Measurement of radioactivity in water by vacuum rotary evaporation

[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 241Am and 90Sr 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. © 2023 Press of Tsinghua University. All rights reserved.