延伸能量之中子偵檢器用於高能電子加速器輻射場的探討

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研究生：	林郁埼 Lin, Yu-Chi
論文名稱：	延伸能量之中子偵檢器用於高能電子加速器輻射場的探討 Study on extended-range neutron detectors used at high-energy electron accelerators
指導教授：	許榮鈞 Sheu, Rong-Jiun
口試委員:	蔡惠予 Tsai, Hui-Yu 林威廷 Lin, Uei-Tyng 劉鴻鳴 Liu, Hong-Ming 洪明崎 Horng, Ming-Chi
學位類別：	博士 Doctor
系所名稱：	原子科學院 - 核子工程與科學研究所 Nuclear Engineering and Science
論文出版年：	2019
畢業學年度：	108
語文別：	中文
論文頁數：	89
中文關鍵詞：	高能電子加速器、蒙地卡羅模擬、光中子、中子量測能譜、中子劑量
外文關鍵詞：	High-energy electron accelerator, Monte Carlo simulation, Photoneutron, Neutron measurement spectrum, Neutron dose assessment
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本研究使用高靈敏度中子波那圓柱量測系統在同步輻射設施的高能電子加速器環境進行中子輻射場的探討，(1)此系統偵測核心體積大，因此靈敏度高，適用於低中子通量的環境；(2)配合同屬高靈敏度的量測參考偵檢器，可降低加速器注射時的動態變化影響；(3)利用中子反應釋放出的能量與光子的沉積能量落差，可清楚鑑別入射中子與光子；(4)搭配不同厚度緩速體與重金屬嵌入物，此系統量測範圍可涵蓋熱中子至GeV等級的高能中子。

於150 MeV線型加速器實驗室之測量過程中，探討電子加速器瞬間脈衝式注射可能引起的時間結構問題，發現量測結果在大於一定量電流(3 nC)後會開始呈現計數率低估情況。此外，主體屏蔽牆外90°方向量測與計算的結果相當一致，0°方向量測結果則會因大量的高能光子與含重金屬的偵檢器產生額外的光中子反應而大於計算結果，且有機會佔全部量測中子的50%以上，此一發現對於延伸能量偵檢器用於光子與中子混和輻射場的高能電子加速器設施相當重要，因延伸能量偵檢器內部之高原子序物質與高能光子額外產生的中子計數並非來自輻射場本身，需考量加以修正。

3 GeV電子加速器中子輻射場的部分，文獻上有些同步輻射設施的中子能譜計算結果，但中子的能譜量測結果則較少，且這些中子能譜量測結果也存在許多改善空間。本研究將計算結果作為初始條件，配合波那圓柱系統的量測數據與參考偵檢器的校正，解析獲得新的量測中子能譜，結果顯示不論是否有增設局部屏蔽，高能中子(>10MeV)皆為中子劑量的主要來源，局部屏蔽增設前後高能中子佔整體中子通量的比例分別為30% 與35%，中子劑量率則分別佔55%與59%。

This study analyzed the responses of two types of neutron detector in mixed photon and neutron radiation fields around a 150-MeV electron linear accelerator (LINAC). The detectors were self-assembled, high efficiency, and designed in two configurations: (1) a conventional moderated-type neutron detector based on a large cylindrical He-3 proportional counter; and (2) an extended-range version with an embedded layer of lead in the moderator to increase the detector’s sensitivity to high-energy neutrons. Two sets of the detectors were used to measure neutrons at the downstream and lateral locations simultaneously, where the radiation fields differed considerably in intensities and spectra of photons and neutrons. Analyzing the detector responses through a comparison between calculations and measurements indicated that not only neutrons but also high-energy photons triggered the detectors because of photoneutrons produced in the detector materials. In the lateral direction, the contribution of photoneutrons to both detectors was negligible. Downstream of the LINAC, where high-energy photons were abundant, photoneutrons contributed approximately 6% of the response of the conventional neutron detector; however, almost 50% of the registered counts of the extended-range neutron detector were from photoneutrons because of the presence of the detector rather than the effect of the neutron field. Dose readings delivered by extended-range neutron detectors should be interpreted cautiously when used in radiation fields containing a mixture of neutrons and high-energy photons.

Accurate neutron spectrum measurements at light source facilities are difficult to perform because of relatively low and time-varying neutron intensities. A homemade Bonner cylinder spectrometer was used to determine the energy spectra of neutrons outside the lateral shielding wall of the Taiwan Photon Source before and after the installation of local injection shielding. The spectrometer, similar to the design of conventional Bonner spheres, features (1) highly sensitive neutron detection and (2) a wide-range response to neutrons with energies up to the GeV range. Neutron measurements were conducted by intentionally parking the injected 3-GeV electrons at the septum of the storage ring. On the basis of high-fidelity FLUKA simulations, neutron spectra at the measurement location under the experimental conditions were obtained and adopted as an initial guess for spectrum unfolding. The neutron spectra determined before and after the local shielding were comprehensively compared in terms of their intensities and characteristics. Both before and after the local shielding, high-energy neutrons (>10 MeV) were the dominant component of the radiation field, which contributed approximately 30% to 35% of the total neutron flux and 55% to 59% of the total neutron dose rate.

摘要............................................................i
Abstract........................................... ............ii
誌謝............................................................iv
目錄............................................................v
表目錄..........................................................vii
圖目錄..........................................................viii
第1章 導論.......................................................1
1.1 國內加速器設施概況............................................1
1.2 電子加速器設施................................................2
1.2.1 線型加速器實驗室............................................2
1.2.2 台灣光子源設施..............................................5
1.3 中子能譜測量文獻探討..........................................8
1.4 高能電子加速器中子量測的困難與挑戰.............................16
1.5 研究流程規劃.................................................17
第2章 實驗儀器設置與模擬分析......................................19
2.1 實驗儀器開發與量測...........................................19
2.1.1 中子能譜量測系統...........................................19
2.1.2 150 MeV線型加速器實驗室之量測環境與設備(PE 與 PE+Pb).........25
2.1.3 3 GeV台灣光子源設施之量測環境與設備(波那圓柱量測系統).........26
2.2 模擬分析程式.................................................30
2.2.1 MCNPX蒙地卡羅程式..........................................31
2.2.2 FLUKA蒙地卡羅程式..........................................31
2.3 偵檢器響應函數驗證與探討......................................33
2.3.1 響應函數之MCNPX計算結果.....................................33
2.3.2標準中子源(Cf-252)之響應函數驗證.............................38
2.3.3中子入射中子偵檢器的模式探討.................................40
第3章 150 MeV線型加速器實驗室中子量測與分析........................42
3.1 線型加速器實驗室射束損失探討與驗證.............................42
3.2 線型加速器實驗室輻射場分析....................................50
3.3 高電流之中子低估現象..........................................55
3.4 光中子之影響.................................................57
第4章 3 GeV台灣光子源設施中子量測與分析............................59
4.1 台灣光子源電子加速器輻射場分析.................................59
4.2 實驗之量測結果...............................................65
4.3 實驗結果之能譜展開............................................68
4.4 加速器引出效率探討與說明......................................71
4.5 注射段局部屏蔽增設前後比較結果.................................73
第5章 結論.......................................................76
5.1 重要成果彙整與說明............................................76
5.2 未來研究方向與討論............................................80
參考文獻.........................................................81
附錄一 偵檢器工程圖(PE+Pb)........................................86
附錄二 期刊論文彙整...............................................89

                                

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