低本底α/β仪测量饮用水中总α放射性活度的实验室质量控制

doi:10.3964/j.issn.1000-0593(2019)07-2166-05

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Abstract 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.

Key words：Gross alpha activity; Volume activity; Drinking water; Quality control chat

Received: 2018-12-07 Accepted: 2019-04-10

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TH843

Corresponding Authors: SU Hui-lan E-mail: tiantian1986315@126.com

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2019)07-2166-05 OR https://www.gpxygpfx.com/EN/Y2019/V39/I07/2166