建築物內宇宙射線牟子與中子的研究

Studies of cosmic-ray muons and neutrons in a concrete building

陳韋霖(Wei-Lin Chen)

指導教授 ： 許榮鈞

國立清華大學/原子科學院/核子工程與科學研究所/博士(2018年)

摘要

地表的宇宙射線劑量以宇宙射線牟子及中子的貢獻為主，因此，過去地表的宇宙射線研究多圍繞著宇宙射線牟子與中子進行探討。然而，卻少有文獻同時探討建築物對宇宙射線牟子與中子的屏蔽效應。基於精進建築物內宇宙射線研究的目的，本研究運用了實驗量測及理論計算方法，同時評估建築物內的宇宙射線牟子與中子的通率/劑量率分佈。本研究採用了柏克萊實驗室宇宙射線偵檢器及高靈敏中子偵檢器，觀測台灣地表及清華大學工科館內宇宙射線牟子與中子隨環境變化的趨勢。利用發展純熟的FLUKA宇宙射線模型，以地球實際尺寸、美國標準大氣模型及理想地磁模型執行蒙地卡羅模擬，獲取台灣地表的宇宙射線牟子與中子資訊。由於宇宙射線模型幾何過於龐大，無法直接用來評估建築物對宇宙射線造成之效應。截至目前為止，亦鮮少有研究在理論計算中，考慮真實的建築物結構。為了突破建築物內宇宙射線理論計算的困窘，本研究提出了二階地表宇宙射線模擬方法。該方法透過完整宇宙射線模擬取得台灣海拔200公尺處的宇宙射線牟子與中子的完整特性，並建立二階地表宇宙射線模型，於海拔200公尺處延續大氣層牟子與中子的發展歷程。透過一系列高空與地表宇宙射線模擬與量測的比較，本研究對理論預測效能進行分析及探討，在利用中子量測結果修正的前提下，於理論預測結果與量測間取得了良好的一致性。
本研究將二階地表宇宙射線模擬方法應用於工科館宇宙射線牟子與中子的研究及建築物屏蔽因數的研究上。於工科館的研究中，本研究展示了工科館內宇宙射線牟子與中子的詳細分布，並決定出宇宙射線牟子和中子於工科館空曠屋頂處的年有效劑量分別為115.2與35.2 μSv a-1，於東側走廊處的劑量率逐層衰減至一樓分別降至97.8與9.9 μSv a-1，每層樓(15公分厚混凝土)對宇宙射線牟子及中子的衰減因數分別決定為0.97與0.78。
建築物屏蔽因數的研究中，本研究探討了兩種常見的建築物型態：20層大樓及5層集合住宅，研究結果充分顯示了側向入射對低樓層的宇宙射線牟子與中子劑量貢獻，有著不可忽略的重要性。本研究決定出宇宙射線牟子與中子的建築物屏蔽因數分別為0.95及0.60。此外，本研究考量宇宙射線劑量組成隨高度變化明顯，以及建築物對不同宇宙射線屏蔽效應的差異，進一步地決定出對整體宇宙射線的建築物屏蔽因數，該因數與海拔高度相依，由海平面的0.79逐漸下降至海拔5000公尺處的0.65。
本研究利用二階地表宇宙射線模擬方法，更合理地估算建築物屏蔽因數，評估結果提供了未來群體劑量評估工作一新參考資訊。此外，本研究所建立的二階地表宇宙射線模擬方法，亦可在未來作為宇宙射線相關應用的研究工具，例如宇宙射線牟子造影、宇宙射線中子水分探測技術等。

關鍵字

宇宙射線牟子 ； 宇宙射線中子 ； 蒙地卡羅模擬 ； 輻射度量 ； 混凝土建築

並列摘要

Cosmic-ray muons and neutrons dominate cosmic-ray dose at terrestrial altitude. Because of the significant of muons and neutrons, most of the past indoor cosmic ray studies focused on cosmic-ray muons or neutrons. However, those studies rarely investigated both of them simultaneously. Therefore, this study thoroughly determined the flux and dose rate distributions of cosmic-ray muons and neutrons in concrete building by comparing measurements with Monte Carlo simulations of cosmic-ray showers. An angular-energy-dependent surface source comprising secondary muons and neutrons at a height of 200 m above ground level was established and verified, which was used to concatenate the shower development in the upper atmosphere with subsequent simulations of radiation transport down to ground level, including the effect of the terrain and studied building. A Berkeley Lab cosmic-ray detector and a highly sensitive Bonner cylinder were used to perform muon and neutron measurements on each building floor. After careful calibration and correction, the measured responses of the two detectors were discovered to be reasonably consistent with the theoretical predictions, thus confirming the validity of the two-step calculation model employed in this study.
This study shows two practical applications of two-step calculation model. The first application is calculation of cosmic-ray muons and neutron flux/dose distribution in a 5-story concrete building, and the second one is evaluation of building shielding factor. In the first application, three-dimension cosmic-ray muons and neutron flux/dose distributions in the concrete building were obtained. The annual effective doses from cosmic-ray muons and neutrons on the open roof of the building were estimated to be 115.2 and 35.2 Sv, respectively. Muons and neutrons were attenuated floor-by-floor with different attenuation factors of 0.97 and 0.78, and their resultant dose rates on the first floor of the building were 97.8 and 9.9 Sv, respectively.
In the second application, shielding effects of two popular buildings on cosmic-ray muons and neutrons were investigated in Taiwan. The results indicated the importance of considering the lateral particle incidences in indoor cosmic ray studies. The building shielding factors for muons and neutrons were determined to be 0.95 and 0.60, respectively. Because of cosmic-ray dose composition varies with altitude, we proposed altitude-dependent building shielding factor be applied to indoor cosmic-ray dose evaluation. The building shielding factor was determined to be 0.79 at sea-level, then decreased down to 0.65 at altitude of 5000 m.
This study adopted a verified two-step calculation model to evaluate building shielding factor more reasonable. The consequents will be helpful to improve the accuracy of population dose evaluation. Furthermore, the two-step calculation model is a useful tool for cosmic-ray application studies, such as muon radiography and moisture detection by cosmic-ray neutron sensors.

並列關鍵字

Cosmic-ray muons ； Cosmic-ray neutrons ； Monte Carlo simulations ； Radiation measurements ； Concrete building

參考文獻

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