Kutcher G, Strubler K, Suntharalingam N
Med Phys. 1977 Sep-Oct;4(5):414-8. doi: 10.1118/1.594327.
The current method for determining absorbed dose from high-energy photons is to use in water a Cobalt-60 exposure-calibrated ionization chamber and Clambda. Recently questions have arisen about the correctness of currently recommended Clambda values and whether or not the Cobalt-60 buildup cap should be used for in-water dose measurements. The effect of the buidup cap on Clambda was obtained by measuring Farmer chamber response in water with and without the buildup cap. Results show no measurable change at Cobalt-60 or 4 MV but an increase of 1%-2% above 20 MV. The effects of the wall materials and the displacement factors on Clambda are also considered. It is shown that at high energies errors of 3%-5% can be made in determining dose in water using commercially available Farmer or Farmer-like chambers and currently recommended Clambda. The exact size of the error depends upon the materials and thicknesses of the wall and cap, the chamber cavity volume, and whether or not the buildup cap is used.
当前用于确定高能光子吸收剂量的方法是在水中使用经钴 - 60照射校准的电离室和 $C_{\lambda}$。最近，关于当前推荐的 $C_{\lambda}$ 值的正确性以及在水中剂量测量时是否应使用钴 - 60积累帽出现了一些问题。通过测量有和没有积累帽时水中 Farmer 电离室的响应，得到了积累帽对 $C_{\lambda}$ 的影响。结果表明，在钴 - 60或4兆伏时没有可测量的变化，但在高于20兆伏时增加了1% - 2%。还考虑了壁材料和位移因子对 $C_{\lambda}$ 的影响。结果表明，在高能情况下，使用市售的 Farmer 或类似 Farmer 的电离室以及当前推荐的 $C_{\lambda}$ 来确定水中剂量时，可能会产生3% - 5%的误差。误差的确切大小取决于壁和帽的材料及厚度、电离室腔体积以及是否使用积累帽。
