臺灣博碩士論文加值系統

本研究主要的探討對象是宇宙射線中子，特別強調其在 空氣/地 與 空氣/水 界面附近的能量與角度分佈。宇宙射線中子的能譜與角度分布在界面附近會有劇烈變化，這是因為界面的存在造成中子截面與中子射源的不連續，破壞了原本大氣層內中子產生與吸收的平衡狀態。海平面宇宙射線中子的強度主要視所在地磁緯度而定，其數量級約在10-3 cm-2s-1左右。因為宇宙射線中子的強度低於市面上一般中子偵檢器的靈敏度，為此我們發展了高靈敏度的波那氏圓柱中子能譜儀（Bonner Cylinders）與八頻道中子偵測儀（8CND），其中波那氏圓柱用來測量中子的能譜，八頻道中子偵測儀用來擷取中子輻射場角度分布的資訊。除了測量之外，我們也建立了一套完整模擬大氣層內宇宙射線中子的計算方法，經由測量與計算的搭配，我們可以得到宇宙射線中子在不同界面附近能譜與角度分佈變化的情形。另外，我們亦將這裡發展的儀器與技術應用於一些高能粒子加速器設施相關的中子輻射場研究。
因為台灣地區接近地磁赤道，有幾乎全球最強之地磁垂直截止剛性，因此宇宙射線強度最弱，但是其平均能量卻最高。儘管在文獻中宇宙射線中子隨高度與緯度的分布有相當多測量或計算的數據，但大多集中在歐美日等中高地磁緯度地區，對於低地磁緯度區域的數據較為缺乏，因此台灣地區的數據正可補其不足。在本研究中，透過計算與測量的搭配，我們決定了台灣地區海平面附近由於宇宙射線中子造成的有效劑量率為26 Sv/yr（空氣/地）與22 Sv/yr（空氣/水），它們對應的宇宙射線中子通率分別為5.610-3 cm-2 s-1（空氣/地）與3.710-3 cm-2 s-1（空氣/水）；而且它們隨著高度的變化為exp(-d/174.5)，d是從大氣層頂端算起的大氣厚度，以g cm-2為單位。除此之外，還有宇宙射線中子在界面附近能譜與角度分佈變動的情形，相較於高空的中子能譜，熱中子會因界面存在而大幅增加。空氣/水 界面附近的中子通率強度雖較 空氣/地 來得弱一些，但中子能譜卻較 空氣/地 來的硬，也就是平均中子能量較高。雖然由上往下與由下往上的宇宙射線中子有明顯不同的能量分佈，整體來說，宇宙射線中子的總通率在 空氣/地 界面附近則接近等向性，而在 空氣/水 界面附近則傾向由上往下。另外，由不同宇宙射線中子能量造成的劑量分佈來看，低能量中子的數目雖然可觀，但是高能量的中子才是其有效劑量的主要來源。

