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.

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