratory in Vienna, Austria, and at the Sahlgrenska University Hospital in Göteborg, Sweden. Calibrations made at the IAEA over several months show variations in the ND,w calibration factors larger than expected, based on previous experiences with PTW–Roos chambers. Measurements of the short-term stability of the Wellhöfer–Roos chambers show a marked increase in chamber response for the time the chambers are immersed in water, pointing to a possible problem in the chamber design. As a consequence of these findings, Wellhöfer is currently working on a re-design of the chamber to solve the stability problem.