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소장정보 검색

국회도서관 홈으로 정보검색 소장정보 검색
결과 내 검색 동의어 포함
결과 내 검색 동의어 포함

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Title Page

ABSTRACT

국문 초록

Contents

Abbreviations 21

1. Introduction 24

2. Neutrino physics 26

2.1. Neutrino history 28

2.2. Neutrino oscillations 29

2.3. Massive neutrino 34

2.4. Neutrinoless double beta decay 37

2.5. Half-life sensitivity 46

3. AMoRE experiment 49

3.1. Overview 49

3.2. Underground laboratory 50

3.2.1. Yangyang underground laboratory 50

3.2.2. Yemi underground laboratory 51

3.3. AMoRE detector 53

3.3.1. ¹⁰⁰Mo as the ββ decay source 53

3.3.2. ¹⁰⁰Mo based scintillating crystals 56

3.4. AMoRE phases 57

3.4.1. AMoRE-I 57

3.4.2. AMoRE-II detector 60

4. AMoRE-II background simulation 62

4.1. Far component background 70

4.1.1. Cosmic muon and muon-induced background 70

4.1.2. Neutron background 76

4.1.3. Lead shield 80

4.1.4. gamma background in rocks 83

4.1.5. Radon from the air surrounding the OVC of the cryostat 84

4.2. Near component background 85

4.3. Crystal internal background 87

4.4. Result 89

5. Solar axion search with the AMoRE-I detector 93

5.1. Introduction of solar axion search 93

5.1.1. CP violation 93

5.1.2. Strong CP problem and Peccei-Quinn mechanism 94

5.1.3. Solar axion 96

5.1.4. Resonant absorption to ⁷Li 97

5.2. AMoRE-I detector and measurement 102

5.3. Signal shape optimization 104

5.4. Energy calibration 105

5.5. Event selection 107

5.5.1. Anti-coincidence 107

5.5.2. alpha event rejection 109

5.5.3. Muon veto coincidence 111

5.5.4. Cut efficiency 111

5.6. Systematic uncertainty 112

5.6.1. Energy bias 112

5.6.2. Resolution 114

5.7. Axion mass limit 116

5.7.1. Counting analysis 117

5.7.2. Shape analysis 118

5.7.3. Result 123

6. Conclusions 125

Bibliography 126

Appendix A. Conference contribution and publication 140

List of Tables

Table 3.1. T₁/₂⁰ν and 〈mββ〉 limits (90% C.L.) sorted by the mass number.[이미지참조] 55

Table 4.1. Radio activities of ²³⁸U and ²³²Th in the detector modules and shield material for AMoRE-II and the flux of the external background source. 69

Table 4.2. The energy and intensity of gammas from ²¹⁴Bi in ²³⁸U decay chain 82

Table 4.3. Radioactivity of the source measured by ICP-MS and HPGe detector and estimated background rate in 3.024 - 3.044 MeV range from near components. 86

Table 4.4. Internal impurity obtained from the LMO crystals of AMoRE-I. The simulation utilized the average values from five crystals. 89

Table 4.5. The background event rates in the ROI for each component, as determined through simulation in the AMoRE experiment. 91

Table 5.1. Exposure and efficiency of each crystal. 104

Table 5.2. Efficiencies for event selection cuts. 111

Table 5.3. Energy bias and systematic errors of each crystal. 112

Table 5.4. Energy resolution and systematic errors of each crystal. 114

List of Figures

Figure 2.1. Elementary particles of the Standard Model (SM). 26

Figure 2.2. Two possible mass hierarchies for neutrinos - the "normal" hierarchy on the left and the "inverted" hierarchy on the right (Adrián-Martínez... 35

Figure 2.3. Mass parabolas for nuclear isobars with even A (Figure is taken from (Saakyan, 2013)). β⁻ decay is not energetically possible from point (a)... 40

Figure 2.4. Feynman diagrams for the two double beta decay modes. (a) is 2νββ decay mode, (b) is 0νββ decay mode in case of light Majorana... 41

Figure 2.5. Schematic view of the 2νββ and the 0νββ spectra. 42

Figure 2.6. Nuclear Matrix Elements calculated in different theoretical models for 0νββ isotopes under investigation by current experiments. An un-... 44

Figure 2.7. Experimentally observable combinations of neutrino mass in 0νββ decay at 2σ as a function of the lightest neutrino mass. The figure... 45

