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研究生:蔡宜璇
研究生(外文):Tsai, Yi-Hsuan
論文名稱:傳統中子偵檢器用於高能中子輻射場的劑量低估修正研究
論文名稱(外文):Spectral Correction Factors for Conventional Neutron Dosemeters Used in High-energy Neutron Environments – A Complete Survey of All Neutron Spectra in IAEA-TRS-403
指導教授:許榮鈞
指導教授(外文):Sheu, Rong-Jiun
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:2016
畢業學年度:104
語文別:英文
論文頁數:116
中文關鍵詞:high-energy neutronsdetectordosedosemeter
外文關鍵詞:高能中子偵檢器劑量劑量計
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高能中子穿透性較強且會在輻射屏蔽外造成不容忽視的劑量,但是對於傳統緩速型中子偵檢器的反應只有微小的貢獻,因此會造成劑量低估的情形。本研究以ISO-8529的劑量校正流程搭配波納球響應函數、通量劑量轉換係數及IAEA Technical Reports Series No, 403 (IAEA-TRS-403) 中的各種能譜為基礎,詳細探討由252Cf校正的偵檢器反應與能譜直接承上通量劑量轉換係數所得之劑量,比較出不同輻射場中子能譜對偵檢器劑量量測正確性的影響,最後得到一系列針對傳統中子偵檢器用於測量高能中子應該使用的劑量修正因子。本研究建議的劑量修正因子可由中子能譜中相對應的高能中子通量百分比查圖表得之,另外,若工作環境的中子能譜不可得,本研究亦建立了合適的高能中子指標(HEN-index)幫助使用者修正中子劑量,本研究建議的高能中子指標為兩個不同設計目的的波納球偵檢器量測讀值比(4P6_8/6”)。後續靈敏度研究亦指出(1) 本研究所得之能譜劑量修正因子與使用下列各個校正射源幾乎無關:252Cf、 241AmBe以及239PuBe;(2) 選用不同大小的波納球(6吋、7吋、8吋及9吋波納球)當作中子劑量劑量計所對應的能譜劑量校正因子的趨勢也大致相似;(3) 輻射工作場所的高能中子指標可以有不同選擇,但是本研究指出應該至少滿足兩個條件,一是延伸能量中子偵檢器的PE厚度應與標準偵檢器相同,另一則是延伸能量偵檢器可以嵌入鉛金屬而非銅金屬,目前發現4P6_8/6” 與3P5_7/5”讀值比可以作為高能中子指標。在此研究中,也將同樣的劑量校正方法應用到實驗室自行組裝的波納圓柱(Bonner cylinder) ,波納圓柱雖然形狀不完全對稱但是對高能中子有20倍高的量測效率,可用於量測低強度中子輻射場。
High-energy neutrons (En > 10 MeV) are relatively penetrating and give a significant dose contribution due to the high fluence-to-dose conversion coefficients, but lead to a negligible response in most conventional moderated-type neutron detectors. A dose correction approach following ISO-8529 calibration method with was proposed based on response functions of Bonner spheres, fluence-to-dose conversion coefficients, and various spectra in workplaces presented in IAEA Technical Reports Series No. 403. By comparing 252Cf-calibrated dose responses with reference values based on fluence-to-dose conversion coefficients, the effects of the neutron spectrum on the accuracy of dose measurements were investigated and workplace-specific correction factors were suggested for conventional neutron dosemeters when used in environments with high-energy neutrons. The suggested spectral correction factors were first established according to the flux percentages of high-energy neutrons in workplaces. Alternatively, another high-energy neutron (HEN) index was also established to facilitate the dose correction in practical situations, where the workplace-specific HEN index was determined based on quick measurements, the ratio between the measured responses of two Bonner spheres (the 4P6_8 extended-range sphere versus the 6” standard sphere) was suggested. A series of sensitivity studies indicated that (1) the spectral correction factors are quite universal and independent of the selection of the following three neutron calibration sources: 252Cf, 241Am and 239Pu; (2) the spectral correction factors for the following four neutron dosemeters (6”, 7”, 8”, and 9” Bonner spheres) are similar in trend; (3) the HEN-index can have several choices, e.g. the ratio of 4P6_8/6” or 3P5_7/5”, provided that the extended-range spheres are lead embedded and have the same thickness of polyethylene as that of the corresponding standard sphere. In the end of this study, similar spectral correction factors were established for the home-made Bonner cylinders, which, although not fully symmetric in geometry, exhibit an efficiency of about 20 times higher than that of the Bonner spheres.
Abstract iii
摘要 v
致謝 vi
Contents vii
List of Tables ix
List of Figures xi
Chapter 1 Introduction 1
Chapter 2 Neutron Detectors 8
2.1 The Principle of Moderated-type Neutron Detectors 8
2.2 Conventional and Extended-range Neutron Detectors 11
2.3 Bonner Spheres as Neutron Dosemeters 13
Chapter 3 Neutron Dose Evaluation 16
3.1 Ambient Dose Equivalent and Fluence-to-dose Conversion Coefficients 16
3.2 Neutron Detector Calibration and Dose Evaluation 20
3.3 Neutron Spectra in Various Workplaces 30
Chapter 4 Spectral Correction Factors 34
4.1 Detector Response Functions and Validation 34
4.2 Dose Correction Factors in Various Neutron Fields 37
4.3 Trends of Dose Correction Factors in Various Neutron Fields 39
4.4 Neutron Spectra that Deviate from the Prediction 48
Chapter 5 Sensitivity Studies of the Spectral Correction Factors 52
5.1 Selections of Calibration Neutron Sources 53
5.2 Selections of Neutron Dosemeters 62
5.3 Selections of High-energy indices 69
5.4 Spectral Correction Factors of Bonner Cylinders 79
Chapter 6 Conclusions and Future work 89
Reference 93
Appendix A. Information of 243 Spectra 96

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