K. Zuber, Uni. Sussex IDEA Meeting, Milano 9. Nov Status of COBRA

COBRA Use large amount of CdZnTe Semiconductor Detectors Array of 1cm 3 CdTe detectors K. Zuber, Phys. Lett. B 519,1 (2001)

Isotopes nat. ab. (%)Q (keV)Decay mode COBRA: CdZnTe semiconductors

Advantages Source = detector Semiconductor (Good energy resolution, clean) Room temperature Tracking („Solid state TPC“) Modular design (Coincidences) Industrial development of CdTe detectors Two isotopes at once 116 Cd above MeV

COBRA collaboration University of Dortmund University of Sussex Laboratori Nazionali del Gran Sasso University of WarwickUniversity of Birmingham University of Liverpool University of York Rutherford Appleton Laboratory University of Bratislava Washington University at St. Louis University of Surrey (UK), University of Hamburg (Germany), Jagellonian University (Poland), University of Prague (Czech Republik), Louisianna State University (USA) Material Research Centre Freiburg

The 2x2 prototype 4 naked 1cm 3 CdZnTe 4.3 kg x days of data accumulated, stopped March 2006 Setup installed at Gran Sasso Underground Laboratory PertinaxCopperCZTWax

Physics Cd T 1/2 = (8.2 ± 0.2 (stat.) (sys)) yrs 113 Cd one of only three 4-fold forbidden  -emitters known in nature C. Goessling et al., Phys. Rev. C 72, (2005)

Latest Limits world best First COBRA Double beta results T. Bloxham et al., submitted

The background model Input: Pertinax (grid, base), detectors, paint and copper Limiting background: Passivation paint on detectors

Coincidences – 214 Bi T 1/2 = 162 ± 19  s Can exclude backgrounds through timing coincidence 214 Bi  214 Po  210 Pb 7.7MeV alpha half-life =  s Beta with endpoint 3.3MeV „self-calibrating“

Strategies We started a long term R& D with eV-Products to find another passivation (they have alternatives) Alternative providers not using any paint (Freiburg Material Research Centre)

The 64 detector array The next step towards a large scale experiment, Scalable modular design, explore coincidences Worldwide largest experiment of 1cm 3 CPG detectors Mass factor 16 higher, about 0.42 kg CdZnTe Physics: - Can access 2 ECEC in theoretically predicted region -Precision measurement of 113Cd - New limits 70 detectors in total available/characterised

Redesigned prototype Delrin holder and kapton foil Scalable design for larger masses

The first layer Installed at LNGS in april 2006

New passivation (4 detectors) Sample measurement at LNGS: U238: 1.4 mBq/kg (before 2.1 Bq/kg) Th232: 0.61 mBq/kg (before 1.1 Bq/kg) Improvement of about 3 orders of magnitude Monte Carlo expectation Paint contribution at 2.8 MeV: about 0.2 counts/keV/kg/yr

New passivation Very preliminary: At least a factor 10 better, lot of construction work around COBRA at LNGS, no coincidences, no nitrogen flushing... Around 10 counts/keV/kg/yr Raw data Cd116

2  - decay S. Elliott, P. Vogel, Ann. Rev. Nucl. Part. Sci Energy resolution (FWHM) important  semiconductor Fraction of 2  in 0  peak: Signal/Background: 2  is ultimate, irreducible background

Energy Resolution Only electron signal read out (CPG technology) Possible improvements: cooling, new grids Better detectors are available  E = 2.8MeV 662keV Resolution of  =0.8% at 2.8 MeV

Back of the envelope  1/2 = ln2 a N A M t / N  (   T) ( Background free)  50 meV implies half-life measurements of yrs 1 event/yr you need source atoms This is about 1000 moles of isotope, implying 100 kg Now you only can loose: nat. abundance, efficiency, background,...

Cd116- Matrix elements =0.4eV V. Rodin et alnucl-th/ , Nucl Phys. A 2006 V. Rodin et al., nucl-th/ , Nucl Phys. A 2006 Phase space: 116Cd/76Ge = 5 116Cd/130Te=1.7

Sensitivity 50 meV

Dimension it right! 116Cd 116In 116SnA real time low-energy solar neutrino experiment? Threshold energy: 464 keV 7Be contribution g.s. alone: 227 SNU  = 14s K. Zuber, Phys. Lett. B 571,148 (2003) e e e Current idea: 40x40x40 CdZnTe detectors = 420 kg, enriched in 116Cd

Shielding and Veto Simulated LNGS neutron flux ~3x10 -7 counts/year/kg/keV in the crystals. <1 neutron per year!<1 neutron per year! (in detectors) D. Stewart et al., acc by Nucl. Inst. Meth A detectors

Monte Carlo Sophisticated MC based on GEANT4, written in C++ Signal (DECAY0) and background

And many more things going on Red = 24°C Blue = 10°C T-measurement Pulse shape analysis Thermal n-capture PL: Zn-content nm

The solid state TPC Energy resolutionTracking Pixellated CdZnTe detectors Massive background reduction Positive signal information

Pixellisation - I Massive BG reduction by particle ID, 200  m pixels (example simulations): eg. Could achieve nearly 100% identification of 214 Bi events ( 214 Bi  214 Po  210 Pb). 0   1-1.5mm ~15  m 3 MeV  7.7MeV  life-time =  s Beta with endpoint 3.3MeV  = 1 pixel,  and  = several connected pixel,  = some disconnected p.

Pixellisation - II Running 256 pixel det with ASIC, 1.6mm pixel size crystal ASIC readout 122 keV 136 keV

Pixellated detectors 2D - Pixelisation on both electrodes Solid state TPC

Rejection power of pixels First (very preliminary) look on rejection power Suggests a background reduction of 1000!

Nobody said it was going to be easy, and nobody was right George W. Bush

Summary COBRA plans to use a large amount of CdZnTe semiconductors for double beta searches Collaboration of about 30 people established Currently preparing a 64 detector array (about 0.5 kg), first 16 installed at LNGS april 2006 Design changed to allow easy upgrade to larger scales Work on signal enhancer/active veto and pixellated detectors has started Progress is fast