水中总α放射性测量的制源厚度及效率校准研究

目的 研究制源厚度及效率校准对水中总α放射性测量结果的影响。方法 分别采用²⁴¹Am和天然铀标准溶液加入水样制备样品源、²⁴¹Am标准物质粉末直接铺样制备标准源，研究质量厚度与探测效率的关系、质量厚度对样品源净计数率的影响、有效厚度的测定以及有效厚度法和厚源法对同一水样测量结果的影响。结果 在水样中分别加入²⁴¹Am和天然铀标准溶液测得水样的有效厚度均与经验值4 mg/cm²基本一致；水样的α计数率在质量厚度为2A~5A mg/cm²时近似处于线性增长趋势，>10A mg/cm²时计数率基本平稳不再增加（A为样品盘面积，cm²）；分别采用有效厚度法和厚源法测量同一水样的总α放射性，铺样在质量厚度为10A mg/cm²时，两种方法的结果最一致。结论 在分析水样总α放射性时，为减少因样品源质量厚度不同引起的总α计数率偏差、因标准源质量厚度不同所致的探测效率偏差，铺样厚度均选择为10A mg/cm²较为合理。

Study on source thickness and counting efficiency calibration for determining gross alpha activity in drinking water

Objective To study the influence of source thickness and counting efficiency calibration on the measurement of gross alpha activity in water. Methods ²⁴¹Am and natural uranium reference materials were spiked in drinking water to prepare source on a planchet with different thickness, for counting alpha activity on the planchet. Results The effective thickness measured by spiking ²⁴¹Am or uranium standard solution in water sample was consistent with the empirical value of 4 mg/cm². The alpha counting rate was in a linear increase trend from 2A-5A mg/cm² and was basically stable and no longer increase when thickness was higher than 10A mg/cm² (A was area of planchet). The result calculated by effective thickness method and thick source method were in good agreement when thickness was 10A mg/cm². Conclusions In order to reduce the deviation of gross alpha counting rate caused by the source thickness and counting efficiency calibration, the source thickness is recommended to be 10A mg/cm².