利用减压旋蒸技术测量水样放射性活度

Measurement of radioactivity in water by vacuum rotary evaporation

在低活度浓度水样的放射性活度测量中,直接取样测量的结果不稳定也不准确。为了提高低活度浓度水样放射性活度测量结果的准确性,该文提出了一种基于减压旋蒸技术的自动浓缩方法。为验证该方法的可行性,设计了一套自动浓缩装置。通过实验对浓缩条件进行研究,以提高浓缩速度、减少浓缩过程中核素损失。实验结果表明:设置浓缩条件为真空2.0~4.0 kPa、冷凝循环机制冷温度-5℃~0℃、蒸发器旋转速度50 r/min,设置水浴温度为初始50℃、50 min后升温至60℃,1 L水样所需浓缩时间大约为70 min。蒸干后选择12 mL 0.05 mol/L硝酸作为洗脱液进行洗脱。该自动浓缩方法对于241Am的平均回收率达到70%以上,90Sr的平均回收率可达到80%。该方法的优点在于能够实现水样浓缩流程的自动化,浓缩速度快且回收率较高。

[Objective]The measurement of radioactivity in water samples with low activity by direct sampling is unreliable and inaccurate.We designed an automatic concentration method based on the vacuum rotary evaporation process to increase the ease and accuracy of such measurements.[Methods]In this method,the water samples were first evaporated to dryness by vacuum rotary evaporation and then washed with dilute nitric acid.The used nitric acid was then sampled and measured by a liquid scintillation spectrometer.An automatic concentration device was designed for these measurements.The optimum water bath temperature for concentrating the samples was determined experimentally.The relationship between the volume of dilute nitric acid used and the proportion of nuclides recovered was studied to improve the yield of the cleaning process.Given that a portion of the residues is likely to adhere to the inner wall of the container during the evaporation process,an experiment was designed to study the efficacy of our procedure to clean these residues using 12mL of 0.05mol/L nitric acid by determining the number of repeated cleanings required for the container to return to the normal background level after evaporating solutions containing ^(241)Am and ^(90)Sr with activitiy concentrations of 20,5,and 1 Bq/L.After the water samples were automatically concentrated,they were measured using a liquid scintillation spectrometer,and the recovery rates of the two nuclides were calculated at different activity concentrations.[Results]Using vacuum rotary evaporation with a vacuum of 2.0-4.0 kPa,a condenser temperature of-5℃-0℃,a rotation speed of 50 r/min,and an initial water bath temperature of 50℃,which was raised to 60℃after 50min,it took about 70min to concentrate 1 L of the water sample.To reduce the post-cleaning residue and avoid contaminating subsequent water samples,the evaporation should be washed with 12 mL of 0.05 mol/L dilute nitric acid before washing with pure water.After evaporating a 1 L water sample with a total activity of less than 5 Bq,two to three cleaning operations were needed,while after evaporating a 1 L water sample with a total activity of 20 Bq,about five cleaning operations were needed.Using 12mL 0.05mol/L nitric acid for elution could get satisfactory elution effects.The average yield of 241Am by the automatic concentration method reached more than 70%,and the average recovery rate of 90Sr reached about 80%.[Conclusions]This paper proposes an automatic concentration method based on the vacuum rotary evaporation process,which has not only a quick turnaround time but also high yield.