The%20COBRA%20Experiment

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The COBRA Experiment

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

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Contents

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

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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
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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)
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Experimental Requirements

• 64,000 1cm3 crystals 418 kg
• 90 enriched in 116Cd
• Backgrounds lt 0.001 count keV-1kg-1year-1
• Energy Resolution lt 2

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Energy Resolution

• 2??? decays
• T1/2(116Cd)2.71019 y
• For ?E 2, fraction of 2??? events in 0???
  window 3 10-9
• 10-5 events/kg/year

S. Elliott, P. Vogel, Ann. Rev. Nucl. Part. Sci.
2002
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Energy Resolution
?E 1.9 _at_ 2.8MeV 2.9 _at_ 662keV

• Only electron signal read out (CPG technology)
• Possible improvements cooling, new grids
• Better detectors are available

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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
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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

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Experimental Status

• Low-background experiments in LNGS (3500 mwe)
• 22 proto-type limits published
• 44 proto-type installed new results
• New laboratory space, clean room conditions,
  fully automated DAQ and environmental
  control/monitoring.
• To be upgraded to 444 (64-Array) next month

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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

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Physics Results - published

• 4.34 kg.days of 22 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 (arXiv0707.2756)

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Physics Results - published
130Te?g.s.
116Cd?g.s.
T. Bloxham et al. Phys. Rev. C 76, 025501, 2007
(arXiv0707.2756)
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Physics Results - published
T. Bloxham et al. Phys. Rev. C 76, 025501, 2007
(arXiv0707.2756)
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Physics Results - published
T. Bloxham et al. Phys. Rev. C 76, 025501, 2007
(arXiv0707.2756)
New limits - A. Barabash et al.
(arXivnucl-ex/0703020)
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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
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New Results

• PRELIMINARY PRELIMINARY PRELIMINARY

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New Results

• PRELIMINARY PRELIMINARY PRELIMINARY
• Some new world-best limits

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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
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Coincidence Searches

• Searching for 1993keV (?E) and 536keV (?E)
  coincident energy deposits in data
• 2 events found ? lt5.32 events (Poissonian 90
  limit)
• Efficiency from simulation
• 0.652
• Leads to a half-life limit of
• Thalf gt 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

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Coincidences

• ßß (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 0?ßß peak
  search
• For 64K array this reduces 232Th chain events
  from crystals by gt50

Simulated 238U chain events inside detectors No
Energy smearing
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Timing Coincidences

• The major contribution to 238U spectrum at 2-3MeV
  is the fast b-a decay
• 214Bi ? 214Po ? 210Pb
• gt40 efficiency for tagging 214Bi events
  originating inside the crystals

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Observation of 214Bi events

• Surface coating ? self calibrating device!
• small dead volume
• Measure of paint activity

T1/2 162 19?s
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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
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Pixellisation - II

• Tests of 1616 1.6mm pixel detectors
• Working on photolithography for 200µm pixel
  detectors.

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Ssssummary
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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.

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• Ssspare Ssslidesss

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Shielding and Veto

• 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

detectors
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?? 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 (VA)
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Measured Backgrounds
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Physics Results - New
PRELIMINARY