利用成對游離腔測量混合輻射場中輻射劑量率

Radiation Dose Rate Measurement in a Mixed Radiation Field Using Paired Ionization Chambers

林怡君(Yi-Chun Lin)

指導教授 ： 周文采 ； 江祥輝 ； 劉淵豪

國立清華大學/原子科學院/生醫工程與環境科學系/博士(2013年)

摘要

游離腔為全球用於定義輻射場絕對劑量的偵檢器，而中子射束往往伴隨光子產生形成複雜的中子、光子混合場。一般游離腔對於光子和中子均會有響應，因此如何在混合場分別量測光子和中子劑量變成相當困難。國際上建議使用成對游離腔方法，利用兩支不同中子靈敏度的游離腔，依據其光子和中子相對響應不同，區分出混合場的光子和中子劑量。本研究目的為建立一套國內準確且完整的成對游離腔技術。
本研究採用的成對游離腔一支為鎂壁填充氬氣(Mg(Ar))，另一支為A150組織等效塑膠壁填充甲烷為基組織等效氣體(TE(TE))游離腔。首先架構游離腔幾何模型，詳細分析討論其響應，進行校正並補齊劑量議定書中所缺乏包含射束品質轉換因子的各項參數，修訂成對游離腔劑量推導數學式，修正活化汙染對游離腔訊號的影響，利用蒙地卡羅方法直接計算游離腔靈敏度。本研究先經由箔片活化分析反應率驗證清華水池式反應器(THOR)硼中子捕獲治療(BNCT)出口面中子射源項後，接著，以成對游離腔實際度量射束在空氣中和假體不同深度位置的光子和中子劑量成份，並與計算結果相比較，本論文同時也對測量結果進行不確定度評估。
關於TE(TE)游離腔的能依響應，蒙地卡羅程式MCNP與其他EGSnrc、FLUKA和GEANT4的計算結果，以及7個光子場(Co-60, keV和MeV等級X射線)和2個MeV等級電子場的量測結果相當一致，Mg(Ar)游離腔MCNP計算結果在低能光子出現響應低估的較大偏差，但對於氫捕獲作用2.2 MeV光子為主的硼中子捕獲治療射束，可以適用。觀察由2009至2011年的歷年校正數據，游離腔響應可能發生變動，顯示定期校正的重要性與必須性。本論文計算出的兩支游離腔個別的能量依存光子和中子靈敏度，可以提供混合場量測時，計算量測位置克馬加權光子和中子靈敏度使用。利用成對游離腔測量BNCT射束在假體中之光子和中子劑量過程中，採用蒙地卡羅計算之光子和中子靈敏度較引用文獻建議之靈敏度數據，所得中子劑量量測值與針對射源和假體的計算值，吻合情況有高達十幾%的改善。此外，在本論文研究過程中，我們也完成了下列幾項研究應用：(1)利用Mg(Ar)游離腔搭配不同厚度增建帽，對BNCT射束出口光子射源項能量分布進行調整；(2)同時測量THOR和歐盟Petten聯合研究中心BNCT射束中子和光子劑量成份並進行比對；以及(3)利用成對游離腔執行每次BNCT臨床試驗射束品質管制和品質保證。

關鍵字

成對游離腔方法 ； 蒙地卡羅計算 ； 混合場劑量 ； 硼中子捕獲治療 ； 清華水池式反應器

並列摘要

Neutron beams adopted in radiobiology and radiotherapy always accompany secondary photons and charged particlesm which lead to a need of mixed radiation dosimetry. Paired ionization chambers method is usually used for dose measurement in mixed neutron/photon fields to separate the photon and neutron dose components which have different relative biological effectiveness. Dosimetry as one of beam characteristics is important because of the correlation with patient safety and radiation protection. Although this technique has been widely used for about 50 years, parameters and corrections involved in the dose derivation are still problematic and need a further and thorough study. Therefore, the purpose of this study is to establish a more accurate, complete and high quality paired ionization chambers detection system, and used in the boron neutron capture therapy (BNCT) beam at the Tsing Hua Open-pool Reactor (THOR).
A magnesium chamber with argon gas (Mg(Ar)) and the other A150 tissue-equivalent plastic chamber with tissue-equivalent gas (TE(TE)) were used to determine the photon and neutron doses. The contents of this Ph. D. thesis work are consist of the following items: (A) establishment of verified chamber models and detailed response analysis including energy, angular and thickness dependent dose responses; (B) calibration and calculation of the lack parameters such as beam conversion factor according to the dosimetry protocol principle; (C) modification of dose derivation, activation contamination correction and determination of neutron and gamma-ray sensitivities; (D) neutron source verification based on activation detector reaction rates; (E) neutron and gamma-ray spectra calculation; and (F) measurements and verifications of the neutron and gamma-ray dose rates and corresponding uncertainty evaluation for measured values.
In this study, the MCNP based TE(TE) chamber model, comparing to EGSnrc, FLUKA, GEANT4 and measurement of 7 realistic photon fields (60Co, keV and MeV level X-rays) as well as two MeV level electron fields, had perfect outcome. In the low energy region, the MCNP based Mg(Ar) chamber underestimated the detector responses, but it is still the most ideal candidate regarding the application of BNCT dosimetry where the hydrogen capture 2.2 MeV photons are dominant.The energy dependent neutron and photon sensitivities of two chambers by using the modified dose derivation were also investigated. The differences of neutron doses between calculations and measurements reduced ~10 percents based on the Monte Carlo calculated sensitivities while compared to those from paper citions. Finally, results of this study were applied to beam photon spectrum adjustment from measurements of Mg(Ar) chamber with different thickness build up caps, inter-center dose comparison between BNCT facilities at the THOR and High Flux Reactor, and beam quality control & quality assurance in the of the THOR BNCT clinical trials.

並列關鍵字

Paired Ionization Chambers Method ； Monte Carlo Calculation ； Mixed Field Dosimetry ； BNCT ； THOR

參考文獻

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