Hakanen Arvi, Siiskonen Teemu, Turunen Jani
Radiation Metrology Laboratory, STUK-Radiation and Nuclear Safety Authority, PO Box 14, FI-00881 Helsinki, Finland.
Radiat Prot Dosimetry. 2009 Jan;133(2):81-8. doi: 10.1093/rpd/ncp021. Epub 2009 Feb 27.
137Cs sources are widely used in calibrations of radiation protection dosimeters and doserate meters. In these calibrations, conversion coefficients from the air kerma to various dose equivalent quantities are needed. These conversion quantities are functions of photon energy. In this work, Monte Carlo simulations and measurements are used for quantifying the energy distributions of the photon fluence from two 137Cs sources used for calibration purposes. The results indicate that the standard set of conversion coefficients, obtained from a monoenergetic spectrum, do not sufficiently take into account the scattered radiation (most of which is scattered by the source itself): For the large (14x15.6 mm) and small (3.5x3 mm) 137Cs sources, the simulated values for the conversion coefficients H'(0.07)/Ka and H*(10)/Ka (which were nearly equal for the both quantities) showed around 1.0 and 1.5% deviations from the ISO standard value 1.20 (for the large and small sources, respectively). Similarly, the conversion coefficient Hp(10)/Ka showed around 1.5 and 2.5% deviations from the ISO standard value 1.21 for the large and small 137Cs sources, respectively, whereas Hp(0.07)/Ka had the values 1.22 and 1.23. The amplitude of the variation of the conversion coefficients due to varying 137Cs source size did not exceed 1%. Thus, the overall uncertainty (with coverage factor k=2) due to varying source size for the average values of the conversion coefficients can be estimated as low as 0.6%.
137Cs源广泛应用于辐射防护剂量计和剂量率仪的校准。在这些校准中，需要空气比释动能到各种剂量当量量的转换系数。这些转换量是光子能量的函数。在本工作中，蒙特卡罗模拟和测量用于量化用于校准目的的两个137Cs源的光子注量能量分布。结果表明，从单能谱获得的标准转换系数集没有充分考虑散射辐射（其中大部分是由源本身散射的）：对于大尺寸（14×15.6 mm）和小尺寸（3.5×3 mm）的137Cs源，转换系数H'(0.07)/Ka和H*(10)/Ka的模拟值（这两个量几乎相等）分别比ISO标准值1.20偏离约1.0%和1.5%（分别对应大尺寸和小尺寸源）。同样，对于大尺寸和小尺寸的137Cs源，转换系数Hp(10)/Ka分别比ISO标准值1.21偏离约1.5%和2.5%，而Hp(0.07)/Ka的值分别为1.22和1.23。由于137Cs源尺寸变化导致的转换系数变化幅度不超过1%。因此，对于转换系数平均值，由于源尺寸变化导致的总体不确定度（包含因子k = 2）可估计低至0.6%。
