A Study of Pulse Shape Discrimination in Scintillator Dark Matter Detectors Daniel Russell Tovey University of Sheffield A thesis submitted for the degree of Doctor of Philosophy in the subject of Physics, October 1998. To Nan. Abstract The existence of `dark matter' throughout the universe is now well established but its form and origin remain one of the greatest problems for modern cosmology. Particle physics posits a solution to this problem in the form of `Weakly Interacting Massive Particles' (WIMPs) which are predicted to exist by many theories extending physics beyond the standard model. The discovery of such particles would consequently have profound implications for both disciplines. Many experiments around the world are now endeavouring to achieve this goal but currently the most successful are those using scintillator detectors. This thesis describes a study of the use of Pulse Shape Discrimination (PSD) techniques to reduce the rate of electron recoil background events in scintillator dark matter experiments. The development of new classes of detector with novel pulse shape properties is described and the results of tests using elastic scattering of monoenergetic neutrons to simulate nuclear recoil signal events are presented. Monte Carlo simulations have been used to assessthe performance of CASPAR, a particularly promising new technique, and the results presented here indicate that this has the potential to considerably improve dark matter sensitivity, particularly for spin dependent WIMP interactions. An analysis of 867 kg. days of data from an operational NaI(Tl) detector is described and the resulting evidence for a small population of events with anomalous pulse shape properties discussed. 1 Acknowledgements I should like to thank the following (in no particular order) for their advice, assistance and support. First and foremost my supervisor Neil Spooner and the rest of the members of the Sheffield dark matter group; John Roberts, Vitaly Kudryavtsev, Matt Lehner, Phil Lightfoot, John MacMillan, Jeff Martoff and Christian Peak. The other members or ex-members of the Sheffield High Energy Physics Group; Craig Buttar, Chris Booth, Susan Cartwright, Fred Combley, Lee Thompson, Mark Lehto, Chris Grigson, Paul Sellin, Andy Beddall, Spyros Manolopoulos, Warrick Newton, Robin Boswell, Sue Walsh, Chris Brew, John Reeve, Mandy Kelly, Debbie Morgan and Paul Hodgson. Peter Smith and everyone else in the UK Dark Matter Collaboration; Nigel Smith, John Alner, Geoff Arnison, Graham Homer, David Lewin, Igor Liubarsky, Jon Barton, Gareth Jones, Gavin Davies, Colin Lally, David Davidge, Alex Howard, Tim Sumner, Tariq Ali, John Quenby, Arthur Bewick, Harry Vine, Manu Joshi, Ben Ahmed and Rupert Smith. John Edgington, Ian Blair and Ann Harris from the Karmen group at Q. M. W.. All the technicians in the Physics Department and the University of Sheffield Central Mechanical Workshop; in particular John Newell, Chris Vickers, Trevor Gamble, John Kelly and Richard Nicholson. Roger Olivant, Alan Bateman, Cathy Haddon and everyone in the Physics De- partment office. Alan Yates, Dawn Bussey and everyone in the Chemistry Department and Sorby Centre for Microanalysis who helped with the CaF2(Eu) synthesis work. Dave Mowbray for his help with the photoluminescencemeasurements. Jim Telfer and Dennis Day at Hilger Analytical Ltd.. The University of Sheffield and Hilger Analytical Ltd. for funding. And finally Mum, Dad and Caroline. October 1998 11 Contents 1 The Dark Matter Problem 1 Introduction 1 1.1 ................ ... ... ..... ... .. Standard Cosmology 1 1.2 The .... ..... ..... ... ... .. Evidence for Dark Matter 4 1.3 Observational ..... ...... ... .. Extragalactic Dark Matter 4 1.3.1 .......... ........ .. Galactic Dark Matter 6 1.3.2 ..... .... ........ ... .. Summary Observational Evidence 7 1.3.3 of . ........ .. .. Arguments for Dark Matter 8 1.4 Theoretical ... ........ ... .. Inflation Cosmological Constant 8 1.4.1 and the ....... ... .. Bang Nucleosynthesis Baryonic Dark Matter 13 1.4.2 Big and ... .. Dark Matter Candidates Searches 15 1.5 and . ..... ...... .. .. Baryonic Dark Matter 15 1.5.1 ... ... ..... ..... .. .. Non-Baryonic Dark Matter 18 1.5.2 .... ........... ... .. Conclusions 26 1.6 ... ..... ..... ... ..... ..... ... .. 2 Supersymmetric Dark Matter 27 2.1 Introduction 27 ......................... ....... The Standard Model 27 2.2 .................... ....... 2.2.1 The Standard Model Lagrangian 27 ..... .... ....... 2.2.2 Deficienciesin the Standard Model 29 .... .... ....... 2.3 Supersymmetry 31 .................... ... ... .... 2.3.1 Motivations for Supersymmetry 31 . ......... ....... 2.3.2 The MSSM 35 ....... ..... ..... ... ..... .. 2.3.3 R Parity 36 ...................... ........ SupersymmetricDark Matter Candidates 37 2.4 ........ ........ Collider Neutralino Searches 38 2.5 ............. .. ... ... .. 2.5.1 Neutralino Mass Limits 38 ...... ..... .. ..... .. 2.5.2 Discovery Potential 39 ............. .. ..... .. 2.6 Conclusions 40 ... ........ ... ....... .. ....... 111 CONTENTS iv 3 Direct Searches for WIMP Dark Matter 41 3.1 Introduction 41 ........................... ..... 3.2 Direct Search Experiments 41 . ..... ..... ... ..... .... 3.2.1 Energy Spectra 41 .......... ...... ... .. .... 3.2.2 Design Considerations 43 ................. ..... 3.2.3 Direct Detection Techniques 44 ............ .. .... 3.3 Couplings, Cross Sections Form Factors 49 and . ... ..... .... 3.3.1 Spin Dependent Neutralino Interactions 49 .... .. ... .. 3.3.2 Spin Independent Neutralino Interactions 56 .... ... ..... 3.3.3 Other WIMPs 57 ..................... ...... 3.4 Nuclear Recoil Kinematics 58 .................. ...... 3.4.1 Halo Models 58 .... ... .... .... .... ... .. 3.4.2 The Scattering Process 59 ................ ...... 3.5 Target Specific Factors 62 ..... ..... .... .... .... .. 3.5.1 Target Composition 62 ...... ..... ..... .. ... .. 3.5.2 Energy Detection Efficiency 63 . ..... ..... .. ... .. 3.6 Detector Specific Factors 65 ....... ..... ....... .... .. 3.6.1 Statistical Effects 65 . ... ..... ......... .... .. 3.6.2 Event Identification Efficiency 68 ....... ..... ...... 3.7 Calculating Dark Matter Limits 69 ... ....... ..... .... .. 3.7.1 High Discrimination Efficiency 70 ..... ..... .. ... .. 3.7.2 Moderate Discrimination Efficiency 72 ..... .... .... .. 3.7.3 Poor Discrimination Efficiency 74 ............ ...... 3.8 Conclusions 75 .......................... ...... 4 Scintillator Detectors and the UKDMC Programme 76 4.1 Introduction 76 ... .... .... ..... ... ..... .. ..... 4.2 Scintillator Detectors 77 ................. ..... ... .. 4.2.1 Crystal Scintillator Theory 77 .... ..... ... ... ... .. 4.2.2 NaI(Tl) Detectors 78 .................... ..... 4.2.3 The UVIS NaI Detector 86 .......... ...... ... .. 4.2.4 The CASPAR Detector 88 . ..... ... ........ ... .. 4.2.5 Xenon Detectors 91 .................... ..... 4.3 Detector Design 93 ..... .................. .. ..... 4.3.1 Light Collection ................ ..... ..... 93 4.3.2 Photodetection 97 ............... ... .. .... 4.3.3 Data Acquisition Systems 102 ............ .. ... .. 4.4 Detector Shielding ....... ..... ... ..... .. ..... 108 CONTENTS v 4.5 Detector Radiopurity 110 ..... ............ .......... 4.6 The Future: Directional Detectors? 111 ..... .... .......... 4.7 Conclusions 113 ........ ..... ... ..... ..... .... 5 Detector Development 114 5.1 Introduction 114 ...................... .......... 5.2 UVIS 114 . ............................ ....... 5.2.1 Crystal Growth 114 .... ... ........... ..... .. 5.2.2 Experimental Apparatus Procedure 117 and .... ....... 5.2.3 Results Interpretation 121 and ...... ..... ..... .. 5.3 CASPAR 124 .......................... ....... 5.3.1 Introduction 124 . ..... ..... ........ ...... 5.3.2 The CASPAR Inorganic Scintillator 125 ........ ....... 5.3.3 The CASPAR Liquid Scintillator 137 ......... ....... 5.3.4 The CASPAR Gelling Agent 141 ..... ....... ....... 5.4 Conclusions 143 ...... .... ... ..... ..... ....... 6 Neutron Beam Tests 144 6.1 Introduction 144 ... ..... ..... .... ...... ....... 6.2 Theory 144 ... ..... ..... ............. ..... .. 6.3 CASPAR Neutron Beam Tests 146 ...... ..... .... ..... .. 6.3.1 Experimental Apparatus Procedure 146 and ..... ..... .. 6.3.2 Data Analysis and Results 150 .... ..... .... ....... 6.4 NaI(Tl) CaF2(Eu) Neutron Beam Tests 153 and ....... ..... .. 6.4.1 Experimental Apparatus Procedure 153 and ..... ..... .. 6.4.2 Data Analysis and Results 159 . ... ..... ... .... ... 6.5 Conclusions 164 ... ..... ...... .... ..... ..... .. 7 CASPAR WIMP Sensitivity 165 7.1 Introduction 165 .............. ........ ..... ..... 7.2 The CASPAR Monte Carlo Simulation 165 .... ............. 7.2.1 The Grain Environment 165 .............. ....... 7.2.2 Recoil Simulation 167 ................. ........ 7.3 CASPARPulse Shape Analysis ............. ........ 169 7.3.1 Optimisation of Detector Parameters 170 ...... ....... 7.3.2 Technique 170 ................... .. .... ... 7.3.3 Origin of Electron Recoils 171 .... ...... .. ....... 7.3.4 Results ............... ..... .. ....... 172 CONTENTS vi 7.4 Detector Sensitivity 172 ......... ... ..... .......... 7.4.1 Nuclear Recoil Signal Rate 172 ............ ........ 7.4.2 Nuclear Recoil Response Matrix . ... ............. 175 7.4.3 Analysis Results 175 and ............... ........ 7.5 Conclusions 178 ................................ 8 NaI(TI) Analysis 179 8.1 Introduction 179 ..... ... ........ ..... ....... .... 8.2 The DM46 Detector 179 ......................