Search for double beta decay of Se-82 with the NEMO-3 detector and development of apparatus

265 Pages·2014·10.7 MB·English

by  © James Mott

82
Search for double beta decay of Se
with the NEMO-3 detector and
development of apparatus for
low-level radon measurements for the
SuperNEMO experiment
James Mott
University College London
Submitted to University College London in fulfilment
of the requirements for the award of the
degree of Doctor of Philosophy
September 25th, 2013
1
Declaration
I, James Mott, confirm that the work presented in this thesis is my own.
Where information has been derived from other sources, I confirm that
this has been indicated in the thesis.
James Mott
2
Abstract
The 2νββ half-life of 82Se has been measured as T2ν = (9.93±
1/2
0.14(stat)±0.72(syst))×1019 yr using a 932 g sample measured for
a total of 5.25 years in the NEMO-3 detector. The corresponding
nuclear matrix element is found to be |M2ν| = 0.0484±0.0018. In
addition, a search for 0νββ in the same isotope has been conducted
and no evidence for a signal has been observed. The resulting half-
life limit of T0ν > 2.18×1023 yr (90% CL) for the neutrino mass
1/2
mechanism corresponds to an effective Majorana neutrino mass of
(cid:104)m (cid:105) < 1.0−2.8 eV (90% CL). Furthermore, constraints on lepton
ββ
number violating parameters for other 0νββ mechanisms, such as
right-handed current and Majoron emission modes, have been set.
SuperNEMO is the successor to NEMO-3 and will be one of the
next generation of 0νββ experiments. It aims to measure 82Se with
an half-life sensitivity of 1026 yr corresponding to (cid:104)m (cid:105) < 50 −
ββ
100 meV. Radon can be one of the most problematic backgrounds
to any 0νββ search due to the high Q value of its daughter
ββ
isotope, 214Bi. In order to achieve the target sensitivity, the radon
concentration inside the tracking volume of SuperNEMO must be
less than 150 µBq/m3. This low level of radon is not measurable
with standard radon detectors, so a “radon concentration line” has
been designed and developed. This apparatus has a sensitivity
to radon concentration in the SuperNEMO tracker at the level
of 40 µBq/m3, and has performed the first measurements of the
radon level inside a sub-section of SuperNEMO, which is under
construction. It has also been used to measure the radon content
of nitrogen and helium gas cylinders, which are found to be in the
ranges 70−120 µBq/m3 and 370−960 µBq/m3, respectively.
3
Acknowledgements
First and foremost, I would like to express my extreme gratitude to my supervisor,
Ruben Saakyan, without whose guidance this thesis would not have been possible.
As annoying as it may be, always knowing the answer to any question I might pose,
no matter how obscure, is certainly a very useful trait to have as a supervisor.
In addition, my thanks go to Stefano Torre, David Waters and Adam Davison for
many constructive conversations on analysis techniques and software issues - I am
sure I would have spent a lot more time groping in the dark without this guiding
influence. In the same vein, collaborative work with fellow NEMO-3 students at
other institutions, in particular Sophie Blondel and Summer Blot, was also pivotal
in producing a coherent analysis.
Away from a computer screen and back in the real world, I am indebted to Derek
Attree and Brian Anderson for their technical expertise and for their patience as
I attempted to develop basic engineering skills. Without them, none of the real
‘screw-driver’ physics in this thesis would have been remotely functional. I am also
truly appreciative of the camaraderie shared with my fellow lab rats, Cristóvão Vilela
and Xin Ran Liu.
Away from phyics, the love and support from my family has been a comforting
presence, not just for this PhD but throughout my life. For this, I feely greatly
blessed. Finally, special thanks go to Clara for her unflinching support throughout
this whole process and for ensuring that my funding lasted as long as necessary with
such rigorous budgeting.
