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

Evaluate Stray Neutron Distribution in Medical Cyclotron Vault Room by Neutron Activation Analysis Approach.

林傑彬(Jye-Bin Lin)

指導教授 ： 陳健懿

中山醫學大學/醫學院/醫學研究所/博士(2009年)

摘要

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。

關鍵字

中子 ； 迴旋加速器 ； 中子活化分析法 ； 銦片 ； 最小可測值 ； 信賴區間

並列摘要

Stray neutron distribution in a medical cyclotron vault room was
evaluated by neutron activation analysis (NAA). Neutrons were generated
in the production of radioactive nuclides, such as 18F, 11C, 13N and 15O, for
diagnostic usage. Indium foil was adopted to evaluate the stray fast and
thermal neutron intensity based on 115In(nf, n’)115mIn and 115In(nth,
γ)116m1In reaction, respectively. The indium foils were weighed sealed and
placed at 62 points around the 6.7×8.2 m2 cyclotron room. Additionally,
each indium foil was exposed for over 80 minutes during cyclotron
operation and γ-peaks were analyzed using an HPGe detector to evaluate
the number of stray fast (Φf) or thermal (Φth) neutrons. The minimum to
maximum 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. The
minimum detectable limit for stray neutrons was included herein to
demonstrate the reliability. Accordingly, 60 and two points, respectively,
the confidence level associated with the reported intensities of fast and
thermal neutrons reached 95%. The low qualified ratio in the evaluation
of stray thermal neutrons might have been caused by either the high
Compton scattering plateau or the low intensity of the gamma-ray peak in
the relevant spectrum.

並列關鍵字

stray neutron ； cyclotron ； neutron activation analysis ； indium foil ； minimum detectable limit ； confidence level

參考文獻

行政院衛生署2009年6月17日記者會發佈資料.

American Association of Physicists in Medicine (AAPM), 1986. Neutron measurements around high energy X-ray radiotherapy machine. AAPM No.19, New York.

Assatel, O.Z., Spyrou, N. M., 1997. Characterisation of mixed radiation field produced in medical linear accelerators using foil activation technique. J. Radioanal. Nucl. Chem., 217(2)255-260.

Carroll, L.R., 1987. Estimating the Radiation Source Term for PET Isotope Targets. Carroll & Ramsey Associates,Berkeley, CA 94710, USA.

Chen, C.Y., Wei, Y.Y., Changlai, S.P., et al., 2003. Concentration of bromine and iodine in Chinese medical herbs determined via epithermal neutron activation analysis. J. Radioanal. Nucl. Chem., 257(2)405-410.