Determination of the Kwall correction factor for a cylindrical ionization chamber to measure air-kerma in 60Co gamma beams

Abstract

The factor Kwall to correct for photon attenuation and scatter in the wall of ionization chambers for 60Co air-kerma measurement has been traditionally determined by a procedure based on a linear extrapolation of the chamber current to zero wall thickness. Monte Carlo calculations by Rogers and Bielajew (1990 Phys. Med. Biol. 35 1065-78) provided evidence, mostly for chambers of cylindrical and spherical geometry, of appreciable deviations between the calculated values of Kwall and those obtained by the traditional extrapolation procedure. In the present work an experimental method other than the traditional extrapolation procedure was used to determine the Kwall factor. In this method the dependence of the ionization current in a cylindrical chamber was analysed as a function of an effective wall thickness in place of the physical (radial) wall thickness traditionally considered in this type of measurement. To this end the chamber wall was ideally divided into distinct regions and for each region an effective thickness to which the chamber current correlates was determined. A Monte Carlo calculation of attenuation and scatter effects in the different regions of the chamber wall was also made to compare calculation to measurement results. The Kwall values experimentally determined in this work agree within 0.2% with the Monte Carlo calculation. The agreement between these independent methods and the appreciable deviation (up to about 1%) between the results of both these methods and those obtained by the traditional extrapolation procedure support the conclusion that the two independent methods providing comparable results are correct and the traditional extrapolation procedure is likely to be wrong. The numerical results of the present study refer to a cylindrical cavity chamber like that adopted as the Italian national air-kerma standard at INMRI-ENEA (Italy). The method used in this study applies, however, to any other chamber of the same type.

Results supporting calculated wall correction factors for cavity chambers

“…This might be the explanation why the calculated wall correction factors reported by Rogers and Treurniet (1999) for the existing cylindrical cavity chambers used as primary standards are throughout greater than those obtained by the extrapolation method. In essence, similar conclusions were drawn in a recent report by Laitano et al (2002) who determined k wall for a cylindrical ionization chamber for measuring air kerma in 60 Co gamma beams using a procedure characterized by a mixture of experimental and analytical parts. From the above discussion it is evident that the magnitude of the differences should roughly scale with the ratio of the inner diameter to the height of the chamber.…”

Air-kerma cavity standards

“…This discussion motivated several NMIs to calculate corrections for their standards using different Monte Carlo codes and to design experiments to test the validity of the different approaches [12,18,55,[80][81][82][83][84][85][86][87][88][89]. It became clear that wall corrections calculated using codes such as ITS3 and MCNP4 [18] and PENELOPE [12,87] yielded similar results to those obtained using EGS4 [79].…”

An EGSnrc Monte Carlo study of the microionization chamber for reference dosimetry of narrow irregular IMRT beamlets

“…As a consequence some standard laboratories have recently announced the change of their 60 Co and 137 Cs air kerma standards to up to 1% as a result of the reevaluation of the wall correction factors. 26,27 Mainegra-Hing, Kawrakow, and Rogers 28 recently published a detailed study of the correction factors involved in the absolute dosimetry using plane-parallel chambers. A latest revision of MC calculated correction factors for primary standards of air kerma with the sensitivity analysis of the derived corrections to the employed cross section and ionization data was carried out by Rogers and Kawrakow.…”