4
Contents
List of Figures 11
List of Tables 16
1. Introduction 18
1.1. NEMO-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.2. SuperNEMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
1.3. Author’s Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . 22
1.3.1. NEMO-3 Contributions . . . . . . . . . . . . . . . . . . . . . . 22
1.3.2. SuperNEMO Contributions . . . . . . . . . . . . . . . . . . . 23
2. Neutrino Phenomenology 25
2.1. Standard Model Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . 25
2.1.1. Discovery of the Neutrino . . . . . . . . . . . . . . . . . . . . 25
2.1.2. Neutrino Interactions . . . . . . . . . . . . . . . . . . . . . . . 25
2.1.3. Neutrino Flavours . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.2. Neutrino Mixing and Oscillations . . . . . . . . . . . . . . . . . . . . 28
2.2.1. Oscillation Phenomenology . . . . . . . . . . . . . . . . . . . . 29
2.2.2. Oscillations in Matter . . . . . . . . . . . . . . . . . . . . . . 30
2.2.3. Measurement of Oscillation Parameters . . . . . . . . . . . . . 31
2.3. Neutrino Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.1. Dirac Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.2. Majorana Mass . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.3.3. See-saw Mechanism . . . . . . . . . . . . . . . . . . . . . . . . 34
2.4. Experimental Constraints on Neutrino Mass . . . . . . . . . . . . . . 36
2.4.1. Tritium Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.4.2. Cosmology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.4.3. Neutrinoless double beta decay (0νββ) . . . . . . . . . . . . . 38
5
Contents 6
2.4.4. Oscillations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.5. Outstanding questions . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.5.1. Number of neutrinos . . . . . . . . . . . . . . . . . . . . . . . 39
2.5.2. Absolute Mass and Mass Hierarchy . . . . . . . . . . . . . . . 40
2.5.3. CP Violation . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.5.4. Nature of the neutrino . . . . . . . . . . . . . . . . . . . . . . 42
3. Double Beta Decay 43
3.1. Beta Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.1.1. Allowed and Forbidden Decays . . . . . . . . . . . . . . . . . 44
3.2. Two Neutrino Double Beta Decay . . . . . . . . . . . . . . . . . . . . 45
3.3. Neutrinoless Double Beta Decay . . . . . . . . . . . . . . . . . . . . . 46
3.3.1. Neutrino Mass Mechanism . . . . . . . . . . . . . . . . . . . . 48
3.3.2. Right-handed Current . . . . . . . . . . . . . . . . . . . . . . 49
3.3.3. Majoron Emission . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.4. Nuclear Matrix Elements . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.4.1. Interacting Shell Model . . . . . . . . . . . . . . . . . . . . . . 55
3.4.2. Quasiparticle Random Phase Approximation . . . . . . . . . . 56
3.4.3. Interacting Boson Model . . . . . . . . . . . . . . . . . . . . . 56
3.4.4. Projected Hartree-Fock-Bogoluibov Method . . . . . . . . . . 56
3.4.5. Energy Density Functional Method . . . . . . . . . . . . . . . 57
3.4.6. Comparison of different NME calculations . . . . . . . . . . . 57
4. Double Beta Decay Experiments 59
4.1. Detector Design Considerations . . . . . . . . . . . . . . . . . . . . . 59
4.1.1. Maximising Signal . . . . . . . . . . . . . . . . . . . . . . . . 60
4.1.2. Minimising Background . . . . . . . . . . . . . . . . . . . . . 62
4.2. Detector Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.2.1. Semiconductor Experiments . . . . . . . . . . . . . . . . . . . 63
4.2.2. Scintillation Experiments . . . . . . . . . . . . . . . . . . . . . 67
4.2.3. Bolometer Experiments . . . . . . . . . . . . . . . . . . . . . . 70
4.2.4. Time Projection Chamber Experiments . . . . . . . . . . . . . 71
4.2.5. Tracker-Calorimeter Experiments . . . . . . . . . . . . . . . . 73
4.3. Double Beta Decay Measurements . . . . . . . . . . . . . . . . . . . . 74
4.3.1. 2νββ Measurements . . . . . . . . . . . . . . . . . . . . . . . 74
