Measurement of absorbed dose to water for medium energy x-rays

This paper evaluates the characteristics of ionization chambers for the measurement of absorbed dose to water for medium energy x-rays. The values of the chamber correction factor, kch, used in the IPEMB 1996 Code of Practice for the UK secondary standard (NE2561/NE2611) ionization chamber are derived. The comparison of the chamber responses in-air revealed that of the chambers tested, only the NE2561, NE2571 and NE2505 exhibit a flat (within 5%) energy response in air. Under no circumstances should the NACP, Sanders electron chamber or any chamber that has a wall made of high atomic number material be used for medium energy x-ray dosimetry. The measurements in water reveal that a chamber that has a substantial housing such as the PTW Grenz chamber, should not be used to measure absorbed dose to water in this energy range. The value of the perturbation correction, pu for a NE2561 chamber was determined by measuring the absorbed dose to water and comparing it with that for a NE2571 chamber, for which pu data has been published. The chamber correction factor varies from (1.023 ± 0.03) to (1.018 ± 0.001) for x-ray beams with HVL between 0.15 and 4 mm Cu. The values agree with that for a NE2571 chamber within the experimental uncertainty. (author)