the cobra experiment jeanne wilson university of sussex, uk on behalf of the cobra collaboration...

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


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


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)


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


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


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


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


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


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


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


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)


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.



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)



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)


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


New ResultsPRELIMINARY PRELIMINARY PRELIMINARY


New ResultsPRELIMINARY PRELIMINARY PRELIMINARY

Some new world-best limits


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


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%


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


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


Observation of 214Bi events Surface coating → self calibrating

device!• small dead volume • Measure of “paint” activity

TT1/21/2 = 162 = 162 ± ± 1919ss


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


Pixellisation - II Tests of 16×16 1.6mm pixel detectors

Working on photolithography for 200μm pixel detectors.


SsssummarySsssummary


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.


Ssspare Ssspare SsslidesssSsslidesss


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


• (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


Measured Backgrounds


Physics Results - NewPRELIMINARY

the cobra experiment jeanne wilson university of sussex, uk on behalf of the cobra collaboration...

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