4.3.2. 0νββ Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Contents 7
4.3.3. Future Measurement Prospects . . . . . . . . . . . . . . . . . 77
I. ββ-decay of 82Se with NEMO-3 79
5. NEMO-3 Detector 80
5.1. Source Foils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.1.1. 82Se Source Foils . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.2. Tracker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.3. Calorimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
5.4. Electronics, DAQ and Trigger . . . . . . . . . . . . . . . . . . . . . . 89
5.4.1. Calorimeter Electronics . . . . . . . . . . . . . . . . . . . . . . 90
5.4.2. Tracker Electronics . . . . . . . . . . . . . . . . . . . . . . . . 90
5.4.3. Trigger System . . . . . . . . . . . . . . . . . . . . . . . . . . 91
5.5. Energy and Time Calibration . . . . . . . . . . . . . . . . . . . . . . 92
5.5.1. Radioactive Sources . . . . . . . . . . . . . . . . . . . . . . . . 92
5.5.2. Laser Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
5.6. Magnetic Coil and Passive Shielding . . . . . . . . . . . . . . . . . . 94
5.6.1. Magnetic Coil . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.6.2. Mount Fréjus . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.6.3. Iron Shield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
5.6.4. Neutron Shielding . . . . . . . . . . . . . . . . . . . . . . . . . 95
5.7. Anti-radon facility . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6. General Analysis Techniques 97
6.1. Monte Carlo Simulation . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.2. Reconstruction of Events . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.3. Particle Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.3.1. Electron Identification . . . . . . . . . . . . . . . . . . . . . . 98
6.3.2. Gamma Identification . . . . . . . . . . . . . . . . . . . . . . 100
6.3.3. Alpha Identification . . . . . . . . . . . . . . . . . . . . . . . . 101
6.4. Time of Flight Information . . . . . . . . . . . . . . . . . . . . . . . . 102
6.4.1. Internal Probability . . . . . . . . . . . . . . . . . . . . . . . . 103
6.4.2. External Probability . . . . . . . . . . . . . . . . . . . . . . . 105
6.5. Analysis Data Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Contents 8
6.6. Statistical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6.6.1. Fitting MC Distributions to Data . . . . . . . . . . . . . . . . 107
6.6.2. Extracting Limits on Signal Strength . . . . . . . . . . . . . . 109
6.7. Half-life Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
7. Estimation of Backgrounds for the 82Se Source Foils 112
7.1. Sources of NEMO-3 Background . . . . . . . . . . . . . . . . . . . . . 113
7.2. NEMO-3 Background Classification . . . . . . . . . . . . . . . . . . . 117
7.2.1. Internal Backgrounds . . . . . . . . . . . . . . . . . . . . . . . 117
7.2.2. External Backgrounds . . . . . . . . . . . . . . . . . . . . . . 119
7.2.3. Radon Backgrounds . . . . . . . . . . . . . . . . . . . . . . . 122
7.3. Channels for Background Measurements . . . . . . . . . . . . . . . . 124
7.3.1. General Quality Criteria . . . . . . . . . . . . . . . . . . . . . 125
7.3.2. Hot Spot Search . . . . . . . . . . . . . . . . . . . . . . . . . 125
7.3.3. 1e1α Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
7.3.4. 1e2γ Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
7.3.5. 1e1γ Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
7.3.6. 1e Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
7.4. Results of Background Measurements . . . . . . . . . . . . . . . . . . 132
7.4.1. Fitting Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 132
7.4.2. Activity Estimations . . . . . . . . . . . . . . . . . . . . . . . 135
7.4.3. Control Distributions . . . . . . . . . . . . . . . . . . . . . . . 144
8. Double Beta Decay Results for 82Se 148
8.1. Optimisation of 2νββ Cuts . . . . . . . . . . . . . . . . . . . . . . . . 149
