| 摘要: | 台灣首座質子治療中心將在未來的幾個月內開始營運，開創國內輻射治療新里程碑。相較於傳統的光子治療，質子治療能減少腫瘤周邊的正常組織受到的輻射劑量，因高能質子射束可以調整質子的能量，使其穿透人體後停留在腫瘤處，不會再繼續穿透造成後方組織的傷害。因此，在質子治療裡，質子射束流的電流監測與其電離室所得到的電離電流之間的關係顯得相當重要。
充氣式電離室被廣泛用來監測質子射束的劑量測量。由於質子的電流密度相對於光子大，電離室內的再結合效應會造成入射質子射束流的訊號失真。根據Boag理論，再結合效應與平行板電離室的間距、電場強度以及通過的電流密度有關。本研究的目的即在探討平行板電離室離子間再結合效應的影響。
本研究以一個帶有225個信號讀値的塊狀(Pad)平行板電離室，用於偵測質子射束流訊號。使用核能研究所能量為30MeV、高斯形狀的質子射束流並探討再結合效應，依據Boag理論中三個變因做為實驗設計的探討: 1. 採用兩種平行板間距, 分別為3mm及6mm; 2. 四種電流強度(1、5、10、15 nA); 3. 平行板間電場強度掃描(電離室間距為3mm時，每100V為間距升壓至1500V; 間距為6mm時，每200V為間距升壓至3000V)。
塊狀(Pad) 充氣式電離室量測之電離電流與電離室內實際游離出的電流大小有著收集效率的關係，而其收集效率與再結合效應有關。實驗過程中使用自製的法拉第杯放置於實驗架設的後端，即時監測質子射束的電流變化，再將測得的電流量減去材料所阻擋的電流修正至電離室內的實際入射電流，此一修正為實驗分析前的重要步驟。藉由此修正得到的實際入射電流，再計算出電離室內實際游離出的電流大小。
實驗結果顯示，在本實驗條件下，General Recombination(Volume recombination)效應為主要成份。在適當的工作電壓區域；越大的電場，越小的電極間距時及質子束電流強度為1 nA時，再結合效應小於百分之零點一。所以，在質子治療裡，質子射束流的電流監測可以忽略此再結合效應，亦即此質子射束流的電流為電離室內實際游離出的電流大小。
;The first Proton Therapy Center in Taiwan will be soon in operation. The superior quality of the depth-dose distribution of protons than electrons or photons to the target volume is well known. Proton therapy can reduce the radiation dose to normal tissue surrounding the tumor due to high-energy proton beam energy can be adjusted so that after it penetrates the body, it stops in the tumor, and will not continue to penetrate to damage the rear of the organ.
However, protons generate more ion pairs in gas filled ionization chamber than electrons or photons, therefore, higher current density in detector. From BOAG theory, recombination effects depend on the electric field applied, the gas width of electrodes and the current density. The purpose of this study is to investigate the recombination effects with parallel plate ionization chamber.
Pad Ionization Chambers were used to study the recombination effect. The experiment was done at Institute of Nuclear Energy Research with 30 MeV protons. We set the experimental steps: (1) two gas gap: 3mm and 6mm, respectively (2) four beam current intensity (1, 5, 10 and 15 nA) (3) scanning electric field between parallel plates (for 3mm, from 100V to 1500V in step of 100 volts; for 6mm, from 200V to 3000V in step of 200 volts).
Collection efficiency means the relations between its ionization current and collection by pad ion chamber due to recombination by ions. Using Faraday cup timely monitoring of their beam current changes, and it measured by subtracting the amount of current material to stop the current correction ionization chamber current actual incident, this amendment before the experiment is an important step before analyzing.
Finally, we verified BOAG theory and compliance with General Recombination (Volume recombination) effects as the dominate ingredient under the experimental conditions. In proper operating voltage region; the larger E field, the smaller electrode spacing and the lower current intensity, the recombination efficiency is less than 0.1%. In proton therapy, we can ignore this recombination phenomenon; the proton beam current means collection ionization current by ion chamber.
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