This study investigated the characteristics of the cosmic-ray neutron field near air/ground and air/water interfaces with emphases on the energy and angular distributions. The cosmic-ray neutron fluence rate at sea level has been reported at the order of 10-3 cm-2s-1. In order to properly measure the low intensity cosmic-ray neutrons, two sets of high-efficiency neutron detecting systems were developed. The first one, called the Bonner Cylinders, was used for measurements of the energy information. The other one, referred to as the 8CND (eight-channel neutron detector), was used to characterize the angular information of the neutron field. The measured results were used to normalize and confirm one-dimensional transport calculations for cosmic-ray neutrons near interfaces. According to these measurements and calculations, both energy spectra and angular distributions of cosmic-ray neutrons near interfaces were resolved. The neutron flux change near interface is owing to the discontinuity in both the neutron interaction and production cross sections. Besides, the instruments and methods developed here were also used to study the neutron field outside the shielding wall of high-energy electron accelerator.
In the present work, annual sea level neutron effective doses in Taiwan were determined to be 26 (air/ground) and 22 Sv (air/water), corresponding to the neutron fluence rates of 5.610-3 (air/ground) and 3.710-3 n cm-2 s-1 (air/water). The dependence of the cosmic-ray neutrons on the atmospheric depth can be represented well by the exponential function of exp(-d/L) with the effective attenuation length L = 175.4 g cm-2 in Taiwan. Since Taiwan is located near the geomagnetic equator, the vertical cutoff of geomagnetic rigidity is almost the highest in the world. The latitude effect, as compared with that reported elsewhere, results in appreciable differences both in absolute values and the altitude dependence. In addition, the energy spectra of cosmic-ray neutrons near air/ground and air/water interfaces were also unfolded. The results show that the neutron fluence rate near the air/ground interface is greater than that near the air/water interface, while the neutron spectrum near the air/water interface is somewhat harder than that near the air/ground interface. Although there is a clear distinction between energy distributions of upward neutrons and downward neutrons near interfaces, the angular distribution of the total neutrons near the air/ground interface is almost isotropic while near the air/water interface more neutrons come downward from the air than upward from the water. In comparison with the in-flight spectrum, the interface has a significant effect on the low-energy part of the spectrum, especially in the thermal region. However, these low-energy neutrons do not make significant contribution to the effective dose rate. On the contrary, high-energy neutrons dominate the effective dose rate from cosmic-ray neutrons.

中文摘要 i
Abstract ii
致謝 iv
目錄 v
圖目錄 vii
表目錄 ix
1 緒論 1
引言 1
1.1 文獻回顧 2
1.2 研究方向 6
1.2.1 高靈敏度中子偵檢器的發展 6
1.2.2 界面附近宇宙射線中子的特性研究 6
2 中子偵檢器之發展與測試 8
2.1 波那氏圓柱（Bonner Cylinders） 8
2.1.1 偵檢器的構造 8
2.1.2 偵檢器的響應函數 10
2.1.3 偵檢器的校正 12
2.2 八頻道中子偵測儀8CND（8-Channel Neutron Detector） 13
2.2.1 偵檢器的構造 13
2.2.2 偵檢器的響應函數 14
2.2.3 偵檢器的校正 15
3 宇宙射線中子之計算模式 17
3.1 計算程式之簡介 17
3.1.1 FLUKA 17
3.1.2 MCNP 17
3.1.3 ANISN 18
3.1.4 MAXED 19
3.2 計算模式與步驟 21
3.2.1 射源項的計算 21
3.2.2 中子遷移計算 23
3.2.3 偵檢器的響應 24
3.2.4 能譜展開與劑量計算 25
4 結果與討論 26
4.1 宇宙射線中子隨高度之變化 26
4.2 宇宙射線中子之能量分布 29
4.3 宇宙射線中子之角度分布 33
4.4 宇宙射線中子之劑量評估 35
5 結論 39
5.1 主要的研究結果 39
5.2 未來研究方向的建議 40
6 其他應用： 同步輻射研究中心的中子輻射場 41
6.1 同步輻射研究中心之輻射場簡介 41
6.2 電子加速器屏蔽牆外中子輻射場之研究 44
6.3 電子儲存環氣體制動輻射引發中子之研究 50
6.4 高效率中子偵檢器監測場界中子天空散射 53
7 參考文獻 54
8 論文發表 58
8.1 Cosmic-Ray Induced Neutron Spectra and Effective Dose Rates near Air/Ground and Air/Water Interfaces in Taiwan 58
8.2 A Study of the Cosmic-Ray Neutron Field near Interfaces 89
8.3 Real-Time Radiation Monitoring System in SRRC 96
8.4 A Study of Neutron Field outside the Shielding Wall at SRRC 103
8.5 Determination of the Effective Z-number of the Residual Gas in the Vacuum Chamber of the Electron Storage Ring of the SRRC：Approach from the Detection of the Photoneutron Yield Produced by Gas Bremsstrahlung 109

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