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The COBRA Experiment
Jeanne WilsonUniversity of Sussex, UK
On behalf of the COBRA CollaborationOn 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 ConceptCadmium Telluride 0-neutrino Beta Decay Research Apparatus
Build up a large array of CdZnTeCdZnTe semiconductor detectors
K. Zuber, Phys. Lett. B 519,1 (2001)K. Zuber, Phys. Lett. B 519,1 (2001)
1 cm
Jeanne Wilson TAUP 2007
0 Candidate Isotopes CdZnTe contains 9 Isotopes
116116Cd, Cd, 130130Te, Te, 114114Cd, Cd, 7070Zn, Zn, 128128TeTe (--), 6464Zn, Zn, 106106Cd, Cd, 108108Cd, Cd, 120120TeTe (++ ,+EC, EC/EC)
Main 0 candidate: 116116CdCd• Endpoint = 2805 keV• Enrichment to 90%• Favourable G0|M0|2 → T1/2 ~1026 years for 50meV
Also interesting: 106106CdCd• 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)M. Hirsch et al., Z. Phys. A 347,151 (1994)
Jeanne Wilson TAUP 2007
Experimental Requirements
64,000 1cm3 crystals = 418 kg
90% enriched in 116116CdCd Backgrounds < 0.001 count keV-1kg-1year-1
Energy Resolution < 2%
EBMtaT
02/1
Jeanne Wilson TAUP 2007
Energy Resolution 2 decays
T1/2(116Cd)=2.7×1019 yFor E = 2%, fraction of 22 events in 00 window ≈ 3 3 × 10× 10-9-9
≈ 1010-5-5 events/kg/year events/kg/year
8Q E 6
me QF =
S. Elliott, P. S. Elliott, P. Vogel, Ann. Rev. Vogel, Ann. Rev.
Nucl. Part. Sci. Nucl. Part. Sci. 20022002
Jeanne Wilson TAUP 2007
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
Jeanne Wilson TAUP 2007
Backgrounds Muons, neutrons sources
• Intrinsic and surface contaminants• Cosmogenics
Crystals238U, 232Th decay chains40K137Cs210Po210Pb on surfaceCosmogenic isotopes
Delrin Holder238U,232Th decay chain40K137Cs
Gas222Rn gas
Chamber walls210Pb on surface
Jeanne Wilson TAUP 2007
SimulationsVENOM 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(D. Stewart et al. NIM A571 2007, 651-662))
Determine material purity requirements
Jeanne Wilson TAUP 2007
Experimental Status Low-background experiments in LNGS (~3500 mwe)(~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 116Cd 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 113Cd decay
C. Gößling et al. Phys. Rev. C, C. Gößling et al. Phys. Rev. C, 2005, 72, 0643282005, 72, 064328
TT1/21/2= (8.2 ± 0.2 (stat.) ± 1.0 (sys))10= (8.2 ± 0.2 (stat.) ± 1.0 (sys))101515 yrs yrs
Likelihood analysis for double-beta decay signaturesT. Bloxham et al. Phys. Rev. C 76,T. Bloxham et al. Phys. Rev. C 76,025501, 2007 (arXiv:0707.2756)025501, 2007 (arXiv:0707.2756)
Jeanne Wilson TAUP 2007
Physics Results - published
T. Bloxham et al. Phys. Rev. C 76,T. Bloxham et al. Phys. Rev. C 76,025501, 2007 (arXiv:0707.2756)025501, 2007 (arXiv:0707.2756)
130Te→g.s.
116Cd→g.s.
Jeanne Wilson TAUP 2007
Physics Results - published
T. Bloxham et al. Phys. Rev. C 76,T. Bloxham et al. Phys. Rev. C 76,025501, 2007 (arXiv:0707.2756)025501, 2007 (arXiv:0707.2756)
Jeanne Wilson TAUP 2007
Physics Results - published
T. Bloxham et al. Phys. Rev. C 76, 025501, 2007 (arXiv:0707.2756)T. Bloxham et al. Phys. Rev. C 76, 025501, 2007 (arXiv:0707.2756)New limits - A. Barabash et al. A. Barabash et al.
(arXiv:nucl-ex/0703020)(arXiv:nucl-ex/0703020)
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 ResultsPRELIMINARY PRELIMINARY PRELIMINARY
Jeanne Wilson TAUP 2007
New ResultsPRELIMINARY PRELIMINARY PRELIMINARY
Some new world-best limits
Jeanne Wilson TAUP 2007
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
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 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%
Jeanne Wilson TAUP 2007
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
detectorsNo Energy smearing
Jeanne Wilson TAUP 2007
Timing Coincidences The major contribution to 238U spectrum at
23MeV is the fast decay:214214Bi Bi 214214Po Po 210210PbPb
>40% efficiency for tagging 214Bi events originating inside the crystals
endpoint 3.3MeV, accounts for >70% events in 2-3MeV region from 238U chain
7.7MeV alpha half-life = 164.3164.3ss
Jeanne Wilson TAUP 2007
Observation of 214Bi events Surface coating → self calibrating
device!• small dead volume • Measure of “paint” activity
TT1/21/2 = 162 = 162 ± ± 1919ss
Jeanne Wilson TAUP 2007
Pixellisation - I Extra information on events with signal
energy.200m pixels (example simulations):
eg. Could achieve nearly 100% identification of 214Bi events.
Bloxham and Freer, NIM A, v572, issue 2, p722-728 (2007)
00
1-1.5mm1-1.5mm ~15~15mm
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
SsssummarySsssummary
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 Ssspare SsslidesssSsslidesss
Jeanne Wilson TAUP 2007
Shielding and Veto Optimise shielding design for neutrons
Size = 18.4 m3, Mass = 64964 kg Active scintillatorActive scintillator component for inner layer
• Veto any residual external background components• Veto s from internal backgrounds• Enhance sensitivity to decays with high energy s
detectorsdetectors
Jeanne Wilson TAUP 2007
• (A,Z) (A,Z-2) + 2 e+ (+2e) ++ • e- + (A,Z) (A,Z-2) + e+ (+2e ) +/EC • 2 e- + (A,Z) (A,Z-2) (+2e) EC/EC
Enhanced sensitivity to right handed weak currents (V+A)
p
p
n
ne+
e+
Q-4mec2
Q-2mec2
Q
++ Modes
Jeanne Wilson TAUP 2007
Measured Backgrounds
Jeanne Wilson TAUP 2007
Physics Results - NewPRELIMINARY