Figure 3.1. Yangyang Underground Laboratory. It is possible to access the underground laboratory by car from the entrance, and the distance is about... 51

Figure 3.2. Overburden of the Y2L in comparison with other underground laboratories. Y2L has a 2000 m water equivalent depth. 52

Figure 3.3. Yemilab 52

Figure 3.4. Detector module used in AMoRE-I. The detector comprises CMO or LMO crystal, a photon detector, and a phonon detector. 58

Figure 3.5. Picture of the AMoRE-I detector tower and schematic view of the detector setup. There are 18 detector modules in four columns in the tower. 59

Figure 3.6. Illustration of the AMoRE-II module and overall structure. The AMoRE-II single module is shown on the left, and the cryostat and surround-... 60

Figure 3.7. Illustration of the AMoRE-II detector substructure and additional facilities. 61

Figure 4.1. The decay chain of ²³⁸U and ²³²Th series. 63

Figure 4.2. The overall view of the geometry implemented in the simulation for the AMoRE-II experiment. 68

Figure 4.3. The AMoRE-II experimental hall geometry for muon simulation. 72

Figure 4.4. The simulation geometry of the AMoRE-II detector. There are two muon veto systems, Cherenkov and plastic scintillator detectors. The... 73

Figure 4.5. The deposited energy spectrum in the muon veto detector 74

Figure 4.6. A time difference distribution between a single crystal hit and a plastic scintillator hit or a water tank hit after the single hit selection. 74

Figure 4.7. Comparison of the energy spectra of background events before and after muon event vetoing. The black dotted line shows the background... 75

Figure 4.8. Design of the AMoRE-II neutron shielding. Boric acid rubber is installed as a neutron shield between the outer PE and lead shields (left).... 77

Figure 4.9. The neutron energy spectrum measured at the A5 laboratory (circles) and the A6 laboratory (squares) in Yangyang underground laboratory. 78

Figure 4.10. The simulation geometry for neutron background simulation. 79

Figure 4.11. Neutron background distribution. 80

Figure 4.12. ²¹⁰Pb source position in the inner lead gives the background event on the crystal detector. 81

Figure 4.13. Background event distribution from the lead shield. 82

Figure 4.14. Simplified rock geometry. We used the ratio between the boundary and background rates from the lead shield simulation for the nor-malization. 83

Figure 4.15. Schematic view of the air space surrounding the outer vessel chamber for radon simulation. 85

Figure 4.16. Schematic view of the detector module for Geant4 simulation. 85

Figure 4.17. Background energy spectrum from near components in region of interest (3.024 - 3.044 MeV). 87

Figure 4.18. Groups within the decay chains of ²³⁸U, ²³²Th, and ²³⁵U decay chains. The groups are divided based on the isotopes with long half-... 88

Figure 4.19. Total background spectra of AMoRE-II. 92

Figure 5.1. Nuclear fusion sequences and neutrino energy spectrum. 98

Figure 5.2. Decay scheme of ⁷Be. 100

Figure 5.3. Ratio of the probabilities of axion and magnetic transitions. The inset illustrates the anticipated spectrum of solar axions emitted in the... 101

Figure 5.4. The schematic view of the crystal tower and the position of LMO crystals. 103

Figure 5.5. The 2.6 MeV energy peak for each crystal. The distribution is before performing the energy calibration and normalized to the height of the peak. 105

Figure 5.6. Comparison of the fitting result between Gaussian shape and crystal ball shape for several energy peaks to determine the shape of the... 106

Figure 5.7. The fitting comparison of the 238.6 MeV gamma peak. The result with the Gaussian and third-order polynomial fitting function was selected. 107

Figure 5.8. The energy calibration function fitting of one of the crystals. 108

Figure 5.9. The light-heat signal ratio distribution as a function of energy. 109

Figure 5.10. The rise-time between 20% - 70% distribution as a function of energy. 110

Figure 5.11. One example of energy bias fitting of LMO4. 112

Figure 5.12. Energy bias of each crystal. The best fit central value versus the literature value for each peak is fitted with second order polynomial.... 113

Figure 5.13. One example of the resolution fitting with high energy region. 114

Figure 5.14. Resolution fitting with σ＝√p₀² + p₁²E + p₂²E² and estimate the resolution at 478 keV.[이미지참조] 115

