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

Jeanne Wilson TAUP 2007 Contents  Concept  Experimental requirements Energy Resolution Backgrounds  Experimental status Published limits New results Coincidences  Future work Shielding Pixellisation

Jeanne Wilson TAUP 2007 COBRA – the Concept Cadmium Telluride 0-neutrino Beta Decay Research Apparatus CdZnTe  Build up a large array of CdZnTe semiconductor detectors K. Zuber, Phys. Lett. B 519,1 (2001) 1 cm

Jeanne Wilson TAUP  Candidate Isotopes  CdZnTe contains 9  Isotopes 116 Cd, 130 Te, 114 Cd, 70 Zn, 128 Te 116 Cd, 130 Te, 114 Cd, 70 Zn, 128 Te (  -  - ), 64 Zn, 106 Cd, 108 Cd, 120 Te 64 Zn, 106 Cd, 108 Cd, 120 Te (  +  +,  + EC, EC/EC) 116 Cd  Main 0  candidate: 116 Cd Endpoint = 2805 keV Enrichment to 90% Favourable G 0 |M 0 | 2 → T 1/2 ~10 26 years for 50meV 106 Cd  Also interesting: 106 Cd 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)

Jeanne Wilson TAUP 2007 Experimental Requirements  64,000 1cm 3 crystals = 418 kg 116 Cd  90% enriched in 116 Cd  Backgrounds < count keV -1 kg -1 year -1  Energy Resolution < 2%

Jeanne Wilson TAUP 2007 Energy Resolution  2  decays T 1/2 ( 116 Cd)=2.7 ×10 19 y 2  0  3 × For  E = 2%, fraction of 2  events in 0  window ≈ 3 × events/kg/year ≈ events/kg/year 8Q  E 6 m e Q F = S. Elliott, P. Vogel, Ann. Rev. Nucl. Part. Sci. 2002

Jeanne Wilson TAUP 2007 Energy Resolution  Only electron signal read out (CPG technology)  Possible improvements: cooling, new grids  Better detectors are available  E = 2.8MeV 662keV

Jeanne Wilson TAUP 2007 Backgrounds  Muons, neutrons   sources Intrinsic and surface contaminants Cosmogenics Crystals 238 U, 232 Th decay chains 40 K 137 Cs 210 Po 210 Pb on surface Cosmogenic isotopes Delrin Holder 238 U, 232 Th decay chain 40 K 137 Cs Gas 222 Rn gas Chamber walls 210 Pb on surface

Jeanne Wilson TAUP 2007 Simulations VENOM VENOM - detailed, Geant4 based Monte Carlo package used to simulate all aspects of background contributions.  Optimise shielding design (D. Stewart et al. NIM A , )  Determine material purity requirements

Jeanne Wilson TAUP 2007 Experimental Status (~3500 mwe)  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

Jeanne Wilson TAUP 2007 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 116 Cd crystals) New contacting methods, cabling to reduce cross-talk and pick-up Materials testing and sourcing Shielding development Pulse shape analysis Coincidences Pixellisation

Jeanne Wilson TAUP 2007 Physics Results - published  4.34 kg.days of 2×2 proto-type data  Measurement of 4-fold forbidden 113 Cd  decay C. Gößling et al. Phys. Rev. C, 2005, 72, T 1/2 = (8.2 ± 0.2 (stat.) ± 1.0 (sys))10 15 yrs  Likelihood analysis for double-beta decay signatures T. Bloxham et al. Phys. Rev. C 76, , 2007 (arXiv: )

Jeanne Wilson TAUP 2007 Physics Results - published T. Bloxham et al. Phys. Rev. C 76, , 2007 (arXiv: ) 130 Te →g.s. 116 Cd →g.s.

Jeanne Wilson TAUP 2007 Physics Results - published T. Bloxham et al. Phys. Rev. C 76, , 2007 (arXiv: )

Jeanne Wilson TAUP 2007 Physics Results - published T. Bloxham et al. Phys. Rev. C 76, , 2007 (arXiv: ) A. Barabash et al. (arXiv:nucl-ex/ ) New limits - A. Barabash et al. (arXiv:nucl-ex/ )

Jeanne Wilson TAUP 2007 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

Jeanne Wilson TAUP 2007 New Results PRELIMINARY PRELIMINARY PRELIMINARY

Jeanne Wilson TAUP 2007 New Results PRELIMINARY PRELIMINARY PRELIMINARY Some new world-best limits

Jeanne Wilson TAUP 2007 Coincidence Searches  Some rare decays can give distinctive coincidence signatures. eg. 130 Te → 130 Xe* + 2e - → 130 Xe + 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

Jeanne Wilson TAUP 2007 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 T half > 2.6 x years (90% C.L.) cf 2.5 x years from single peak search  Efficiencies will increase significantly for larger array. 64 array (2% resolution), efficiency = 3.2%

Jeanne Wilson TAUP 2007 Coincidences  ββ (to g.s.) is normally single crystal event  Intrinsic 238 U and 232 Th could be major backgrounds, but more likely to trigger multiple crystals.  Select only single site events for 0ν ββ peak search For 64K array – this reduces 232 Th chain events from crystals by >50% Simulated 238 U chain events inside detectors No Energy smearing

Jeanne Wilson TAUP 2007 Timing Coincidences  The major contribution to 238 U spectrum at 2  3MeV is the fast  decay: 214 Bi  214 Po  210 Pb  >40% efficiency for tagging 214 Bi events originating inside the crystals  endpoint 3.3MeV, accounts for >70% events in 2-3MeV region from 238 U chain  s 7.7MeV alpha half-life =  s

Jeanne Wilson TAUP 2007 Observation of 214 Bi events  Surface coating → self calibrating device! small dead volume Measure of “paint” activity T 1/2 = 162 ± 19  s

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

Jeanne Wilson TAUP 2007 Pixellisation - II  Tests of 16×16 1.6mm pixel detectors  Working on photolithography for 200μm pixel detectors.

Jeanne Wilson TAUP 2007 Ssssummary Ssssummary

Jeanne Wilson TAUP 2007 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.

Jeanne Wilson TAUP 2007 Ssspare Ssslidesss Ssspare Ssslidesss

Jeanne Wilson TAUP 2007 Shielding and Veto  Optimise shielding design for neutrons Size = 18.4 m 3, Mass = kg  Active scintillator  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 detectors

Jeanne Wilson TAUP 2007 (A,Z)  (A,Z-2) + 2 e + (+2 e )  +  + e - + (A,Z)  (A,Z-2) + e + (+2 e )  +/EC 2 e - + (A,Z)  (A,Z-2) (+2 e ) EC/EC Enhanced sensitivity to right handed weak currents (V+A) p p n n e+e+ e+e+ Q-4m e c 2 Q-2m e c 2 Q  +  + Modes

Jeanne Wilson TAUP 2007 Measured Backgrounds

Jeanne Wilson TAUP 2007 Physics Results - New PRELIMINARY