非准直点源透射法测量体源样品的线性衰减系数

体源样品的线性衰减系数是计算自吸收修正因子最重要的参数。本文针对未知体源样品的线性衰减系数测量问题，提出了非准直点源透射法。该方法直接把点源置于样品盒上方一定距离或直接放置在样品盒上方进行透射，对蒙特卡罗模拟透射率和实验测量透射率进行比较，反推待测体源样品的线性衰减系数。本文详细介绍了该方法的基本原理和计算过程，选取了5种不同密度的物质作为待测对象，采用3种不同能量的点源，对该方法进行了实验验证。研究结果表明，不同能量的γ射线下，该方法的实验结果和XCOM平台理论计算结果吻合程度好（＜8%），能方便计算出体源样品的线性衰减系数。该方法不受准直器以及探测器和样品尺寸、形状的限制，简单易行，便于各实验室推广使用。

Measurement of Linear-attenuation Coefficient of Body Source Using Non-collimated Point Source Transmission Approach

Linear-attenuation coefficient of body source is an important parameter for the calculation of the self absorption correction factor. In this work, the non collimated point source transmission method was introduced for the determination of the linear attenuation coefficient of unknown body source. The point source was directly placed atop, or at certain distance above the source container. By the comparison of the simulated transmission ratio with the experimental results, the linear attenuation coefficient of this body source was deduced. The principle and calculation process were presented in detail in the text. To test this method, five different substances with different densities were chosen as test subjects, and non collimated transmission experiments were conducted using three different point sources. It shows that the experimental results of this method agree well with the theoretical values given by the XCOM platform, and this method can provide the linear attenuation coefficients of body sources conveniently. This method avoids the need for collimator, and limitations posed by size and shape of detector and sample. It is simple and easy to be used in laboratories.