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研究生:林子祺
研究生(外文):Lin, Tzu-Chi
論文名稱:發展用於電子束劑量量測之平板型游離腔
論文名稱(外文):Development of a parallel plate ionization chamber for electron dose measurement
指導教授:林明緯林明緯引用關係
指導教授(外文):Lin, Ming-Wei
口試委員:林怡君蔡惠予
口試委員(外文):Lin, Yi-ChunTsai, Hui-Yu
口試日期:2022-07-26
學位類別:碩士
校院名稱:國立清華大學
系所名稱:核子工程與科學研究所
學門:工程學門
學類:核子工程學類
論文種類:學術論文
論文出版年:2022
畢業學年度:110
語文別:中文
論文頁數:70
中文關鍵詞:平板型游離腔電子劑量輻射度量
外文關鍵詞:Parallel plate ionization chamberElectron doseRadiation measurement
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本研究發展可用以量測電子劑量的平板型游離腔。自製的平板型游離腔主要由腔室主體、入射窗口、收集電極、護極、三軸電纜以及通風口組成,並參考商售模組及文獻所列之規格與材料來設計與製造。腔室主體使用PMMA材料並以半徑10mm的鍍鋁聚酯薄膜作為入射窗口,半徑5 mm的石墨薄片作為收集電極,寬度4.5 mm的石墨薄片作為護極,兩平行電極之間距為2 mm,敏感體積為 0.157 cm3。自製平板型游離腔為直徑46 mm,高度22 mm的圓柱體。
本研究使用直線加速器提供之6 MeV電子束對自製平板型游離腔進行短期穩定性測試、中期穩定性測試、漏電流測試、離子收集效率、角度依賴性測試、極性影響以及電纜線影響等性能測試。實驗結果顯示自製平板型游離腔在短期及中期穩定性測試中,相對標準偏差約為0.05%至0.06%。漏電流約落在〖10〗^(-13)至〖10〗^(-14)安培使其在400伏特工作電壓下對量測電流之比值為0.15%,離子收集效率約為97.2%,且約在±10度內沒有角度依賴性。在極性影響中,將游離腔置於固態水假體並於0.3至0.7倍的電子射程深度下量測,結果顯示正負電壓的比值落在1.04至1.06。在5×5 〖cm〗^2、10×10 〖cm〗^2、15×15 〖cm〗^2、20×20 〖cm〗^2的照野下,正負電壓的比值約落在0.99至1.03,可使用極性修正因子來修正極性影響。自製平板型游離腔並無電纜線影響。後續製作將敏感體積由0.157 cm3增加至0.226cm3的游離腔證實可進一步降低漏電流對量測電流之比值至< 0.1%。而在使用電極間距由2mm縮減為1mm之游離腔時,也驗證離子收集效率可提升至98.9%。
實驗也使用固態水假體進行60Co水吸收劑量校正以將游離腔所量測之電荷量轉換為水吸收劑量,而所獲得之游離腔校正因子為1.426×〖10〗^8 (Gy⁄C)。此外,實驗上亦發展由靜電計(electrometer)與相關量測電路之量測系統並發展控制靜電計並記錄電流量測結果之電腦控制程式。本研究已成功自製出平板型游離腔,掌握設計、製造與測試的技術,並掌握其性能表現,作為後續進一步改良與發展的基礎。
This study focuses on developing a parallel plate ionization chamber (PPIC) for electron dose measurement. The implemented PPIC is mainly composed of a chamber body, entrance window, collecting electrode, guard ring, triaxial cable, and a vent hole, while their geometric parameters are specified by referring to commercial models and/or these introduced in previously publications. In this study, the default chamber was fabricated with the material of PMMA and its 20-mm diameter entrance window was realized with aluminized Mylar, in addition to the 10-mm diameter collecting electrode and the 4.5-mm wide guard ring formed by using 25-µm thick graphite sheet. The distance between the two parallel electrodes is 2 mm and the sensitive volume is 0.157 cm3. The outer shape for the default model is a cylinder with a diameter of 46 mm and a height of 22 mm.

