Takata N, Begum A
Ionizing Radiation Section, NMIJ, AIST, Tsukuba, Ibaraki 305-8568, Japan.
Radiat Prot Dosimetry. 2008;130(4):410-8. doi: 10.1093/rpd/ncn093. Epub 2008 Apr 8.
The signal charge from a free air ionisation chamber for the measurement of air kerma and exposure consists of not only the charge of ion pairs produced by secondary electrons (i.e. photoelectrons, Compton electrons and Auger electrons), but also the charge of the secondary electrons and single and multiple charged ions formed by the release of the secondary electrons. In the present work, correction factors for air kerma and exposure for the charge of the secondary electrons and ions were calculated for photons with energies in the range from 1 to 400 keV. The effects of an increase in the W value of air for low-energy electrons were also taken into consideration. It was found that the correction factors for air kerma and exposure have a maximum value near a photon energy of 30 keV; in the lower energy region, the correction factor for exposure monotonically decreases with a decrease in photon energy except for a small dip due to K-edge absorption by argon atoms in air. The values of the correction factors were found to be 0.9951 and 0.9892, respectively, for a spectrum with a mean energy of 7.5 keV, the reference X-ray spectrum with the lowest mean energy in ISO 4037-1. The air kerma correction is smaller than that for exposure, because for air kerma the signal due to the charge of secondary electrons and ions is partly compensated by the decrease in the number of ion pairs produced by the secondary electrons due to the increase of the W value of air for lower energy electrons.
用于测量空气比释动能和照射量的自由空气电离室的信号电荷，不仅包括由次级电子（即光电子、康普顿电子和俄歇电子）产生的离子对电荷，还包括次级电子以及由次级电子释放形成的单电荷和多电荷离子的电荷。在本工作中，针对能量范围为1至400 keV的光子，计算了空气比释动能和照射量对于次级电子和离子电荷的修正因子。同时也考虑了低能电子情况下空气W值增加的影响。结果发现，空气比释动能和照射量的修正因子在光子能量接近30 keV时具有最大值；在较低能量区域，照射量修正因子除了因空气中氩原子的K边吸收导致的小波动外，随光子能量降低单调减小。对于平均能量为7.5 keV的能谱（ISO 4037-1中平均能量最低的参考X射线能谱），修正因子的值分别为0.9951和0.9892。空气比释动能修正小于照射量修正，因为对于空气比释动能，由于低能电子空气W值增加导致次级电子产生的离子对数量减少，部分抵消了次级电子和离子电荷产生的信号。
