某质子治疗系统三维水箱测量用水中^(7)Be和另一全能峰的发现与探讨

Discovery and discussion of^(7)Be and another full-energy peak in water from 3D water phantom of a proton therapy system

目的对某质子治疗系统三维水箱测量用水中发现的^(7)Be和短半衰期77.2 keV全能峰进行了初步探讨。方法依据《水中放射性核素的γ能谱分析方法》(GB/T 16140—2018)对某质子治疗系统三维水箱测量用水样品进行测量。结果2018年12月24日三维水箱测量用水样品中^(7)Be的活度浓度为1.30×10^(1)Bq·L^(−1),2019年3月22日其活度浓度为4.3 Bq·L^(−1);采样时(2018年12月19日)其活度浓度为1.41×10^(1)Bq·L^(−1)。2018年12月24日样品中77.2 keV全能峰的净峰面积为(683±45),测量时间为26123.02 s;2019年3月22日其净峰面积已降至本底水平(194±49),测量时间为86400.00 s。结论质子治疗系统三维水箱测量用水^(7)Be的产生,主要与高能中子与水中的氧发生散裂反应有关;另外,还发现了短半衰期77.2 keV全能峰。该样品中^(7)Be的活度浓度低于豁免水平,取样时其活度浓度未必是质控过程中的最大活度浓度。质子治疗系统放射性职业病危害评价应对三维水箱测量用水产生的感生放射性核素^(7)Be进行识别并适当评价。

Objective To discuss 7 Be and a 77.2 keV full-energy peak with short half-life found in the water sample from the 3D water phantom of a proton therapy system.Methods We measured the water sample from the 3D water phantom of a proton therapy system according to Determination of Radionuclides in Water by Gamma Spectrometry(GB/T 16140—2018).Results The activity concentration of 7 Be in the water sample was 1.30×10^(1)Bq·L^(−1)on December 24,2018;4.3×10^(1)Bq·L^(−1)on March 22,2019;and 1.41×10^(1)Bq·L^(−1)at the time of sampling on December 19,2018.On December 24,2018,the net peak area of the 77.2 keV full-energy peak in the sample was 683±45,and the measurement time was 26123.02 s;on March 22,2019,the net peak area decreased to the background level of 194±49,and the measure-ment time was 86400.00 s.Conclusion In the 3D water phantom of the proton therapy system,7 Be can be generated from the spallation reaction between high-energy neutrons and oxygen in water.In addition,we find a full-energy peak at 77.2 keV with short half-life.The activity concentration of 7 Be in the water sample is lower than the exemption level,but the activity concentration at sampling may not be the maximum activity concentration in the process of quality control.The in-ductive radionuclide 7 Be produced in the 3D water phantom should be identified and properly evaluated in the assessment of occupational radiation hazards of proton therapy system.