Lempert G D, Nath R, Schulz R J
Med Phys. 1983 Jan-Feb;10(1):1-3. doi: 10.1118/1.595288.
The accuracy of high-energy x-ray dosimetry can be improved by taking account of differences between the compositions of the chamber wall and the buildup cap or dosimetry phantom. The fraction of the ionization due to secondary electrons arising in the chamber wall has been determined as a function of wall thickness for 60Co gamma rays and x rays in the range of 2-25 MV for Farmer-type chambers. Secondary electrons arising in the accelerator head were removed from the x-ray beams by a magnetic field placed just in front of the ionization chamber. For 60Co gamma rays, the fraction increases from 40% to 100% as the wall thickness increases from 0.05 to 0.55 g cm-2. For a 0.05 g cm-2 wall, fraction decreases from 60% to 10% as the x-ray energy is increased from 2 to 25 MV. Limited data obtained with different chambers suggest that the fraction is independent of chamber wall composition when the thickness is expressed in g cm-2.
通过考虑电离室壁、积累帽或剂量测定体模的成分差异，可以提高高能X射线剂量测定的准确性。对于Farmer型电离室，已确定了60Coγ射线和2 - 25 MV范围内X射线在电离室壁中产生的二次电子引起的电离分数与壁厚度的函数关系。通过放置在电离室正前方的磁场，将加速器头部产生的二次电子从X射线束中去除。对于60Coγ射线，随着壁厚度从0.05增加到0.55 g/cm²，该分数从40%增加到100%。对于0.05 g/cm²的壁，随着X射线能量从2增加到25 MV，该分数从60%降低到10%。用不同电离室获得的有限数据表明，当厚度以g/cm²表示时，该分数与电离室壁成分无关。
