Direct measurement of ionization chamber absorbed dose kQ factors in clinical electron beams

Abstract

An electron water calorimeter is described and is used to experimentally determine beam quality dependent conversion factors, kQ, for two ionization chamber types in high-energy electron beams. The current water calorimeter was upgraded to obtain reproducible measurements in electron beams from 10 MeV to 22 MeV (corresponding beam quality index R50 ranging over 4.20–8.97 g cm−2). Measurements of radiation induced temperature rise are made using a plane-parallel glass vessel with calibrated thermistor probes in a 4 °C water phantom. Three types of plane-parallel chambers (PTW Roos, IBA PPC40 and SNC 1045) and two types of cylindrical chambers (PTW30013 and IBA FC65-G) are measured to directly derive beam quality correction factors in the same water phantom. Relative combined standard uncertainty for kQ factors of 0.56% for 10 MeV beams and above were achieved. General agreement is obtained between the relative electron energy dependencies of the kQ factors measured in this work and those derived from TRS-398 of the IAEA and recent other experimental investigations. Furthermore, the experimental kQ values obtained here will impact future updates of reference dosimetry protocols, and the current standard uncertainties in electron reference dosimetry can be reduced by the method proposed in this work.