準確的量測游離輻射在生物體內所造成的劑量是十分重要的工作。要精確計算人體內所接受的輻射劑量，可以用游離腔在水假體中以同樣的照射條件來模擬。但是游離腔量測到的電量訊號必須經過準確的轉換因子的計算，才能得到劑量的單位。早期，要求得這些轉換因子，游離腔必須在國家游離輻射標準實驗室經過鈷60射束空氣克馬（air Kerma）校驗法校驗得到鈷60射束空氣克馬轉換因子，再藉由某些經過仔細驗證過的劑量計算議定書；像是美國醫學物理師協會（American Association of Physicists in Medicine, AAPM）於1984年發表的TG-21號議定書；根據實際射束的射質及量測儀器的材料，逐項求得不同的轉換因子。但是此種複雜的轉換方法使得不確定度高達4%。1999年美國醫學物理師協會發表TG-51號議定書，將此過程簡化，直接由國家游離輻射標準實驗室提供使用者游離腔的鈷60水中劑量轉換因子，藉以降低不確定度。據此，本研究與國家游離輻射標準實驗室研究人員合作，以原級方式製作圓盤狀平行板式游離腔，仔細求得各項修正因子後，置於水假體中直接量取標準實驗室的鈷60水中劑量。經過比對的過程，將此游離腔定為日後鈷60水中劑量校驗的原級標準件。本研究之進行將可提升國內游離輻射吸收劑量計算的準確度。尤其對高能放射治療，在劑量分布日求精確的同時，本研究將有助於醫療品質的提昇。

In the field of ionizing radiations, the absorbed dose is closely related to the biological effects of radiation. Accurately determine the absorbed dose to a human body is very important. Water is of special interest as an absorbing material because it is very similar to human tissue. So, the absorbed dose to human tissue may be obtained by measuring the dose distribution in water under same condition. For clinical reference dosimetry of external beam radiation therapy, a protocol written by Task Group 51 of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) uses ion chamber with absorbed-dose-to-water calibration factor, which is traceable to national primary standards. This protocol represents a major simplification compared to the AAPM’s TG-21 protocol in the sense that large tables of stopping-power ratios and mass-energy absorption coefficients are not needed and the user dose not need to calculate any theoretical dosimetry factors. According the requirement, one of the duties of the national standards laboratory is to establish standards for absorbed dose to water. This study cooperate with national standards laboratory for developing an ionometric dosimetry system for the determination of absorbed dose to water for Co-60 gamma rays, which will be the national primary standard. This system consists of a cubic perspex water phantom and a graphite cylindrical ionization chamber. Reliable estimation of the perturbation corrections, which account for the presence of the chamber inside the water phantom, allows an accurate determination of absorbed dose. This study may improve the accuracy of calibrating photon and electron beams for radiation therapy.

1.前言 -----------------------------------------------5
2.原級標準輻射度量 -----------------------------------8
2.1卡計 --------------------------------------------8
2.2游離腔 ------------------------------------------12
2.2.1游離腔輻射度量原理 ---------------------------12
2.2.2空氣克馬度量原理 -----------------------------18
2.2.3水中吸收劑量度量原理 -------------------------24
3.游離腔之結構與性能 ---------------------------------27
3.1 游離腔構造 -------------------------------------27
3.2 游離腔空腔體積----------------------------------28
3.3 游離腔性能 -------------------------------------34
4.水假體及水密封套之設計 -----------------------------37
5.空氣克馬之測量 -------------------------------------40
5.1修正因子及物理參數-------------------------------40
5.2量測結果與比對 ----------------------------------45
6.水中吸收劑量之測量 ---------------------------------51
6.1修正因子及物理參數 ------------------------------52
6.2量測結果與比對 ----------------------------------54
7. 比對與不確定度分析 --------------------------------58
8. 討論 ----------------------------------------------63
9. 結論 ----------------------------------------------66
參考文獻 ---------------------------------------------67
附錄一 -----------------------------------------------70
附錄二 -----------------------------------------------72

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