Figure 5.15. The energy distribution in the region of the interest. The orange dashed line represents where the axion's signal should appear. We counted... 118

Figure 5.16. The energy distribution with summed fitting result. The dashed line represents where the signal from the axion should appear. We observed... 119

Figure 5.17. The distribution of △X², Probability Density Function and Cumulative Distribution Function of axion mass. The dashed line with red... 120

Figure 5.18. The energy distribution with summed fitting result. We ob- served zero events from axion with X²/ν＝1.02. 121

Figure 5.19. The distribution of △X², CDF of axion mass. The black solid line is the CDF and the blue dashed line is the △X². The black dotted line... 121

Figure 5.20. The energy distribution of each crystal. The dashed line represents where the signal from the axion should appear. 122

Figure 5.21. The comparison of the axion mass limits from different experiments, with this measurement achieving the lowest limit to date. 124

초록보기

 AMoRE (Advanced Mo-based Rare process Experiment)은 중성미자 미방출 이중 베타 붕괴를 조사하기 위한 실험이다. 이러한 희귀 반응을 탐지하는 실험에서 배경 잡음을 최소화하는 것이 중요하다. 중성미자 미방출 이중 베타 붕괴 실험에서 기여하는 배경 원인에는 실험 환경에서의 뮤온, 중성자, 감마선 등이 포함된다. 이중 베타 붕괴나 어둠 물질 탐색을 위한 많은 실험들은 뮤온에 의한 배경을 줄이기 위해 지하 실험실에서 진행된다. 현재 Yemilab에서 준비 중인 AMoRE-II는 지하 1000미터에 위치한다. Yemilab의 AMoRE-II 실험에서 감도를 극대화하기 위한 원하는 배경 한계는 10-4 counts/kg/keV/year이다. 그러나 이러한 지하 환경에서도 주변 암석으로부터 발생하는 뮤온과 뮤온에 의해 유발되는 배경, 그리고 감마선과 중성자가 배경에 기여할 수 있다. 따라서 이러한 배경원을 차단하기 위해 검출기 주변에 방패 재료가 설치된다. 방패 재료는 환경 배경을 줄이는 데 중요한 역할을 한다. 그러나 방패 재료는 스스로 배경을 생성할 수도 있으므로 실험에 대한 방패 재료의 배경 영향을 연구하는 것 또한 필요하다. 또한 검출기 재료에 존재하는 우라늄과 토륨과 같은 장기적인 방사성 동위원소로부터 생성되는 배경도 잘 알려진 배경 원인이다. 그러므로 외부환경에서부터 검출기의 구성요소까지 다양한 백그라운드 소스가 주는 영향을 연구하여 실험이 시작되기 전에 백그라운를 줄여나가는 노력이 필요하다.

이 논문의 전반부는 AMoRE-II 실험에 영향을 줄 수 있는 다양한 백그라운드를 이해하고 관심 에너지 영역에서의 백그라운드 이벤트 레이트를 예측하기 위해 Geant4 툴킷을 사용하여 배경 시뮬레이션 연구를 수행하는 것에 초점을 맞추고 있다. 시뮬레이션은 검출기 근처와 먼 곳에 있는 백그라운드를 포함하며, 상세한 시뮬레이션 방법론이 제시된다.

논문의 후반부에서는 AMoRE 실험에서 태양 액시온 공명 사건을 탐색하는 것을 소개한다. 액시온은 물리학에서 여러 미해결 문제들을 해결하기 위해 제안된 가상 입자이다. AMoRE에서 사용되는 리튬(Li)은 태양 주기 반응에 의해 생성된 흥분된 핵의 M1 전이에 의해 생성된 액시온을 공명 흡수할 수 있는 동위원소 중 하나이다. 따라서 Li₂MoO₄ 결정에서 공명 흡수된 액시온이 방출하는 감마선을 검출함으로써 액시온의 질량을 결정할 수 있다. 후반부의 시작은 액시온에 대한 간단한 소개를 제공하며, 이어서 AMoRE-I에서 사용된 다섯 개의 Li₂MoO₄ 결정 검출기를 활용하여 7Li를 대상으로 하는 태양 액시온 공명 사건을 분석하는 과정을 설명한다. 마지막으로 AMoRE-II에서 액시온의 질량을 추정하는 예측을 제시한다.

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