以中子活化分析法評估醫用迴旋加速器室內的中子分佈情形

以中子活化分析法(Neutron Activation Analysis, NAA)評估中山醫學大學附設醫院迴旋加速器室內中子(neutron)分佈情形，放射製藥生產的過程中產生中子，例如：核醫在診斷時使用的18F, 11C, 13N , 15O，當迴旋加速器運轉，我們以中子與銦片進行115In(nf,n’)115mIn 及115In(nth,γ)116m1In 核反應，分別來評估快中子與熱中子的通率(flux)。將銦片經稱重、密封、佈點於迴旋加速器室內的62 個位置，另外每片銦片需在迴旋加速器的運轉中，被照射超過80 分鐘，再使用高純度鍺型偵檢器(HPGe)量測加馬射線去解算快中子和熱中子的通率。本論文發現快中子和熱中子的通率由小至大分別為(3.47±0.11)×103 至(1.06±0.21)×104 n⋅cm-2/s，9 至965n⋅cm-2/s。在此，包含的中子的最小可測值(Minimum Detectable Limit, MDL)，說明了本方法的可適性。相對應地，在測量62 點中快中子和熱中子強度相關的信賴區間(confidence level)分別達到95%。而若能圖譜裡的高能康普吞散射或較低強度γ 射線波峰，會導致熱中子通率可以量測的比率降低。

Stray neutron distribution in a medical cyclotron vault room wasevaluated by neutron activation analysis (NAA). Neutrons were generatedin the production of radioactive nuclides, such as 18F, 11C, 13N and 15O, fordiagnostic usage. Indium foil was adopted to evaluate the stray fast andthermal neutron intensity based on 115In(nf, n’)115mIn and 115In(nth,γ)116m1In reaction, respectively. The indium foils were weighed sealed andplaced at 62 points around the 6.7×8.2 m2 cyclotron room. Additionally,each indium foil was exposed for over 80 minutes during cyclotronoperation and γ-peaks were analyzed using an HPGe detector to evaluatethe number of stray fast (Φf) or thermal (Φth) neutrons. The minimum tomaximum numbers of fast and thermal neutrons were (3.47±0.11)×103 to(1.06±0.21)×104 n⋅cm-2⋅sec-1 and 9 to 965 n⋅cm-2⋅sec-1, respectively. Theminimum detectable limit for stray neutrons was included herein todemonstrate the reliability. Accordingly, 60 and two points, respectively,the confidence level associated with the reported intensities of fast andthermal neutrons reached 95%. The low qualified ratio in the evaluationof stray thermal neutrons might have been caused by either the highCompton scattering plateau or the low intensity of the gamma-ray peak inthe relevant spectrum.