The magnetic field dependent displacement effect and its correction in reference and relative dosimetry

Objective This study investigates the perturbation correction factors of air-filled ionization chambers regarding their depth and magnetic field dependence. Focus has been placed on the displacement or gradient correction factor Pgr. Besides, the shift of the effective point of measurement Peff that can be applied to account for the gradient effect has been compared between the cases with and without magnetic field. Approach The perturbation correction factors have been simulated by stepwise modifications of the models of three ionization chambers (Farmer 30013, Semiflex 3D 31021 and PinPoint 3D 31022, all from PTW Freiburg). A 10 cm x 10 cm 6 MV photon beam perpendicular to the chamber’s axis was used. A 1.5 T magnetic field was aligned parallel to the chamber’s axis. The correction factors were determined between 0.4 and 20 cm depth. The shift of Peff from the chamber's reference point Pref, ∆z, was determined by minimizing the variation of the ratio between dose-to-water Dw(zref+∆z) and the dose-to-air Dair(zref) along the depth. Main Results The perturbation correction factors with and without magnetic field are depth dependent in the build-up region but can be considered as constant beyond the depth of dose maximum. Additionally, the correction factors are modified by the magnetic field. Pgr at the reference depth is found to be larger in 1.5 T magnetic field than in the magnetic field free case, where an increase of up to 1% is obserbed for the largest chamber (Farmer 30013). The magnitude of ∆z for all chambers decreases by 40% in a 1.5 T magnetic field with the sign of ∆z remains negative. Significance In reference dosimetry, the change of Pgr in a magnetic field can be corrected by applying the magnetic field correction factor kB Qmsr when the chamber is positioned with its Pref at the depth of measurement. However, due to the depth dependence of the perturbation factors, it is more convenient to apply the ∆z-shift during chamber positioning in relative dosimetry.

APMP key comparison report of air kerma for 137Cs (APMP.RI(I)-K5)

Main text The APMP/TCRI Dosimetry Working Group performed the APMP.RI(I)-K5 key comparison of the air kerma for 137Cs in 2014. Five national metrology institutes (NMIs) took part in the comparison. Two commercial ionization chambers were used as transfer instruments and circulated among the participants. The results showed that the maximum difference between the participants and the Bureau International des Poids et Mesures, evaluated using the comparison data of the linking laboratories of the Korea Research Institute of Standards and Science and the National Metrology Institute of Japan, was less than 0.5% within the expanded uncertainty. This comparison supports the equivalence of the calibration capabilities of the participating laboratories. The results predate the publication of ICRU report 90, therefore, the revision of the data reflecting the effects of the ICRU report 90 on the degrees of equivalences of the participant laboratories is presented in Appendix C. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/. The final report has been peer-reviewed and approved for publication by the CCRI, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).