Rosser K E
National Physical Laboratory, Teddington, Middlesex, UK.
Phys Med Biol. 1998 Nov;43(11):3195-206. doi: 10.1088/0031-9155/43/11/003.
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, k(ch), used in the IPEMB code of practice for the UK secondary standard (NE2561/NE2611) ionization chamber are derived and their constituent factors examined. The comparison of the chambers' 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 k(ch) for an NE2561 chamber was determined by measuring the absorbed dose to water and comparing it with that for an NE2571 chamber, for which k(ch) data have 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 an NE2571 chamber within the experimental uncertainty. The corrections due to the stem, waterproof sleeve and replacement of the phantom material by the chamber for an NE2561 chamber are described.
本文评估了用于测量中能X射线水吸收剂量的电离室特性。推导了英国二级标准（NE2561/NE2611）电离室在IPEMB实用规程中使用的室修正因子k(ch)的值，并对其组成因素进行了研究。电离室在空气中响应的比较表明，在所测试的电离室中，只有NE2561、NE2571和NE2505在空气中呈现平坦（在5%以内）的能量响应。在任何情况下，都不应将NACP、桑德斯电子室或任何具有高原子序数材料制成的壁的室用于中能X射线剂量测定。在水中的测量表明，具有较大外壳的室，如PTW Grenz室，不应在该能量范围内用于测量水吸收剂量。通过测量水吸收剂量并将其与已公布k(ch)数据的NE2571室的剂量进行比较，确定了NE2561室的k(ch)值。对于半值层在0.15至4毫米铜之间的X射线束，室修正因子在1.023±0.03至1.018±0.001之间变化。在实验不确定度范围内，这些值与NE2571室的值一致。描述了NE2561室由于杆、防水套以及室对模体材料的替代所引起的修正。
