Zoetelief J, Engels A C, Broerse J J, Mijnheer B J
Phys Med Biol. 1980 Nov;25(6):1121-31. doi: 10.1088/0031-9155/25/6/010.
The effective point of measurement for spherical ionisation chambers was determined free-in-air for d + T neutrons and inside a human water phantom for d + T neutrons and 60Co gamma rays. Measurements were performed with spherical chambers of different diameters and a disc-type chamber. For spherical ion chambers free-in-air, the geometrical centre of the chamber was found to be the point of measurement for sufficiently great distances from the neutron source. In-phantom for spherical ion chambers, displacement correction factors of 1 - (0.25 +/- 0.06) x 10(-2) r for d + T neutrons and of 1 - (0.37 +/- 0.04) x 10(-2) r for 60Co gamma rays (r being the cavity radius in mm) were observed. The differences in displacement for d + T neutrons and 60Co gamma rays can be attributed to differences in attenuation and scatter characteristics of these radiation qualities. The replacement of phantom material by the cavity of an ion chamber was simulated by introducing Styrofoam spheres of comparable dimensions into the phantom. The dose distributions measured over the cavities could explain the different displacement corrections for 60Co gamma rays and 15 MeV neutrons.
对于球形电离室，测量的有效点是在空气中针对d + T中子确定的，而对于d + T中子和60Co伽马射线，则是在人体水模体内部确定的。测量使用了不同直径的球形电离室和一个盘形电离室。对于空气中的球形电离室，发现对于距中子源足够远的距离，电离室的几何中心是测量点。对于球形电离室在模体中，观察到d + T中子的位移校正因子为1 - (0.25 ± 0.06)×10⁻²r，60Co伽马射线的位移校正因子为1 - (0.37 ± 0.04)×10⁻²r（r为腔半径，单位为mm）。d + T中子和60Co伽马射线在位移上的差异可归因于这些辐射品质在衰减和散射特性上的差异。通过将尺寸相当的聚苯乙烯泡沫塑料球引入模体，模拟了用电离室的腔取代模体材料的情况。在腔上测量的剂量分布可以解释60Co伽马射线和15 MeV中子不同的位移校正。