8.1.1. Starting Point for Optimisation . . . . . . . . . . . . . . . . . 149
8.1.2. Optimisation Procedure . . . . . . . . . . . . . . . . . . . . . 152
8.1.3. Optimisation Results . . . . . . . . . . . . . . . . . . . . . . . 154
8.2. 2νββ Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
8.2.1. Re-measurement of Background Activities . . . . . . . . . . . 159
8.2.2. 2νββ Distributions . . . . . . . . . . . . . . . . . . . . . . . . 163
8.2.3. 2νββ systematics . . . . . . . . . . . . . . . . . . . . . . . . . 167
8.2.4. 2νββ Half-life Measurement . . . . . . . . . . . . . . . . . . . 170
8.3. Optimisation of 0νββ Cuts . . . . . . . . . . . . . . . . . . . . . . . . 171
8.4. 0νββ Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
8.4.1. Mass Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . 176
Contents 9
8.4.2. Right-handed Current . . . . . . . . . . . . . . . . . . . . . . 178
8.4.3. Majoron Emission . . . . . . . . . . . . . . . . . . . . . . . . . 181
II. Radon Research and Development for SuperNEMO 184
9. The SuperNEMO Experiment 185
9.1. SuperNEMO Baseline Design . . . . . . . . . . . . . . . . . . . . . . 185
9.2. Research and Development . . . . . . . . . . . . . . . . . . . . . . . . 187
9.3. Timescale and Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . 189
10.Radon and SuperNEMO 192
10.1.Properties of Radon . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
10.2.Radon as a SuperNEMO Background . . . . . . . . . . . . . . . . . . 194
10.3.Radon Suppression with Gas Flow . . . . . . . . . . . . . . . . . . . . 194
10.4.222Rn Decay Chain to 214Po . . . . . . . . . . . . . . . . . . . . . . . 197
10.4.1. 222Rn Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
10.4.2. 218Po Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
10.4.3. 214Pb Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
10.4.4. 214Bi Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
10.4.5. 214Po Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
10.4.6. Decay chain activities . . . . . . . . . . . . . . . . . . . . . . . 203
11.Electrostatic Detector and Radon Concentration Line 204
11.1.Electrostatic Detector . . . . . . . . . . . . . . . . . . . . . . . . . . 205
11.1.1. Detector Signal . . . . . . . . . . . . . . . . . . . . . . . . . . 207
11.1.2. Detector Efficiency Calibration . . . . . . . . . . . . . . . . . 211
11.1.3. Detector Background Measurement . . . . . . . . . . . . . . . 214
11.2.Radon Concentration Line (RnCL) . . . . . . . . . . . . . . . . . . . 215
11.2.1. Design and Construction . . . . . . . . . . . . . . . . . . . . . 216
11.2.2. Trapping Efficiency . . . . . . . . . . . . . . . . . . . . . . . . 218
11.2.3. Trap Background . . . . . . . . . . . . . . . . . . . . . . . . . 223
12.Radon Concentration Line Sensitivity Estimates 225
12.1.Minimum Detectable Activity . . . . . . . . . . . . . . . . . . . . . . 225
12.1.1. Normal Approximation . . . . . . . . . . . . . . . . . . . . . . 227
12.2.Electrostatic Detector Sensitivity . . . . . . . . . . . . . . . . . . . . 228
Contents 10
12.3.Uniform Gas Measurement . . . . . . . . . . . . . . . . . . . . . . . . 231
12.4.C-Section Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 232
13.Low-level Radon Measurements 236
13.1.Gas System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
13.2.Gas Cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
13.2.1. Modelling Cylinder Activity . . . . . . . . . . . . . . . . . . . 238
13.2.2. Measuring Full Cylinders . . . . . . . . . . . . . . . . . . . . . 240
13.2.3. Measuring Used Cylinders . . . . . . . . . . . . . . . . . . . . 242
13.3.C-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
13.3.1. Measurement Starting Point . . . . . . . . . . . . . . . . . . . 244
13.3.2. Extracting C-section Activity . . . . . . . . . . . . . . . . . . 246
13.3.3. Anti-Radon Tent . . . . . . . . . . . . . . . . . . . . . . . . . 247
13.3.4. C-section Results . . . . . . . . . . . . . . . . . . . . . . . . . 247
14.Conclusion 251
Bibliography 255