The performance of the developed PPIC was characterized by the tests of short-term stability, mid-term stability, leakage current, ion collection efficiency, angle dependence, polarity effect, and cable effect. Using the 6-MeV electron beam produced from the linear accelerator for test, the results show that the relative standard deviations were 0.05% to 0.06% for the short-term and mid-term stabilities, respectively. With the applied voltage of 400 V, the leakage current was measured to be 〖10〗^(-13) to 〖10〗^(-14)A that corresponded to the ratio of 0.15 % with respect to the current when measuring the electron beam, while the ion collection efficiency is estimated to be about 97.2%. The angle dependence was not obvious within ±10 degrees. For the tests of the polar effect, the PPIC was placed in a solid water phantom with the depths between 0.3-0.7 of the electron range, from which the ratio of positive and negative voltage was measured to be within about 1.04 to 1.06; in addition, this ratio was about 0.99 to 1.03 when the field size was set at 5×5, 10×10, 15×15, 20×20 〖cm〗^2. No cable effect was observed for the developed PPIC. Results also showed that, the modified PPIC with an increased sensitive volume from 0.157 cm3 to 0.226 cm3 can lower the leakage current to be < 0.1 % of the current for measuring the electron beam. When using the PPIC with electrode space of 1mm, the ion collection efficiency can be raised to 98.9%, accordingly.
To convert the measured current for the ion chamber to the water absorbed dose, a solid water phantom was used to calibrate the water absorbed dose with a 60Co source, from which the calibration factor was estimated to be 1.426×〖10〗^8 (Gy⁄C). In addition, a measurement system consisting of an electrometer with related circuits and a control program was developed for setting up the electrometer and recording the current/charge data. This study enables the knowledge of design, manufacturing, and testing a PPIC to accumulate and represents a key foundation to further improve the performance of a PPIC in the future.
摘要 i
Abstract ii
誌謝 iv
目錄 v
表目錄 viii
圖目錄 x
第一章 緒論 1
1.1 研究背景 1
1.2 研究架構 2
第二章 游離腔簡介 4
2.1 游離腔原理 4
2.1.2 工作電壓 5
2.1.3 量測訊號模式 8
2.2 游離腔種類 10
2.2.1 自由空氣游離腔 10
2.2.2 圓柱型游離腔 11
2.2.3 平板型游離腔 12
2.2.3.1 結構介紹 13
2.2.3.2 擾動效應 14
2.2.3.3 適用材質 15
2.2.3.4 規格國際規範 16
2.2.3.5 參考平板型游離腔規格統整 17
2.2.3.6 性能測試 17
2.3 游離腔劑量量測 18
2.3.1 電子劑量量測 18
2.3.1.1 電子射束品質 19
2.3.2 水吸收劑量 20
第三章 自製平板型游離腔及實驗設計 25
3.1 設計概念及重點 25
3.1.1 入射窗口及偏壓設計 26
3.1.2 收集電極 26
3.1.3 護極 26
3.1.4 三軸電纜及接線設計 27
3.2 游離腔電流量測系統建立 29
3.2.1 轉接盒及電容盒設計 29
3.2.2 電流量測控制系統 30
3.3 固態水假體 31
3.4 直線加速器 33
3.5 量測方法 33
3.5.1 固定MU量測方法 34
3.6 實驗架設 34
3.7 60CO水吸收劑量校正方法 36
3.8 防水平板型游離腔設計與製作 38
第四章 結果與討論 41
4.1 自製平板型游離腔性能測試 41
4.1.1 電壓曲線 41
4.1.2 穩定性測試 42
4.1.2.1 短期穩定性測試 42
4.1.2.2 中期穩定性測試 44
4.1.3 漏電流測試 45
4.1.4 離子收集效率 47
4.1.5 角度依賴性測試 48
4.1.6 極性影響 49
4.1.6.1 不同深度下之極性影響 49
4.1.6.2 不同照野下之極性影響 51
4.1.7 電纜線影響 52
4.2 自製平板型游離腔60CO水吸收劑量校正 53
4.3 性能改善 54
4.3.1 漏電流改善 55
4.3.2 離子收集效率改善 58
4.3.3 防水平板型游離腔之角度依賴性測試 59
4.4 驗證自製游離腔電流量測系統 60
第五章 結論與未來研究方向 62
5.1 結論 62
5.2 未來工作 65
參考文獻 67
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