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The COBRA Experiment
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  1. The COBRA Experiment Jeanne Wilson University of Sussex, UK On behalf of the COBRA Collaboration TAUP 2007, Sendai, Japan

  2. Contents • Concept • Experimental requirements • Energy Resolution • Backgrounds • Experimental status • Published limits • New results • Coincidences • Future work • Shielding • Pixellisation

  3. COBRA – the Concept Cadmium Telluride 0-neutrino Beta Decay Research Apparatus • Build up a large array of CdZnTe semiconductor detectors K. Zuber, Phys. Lett. B 519,1 (2001) 1 cm

  4. 0 Candidate Isotopes • CdZnTe contains 9  Isotopes 116Cd, 130Te, 114Cd, 70Zn, 128Te(--), 64Zn, 106Cd, 108Cd, 120Te(++ ,+EC, EC/EC) • Main 0candidate: 116Cd • Endpoint = 2805 keV • Enrichment to 90% • Favourable G0|M0|2→ T1/2 ~1026 years for 50meV • Also interesting:106Cd • Endpoint = 2771 keV • EC/EC, +/EC and ++ modes • +/EC - enhanced sensitivity to RH weak currents M. Hirsch et al., Z. Phys. A 347,151 (1994)

  5. Experimental Requirements • 64,000 1cm3 crystals = 418 kg • 90% enriched in 116Cd • Backgrounds < 0.001 count keV-1kg-1year-1 • Energy Resolution < 2%

  6. 8Q E 6 F = me Q Energy Resolution • 2 decays T1/2(116Cd)=2.7×1019 y For E = 2%, fraction of2 events in 0 window ≈ 3 × 10-9 ≈ 10-5 events/kg/year S. Elliott, P. Vogel, Ann. Rev. Nucl. Part. Sci. 2002

  7. Energy Resolution • Only electron signal read out (CPG technology) • Possible improvements: cooling, new grids • Better detectors are available E = 1.9% @ 2.8MeV =2.9% @ 662keV

  8. Backgrounds • Muons, neutrons • a, b, g sources • Intrinsic and surface contaminants • Cosmogenics Crystals 238U, 232Th decay chains 40K 137Cs 210Po 210Pb on surface Cosmogenic isotopes Gas 222Rn gas Delrin Holder 238U,232Th decay chain 40K 137Cs Chamber walls 210Pb on surface

  9. Simulations VENOM - detailed, Geant4 based Monte Carlo package used to simulate all aspects of background contributions. • Optimise shielding design (D. Stewart et al. NIM A571 2007, 651-662) • Determine material purity requirements

  10. Experimental Status • Low-background experiments in LNGS (~3500 mwe) • 2×2 proto-type – limits published • 4×4 proto-type installed – new results • New laboratory space, clean room conditions, fully automated DAQ and environmental control/monitoring. • To be upgraded to 4×4×4 (64-Array) next month

  11. Measured Backgrounds • Passivation paint forms dominant background. • Many steps being taken to reduce backgrounds • New passivation coatings being tested • New crystal production at Freiburg materials institute (towards enriched 116Cd crystals) • New contacting methods, cabling to reduce cross-talk and pick-up • Materials testing and sourcing • Shielding development • Pulse shape analysis • Coincidences • Pixellisation

  12. Physics Results - published • 4.34 kg.days of 2×2 proto-type data • Measurement of 4-fold forbidden 113Cd  decay C. Gößling et al. Phys. Rev. C, 2005, 72, 064328 T1/2= (8.2 ± 0.2 (stat.) ± 1.0 (sys))1015 yrs • Likelihood analysis for double-beta decay signatures T. Bloxham et al. Phys. Rev. C 76, 025501, 2007 (arXiv:0707.2756)

  13. Physics Results - published 130Te→g.s. 116Cd→g.s. • T. Bloxham et al. Phys. Rev. C 76, • 025501, 2007 (arXiv:0707.2756)

  14. Physics Results - published • T. Bloxham et al. Phys. Rev. C 76, • 025501, 2007 (arXiv:0707.2756)

  15. Physics Results - published • T. Bloxham et al. Phys. Rev. C 76, 025501, 2007 (arXiv:0707.2756) • New limits - A. Barabash et al. (arXiv:nucl-ex/0703020)

  16. Physics Results - New • 7.85 kg-days of data • ~12 detectors operating simultaneously • Different backgrounds, thresholds, livetimes, efficiencies, resolutions etc. for each detector, all folded into likelihood function. PRELIMINARY

  17. New Results PRELIMINARY PRELIMINARY PRELIMINARY

  18. New Results PRELIMINARY PRELIMINARY PRELIMINARY Some new world-best limits

  19. Coincidence Searches • Some rare decays can give distinctive coincidence signatures. eg.130Te → 130Xe* + 2e- → 130Xe + 536keV Q – 536 = 1993keV, most likely to stay in same crystal Strong chance of escaping to be detected in nearby crystal Simulated events, calibrated resolution functions applied

  20. Coincidence Searches • Searching for 1993keV (±ΔE) and 536keV (±ΔE) coincident energy deposits in data: 2 events found → <5.32 events (Poissonian 90% limit) • Efficiency from simulation: 0.652% • Leads to a half-life limit of Thalf > 2.6 x 1019 years (90% C.L.) cf 2.5 x 1020 years from single peak search • Efficiencies will increase significantly for larger array. 64 array (2% resolution), efficiency = 3.2%

  21. Coincidences • ββ (to g.s.) is normally single crystal event • Intrinsic 238U and 232Th could be major backgrounds, but more likely to trigger multiple crystals. • Select only single site events for 0νββ peak search • For 64K array – this reduces 232Th chain events from crystals by >50% Simulated 238U chain events inside detectors No Energy smearing

  22.  endpoint 3.3MeV, accounts for >70% events in 2-3MeV region from 238U chain 7.7MeV alpha half-life = 164.3s Timing Coincidences • The major contribution to 238U spectrum at 2-3MeV is the fast b-a decay: 214Bi 214Po 210Pb • >40% efficiency for tagging 214Bi events originating inside the crystals

  23. Observation of 214Bi events • Surface coating → self calibrating device! • small dead volume • Measure of “paint” activity T1/2 = 162 ± 19s

  24. Pixellisation - I • Extra information on events with signal energy. 200m pixels (example simulations): • eg. Could achieve nearly 100% identification of 214Bi events. Bloxham and Freer, NIM A, v572, issue 2, p722-728 (2007)  0 ~15m 1-1.5mm

  25. Tests of 16×16 1.6mm pixel detectors Working on photolithography for 200μm pixel detectors. Pixellisation - II

  26. Ssssummary

  27. Join the party! • Learnt a lot from small proto-type experiments, including some interesting physics results. • Required energy resolutions can be achieved. • Lots of work in progress to reduce backgrounds.

  28. Ssspare Ssslidesss

  29. Optimise shielding design for neutrons Size = 18.4 m3, Mass = 64964 kg Active scintillator component for inner layer Veto any residual external background components Veto s from internal backgrounds Enhance sensitivity to  decays with high energy s Shielding and Veto detectors

  30. ++ Modes p n e+ e+ n p • (A,Z)  (A,Z-2) + 2 e+ (+2e) ++ Q-4mec2 Q-2mec2 • e- + (A,Z)  (A,Z-2) + e+ (+2e )+/EC Q • 2 e- + (A,Z)  (A,Z-2) (+2e) EC/EC Enhanced sensitivity to right handed weak currents (V+A)

  31. Measured Backgrounds

  32. Physics Results - New PRELIMINARY