Air kerma calibration factors and chamber correction values for PTWsoft x-ray, NACP and Roos ionization chambers at very low x-rayenergies

Abstract

This paper evaluates the characteristics of ionization chambers for themeasurement of absorbed dose to water using very low-energy x-rays. The valuesof the chamber correction factor, kch, used in the IPEMB 1996 code ofpractice for the UK secondary standard ionization chambers (PTW type M23342and PTW type M23344), the Roos (PTW type 34001) and NACP electron chambers arederived.The responses in air of the small and large soft x-ray chambers (PTWtype M23342 and PTW type M23344) and the NACP and Roos electron ionizationchambers were compared. Besides the soft x-ray chambers, theNACP and Roos chambers can be used for very low-energy x-ray dosimetryprovided that they are used in the restricted energy range for which theirresponse does not change by more than 5%.The chamber correction factor was found by comparing the absorbed dose towater determined using the dosimetry protocol recommended for low-energyx-rays with that for very low-energy x-rays. The overlap energy range wasextended using data from Grosswendt and Knight. Chamber correction factorsgiven in this paper are chamber dependent, varying from 1.037 to 1.066 for aPTW type M23344 chamber, which is very different from a value of unity givenin the IPEMB code. However, the values of kch determined in thispaper agree with those given in the DIN standard within experimentaluncertainty. The authors recommend that the very low-energy section of theIPEMB code is amended to include the most up-to-date values of kch.