总α放射性活度检测可避免繁琐的低水平放射性核素鉴定, 是饮用水放射性水平的初筛监测手段之一。 近年调查显示： 我国饮用水总α放射性活度稳定保持在较低水平。 低水平的α放射性活度检测需要高质量实验室质量控制, 以保证检测结果的准确度。 实验采用低本底α/β仪, 以α闪烁体为探测器, 吸收样品核辐射α粒子能量, 使有机闪烁体分子ZnS(Ag)发射荧光, 通过统计单位时间内的闪烁体发射荧光数目正比于核衰变数目, 由此感应有效厚度样品层辐射的α粒子计数信号, 对饮用水中总α放射性活度浓度检测。 首先, 实验以表面α粒子发射率为2～20粒子数/s(2π 方向)的电镀源测定本底α计数效率(CPS)。 然后, 在最优化的本底值、 工作源效率、 串道率等参数条件下, 运用标准曲线法测定标准源α计数效率(ε)。 最后, 结合CPS和ε代值计算质控样总α体积活度、 计数平均值(或总体积活度平均值)和标准偏差s, 并以±s为上辅助线(upper auxiliary limit, UAL)和下辅助线(lower auxiliary limit, LAL), 以±2s为上警告线(upper warning limit, UWL)和下警告线(lower warning limit, LWL), 以±3s为上控制线(upper control limit, UCL)和下控制线(lower control limit, LCL), 绘制本底α计数、 标准源α计数和质控水样总α体积活度的均数质量控制图, 以考察CPS和ε对质控水样总α体积活度测量质量控制的影响。 数据结果显示, 以电镀239Pu为工作源, 以241Am为标准源, 样品放置时间24 h, 测量时间60 min, 铺样厚度4 mg·cm-2时, 本底α计数率CPS=0.000 37 s-1, 工作源探测效率η=94.35%, α→β串道率Fα=0.41%, 标准源计数效率ε=7.25%(Y=1.323X-5.285, R2=0.991 5)。 统计结果显示： 40份空盘本底α计数的受控范围为-1.61~5.82, 33个点落在UAL与LAL范围内, 2个点落在UWL与UAL范围内, 3个点落在LWL与LAL范围内, 2个点落在UWL与UCL之间, 本底测量受控良好; 24份标准源α计数的受控范围523.7~644.3, 14个点落在UAL与LAL范围内, 5个点落在UWL与UAL范围内, 5个点落在LWL与LAL范围内, 标准源测量受控良好; 20份质控水样总α体积活度受控范围为0.007 91~0.057 86 Bq·L-1, 11个点在UAL与LAL范围内, 5个点在UWL与UAL范围内, 3 个点在LWL与LAL范围内, 1个点在LWL与LCL之间, 质控水样测量实验室质量控制良好。 因此, 以α闪烁探测器对低水平α放射性计数测量时, 控制α本底计数和标准源计数效率, 这两个α统计计数中的主要不确定度来源, 可以实现水样中总α放射性活度检测的有效实验室质量控制。

The gross alpha activity assay generally serves as one of screening approaches on gross radiation level to avoid the cumbersome radionuclide identification at low radioactivity level. Since the radioactivity investigation in China showed that the gross alpha activity in drinking water maintained a low level, the high-efficient quality control in laboratory should be provided in gross alpha activity assay to guarantee the accuracy. In this experiment, the low-background gross alpha/beta counter was employed with alpha scintillation as probe to detect the gross alpha activity in drinking water. The energy of alpha particles emitted from analyte sample was absorbed by alpha scintillation and transferred to organic scintillation with fluorescence emission on the probe, by which the nuclear radiation was converted into the flicker of fluorescence. The number of flickers is proportional to the number of kernel decay per unit time for counting the alpha particles numbers emitted from analyte sample layer. At first, the electroplating source with alpha particle emissivity of 2~20 particles numbers per second in 2π direction vector was used to determinate the alpha background counting rate (CPS). Then the alpha background count, the detection efficiency of work source (η), the common background beta to alpha ratio (Fα) were optimized in the experiment. Upon these optimized parameters, the standard source counting rate (ε) was calculated by fitting calibration curve. Finally, combining the value of CPS and ε, the gross alpha volume activity in controlled water samples was calculated as mathematical model. Based on these data statistics, the quality control on alpha background count, alpha count of standard source and the gross alpha volume activity were investigated for evaluating the influence of CPS and ε on the quality control of gross alpha volume activity assay. The results showed that while the spread sample with the density of 4 mg·cm-2 was placed for 24 h and measured with 239Pu as work source and 241Am as standard source for 60 min, the CPS could be obtained as 0.000 37 s-1 upon the η of 94.34%, the Fα of 0.41% and ε of 7.25% (Y=1.323X-5.285, R2=0.991 5). And the alpha backgrounds of 40 blank panel samples count over the range of -1.61~5.82. Among these samples, 33 of samples count in the controlled scope of upper auxiliary limit (UAL) and lower auxiliary limit (LAL). 2 of samples count in the controlled scope of upper warning limit (UWL) and UAL. 3 of samples count in the controlled scope of lower warning limit (LWL) and LAL. 2 of samples count in the controlled scope of UWL and upper control limit (UCL). The alpha background was well controlled. And the alpha particle numbers of 24 standard source samples count over the range of 523.7~644.3. Among these samples, 14 of samples count in the controlled scope of UAL and LAL. 5 of samples count in the controlled scope of UWL and UAL. 5 of samples count in the controlled scope of LWL and LAL. The alpha count of standard source was well controlled. Moreover, the gross alpha volume activity of 20 drinking-water samples distributed over the range of 0.007 91~0.057 86 Bq·L-1. Among these water samples, 11 of samples dispersed in the controlled scope of UAL and LAL. 5 of samples dispersed in the controlled scope of UWL and UAL. 3 of samples dispersed in the controlled scope of LWL and LAL. Only one sample dispersed in the scope of LWL and lower control limit (LCL). The gross alpha activity detection was well controlled in drinking-water samples. Therefore, while the gross alpha activity at low level was detected by using alpha scintillation probe, controlling two main uncertainty source of alpha background and standard source counting rate was an effective strategy in quality control of gross alpha activity assay in laboratory.