强X(γ)辐射场中的低中子剂量监测技术研究

X-rays produced by medical accelerators have been widely used in radiotherapy. During the process of therapy, a certain level of photonuclear neutrons may be produced by the (γ,n) and (γ,2n) reactions on elements in surrounding materials. These neutrons are potentially harmful for medical personnel and patients. It has been a difficult problem to evaluate the neutron radiation dose levels of medical accelerator and the occupational health effects on related personnel, so as to effectively control the neutron dose received by the staff and patients. Bubble Neutron detector (BND) is one of the most promising neutron dosimeters to solve the above-mentioned problem for its stable energy response over wide range of neutron energy, low detection limit, high detection sensitivity, real-time monitoring of neutron dose and "insensitivity to γ rays". However,there is so far no experimental confirmation on whether BND is responsive to γ rays with the energy and dose higher than 3 MeV and 1Gy. The aims of this study are: (1)To study the prescription for making NBD insensitive to γ rays over wide energy range. (2)To determine the γ-ray dose and energy threshold, above which BND is responsive, and the detection limit of BND for neutrons. (3)To clarify whether photonuclear neutrons exist and how they are distributed (if they do exist) in the treatment plane of 6 and 10 MV medical accelarators. A method for measuring low neutron dose in intense high energy and dose X(γ) radiation field is expected to be established after the completion of the project.

医用电子直线加速器放疗过程中，存在于X射线场中与治疗无关的中子辐射剂量影响病患的治疗和工作人员的健康。由于没有对强X射线辐射场背景下低中子剂量的理想监测方法，上述的污染中子剂量测定是长期困扰放疗的难题之一。气泡中子探测器（BND）具有宽能区响应、高灵敏度、直读、"对γ射线不灵敏"等优势， 是解决上述问题最有希望的仪器之一，然而，BND对3 MeV、1Gy以上γ射线是否有响应，迄今尚无实验研究。本工作的研究内容是：1)优化室温条件下"宽能区γ射线不灵敏"的BND过热液体配方；2)测量BND对γ射线响应的阈能和阈剂量，以及中子剂量探测限；3)6和10MV医用加速器治疗室内有无光核中子，及其剂量分布测量(若有的话)。本工作的完成将首次提供一种高能高剂量X(γ)射线辐射场内低中子剂量的监测方法，为解决多种射线混合辐射场中的中子剂量选择性监测提供技术基础，为辐射防护和剂量学发展做出贡献。

医用电子直线加速器放疗过程中，存在于X 射线场中与治疗无关的中子辐射剂量影响病患的治疗和工作人员的健康。由于没有对强X 射线辐射场背景下低中子剂量的理想监测方法，上述的污染中子剂量测定是长期困扰放疗的难题之一。气泡中子探测器（BND） 具有宽能区响应、高灵敏度、直读、“对γ 射线不灵敏”等优势， 是解决上述问题最有希望的仪器之一，然而，BND 对3 MeV、1Gy 以上γ 射线是否有响应，迄今尚无实验研究。本工作通过测量BND 对γ 射线响应的阈能和阈剂量，以及中子剂量响应。得出如下四个结论：.1）气泡中子探测器对高能、高剂量率γ 射线有响应，能量越高，响应系数越大；剂量率越大，响应系数越大，但增幅不同，并初步分析响应机制。.2）探测器剂量率小于17.43 Gy/h，剂量小于4Gy，能量小于1332 keV的 γ 射线不响应.3）对能量大于等于6MeV的γ 射线有响应，在3.0Gy/min 剂量率，2.0Gy γ 剂量点条件下，探测器γ 射线响应系数约为26Bub/Gy，阈剂量约为38mGy。.4）证明6MV电子直线加速器无污染中子产生，回答了先期气泡探测器在6MV 能量档的医用电子直线加速器上产生的信号是中子还是γ 信号的疑问。.本工作的完成将首次提供一种高能高剂量X(γ ) 射线辐射场内低中子剂量的监测方法，为解决多种射线混合辐射场中的中子剂量选择性监测提供技术基础，为辐射防护和剂量学发展做出贡献。