1. COBRA – CZT detectors for neutrinoless double beta decay measurements Martin Freer, University of Birmingham

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

3. Fundamental neutrino properties What is the absolute neutrino mass? Are neutrinos their own antiparticles? β-β- β-β- Energy β-β- β-β-

4. Sensitivity 0  : Peak at Q-value of nuclear transition Half-life factors: Half-life and neutrino mass Critical factors (factor of 2 in neutrino mass requires factor of 16 improvement): Background Energy resolution Mass time

5. 2  - decay S. Elliott, P. Vogel, Ann. Rev. Nucl. Part. Sci. 2002 Energy resolution important  semiconductor Fraction of 2  in 0  peak: Signal/Background: 2  is the ultimate irreducible background

6. How big does a detector need to be? ( Background free)  50 meV implies half-life measurements of 10 26-27 yrs 1 event/yr - need 10 26-27 source atoms This is about 1000 moles of isotope, implying 100 kg Need to get backgrounds to below the above level! (currently achieved levels about 0.1 ct/keV/kg/yr)

7. COBRA (Source = Detector) CdZnTe (room temp semiconductor) nat. ab. (%)Q (keV)Decay mode

8. Resolution < 1% @ 662 keV 32,768 pieces of 4 x 4 x 2 mm (CZT) form a 1.2 x 0.6 m sensitive area Burst Alert Telescope (BAT) Gamma-ray applications

9. CZT for COBRA Isotopic abundance (90%, i.e. enrichment) Detection efficiency (65%) Mass (420 kg CdZnTe, about 180 kg Cd) Energy resolution (1-2% at 2.8 MeV) Background at peak position (10 -3 -10 -4 cts/kg/keV/yr) To probe neutrino masses below 50 meV Two strands:- (i)large array of CPG CZT detectors at Gran Sasso (Current) (ii)Development of Pixel CZT detector (Future – PPRP)

10. Gran Sasso (GPG –CZT)

11. Half-life around 10 15 yrs. Only three isotopes of this type are known, 50 V, 113 Cd and 115 In. Measurement of 4-fold forbidden decay of 113 Cd

12. Sensitivity 50 meV (background limited) (background free) Crucial items (for every experimemt): - mass (enrichment) - energy resolution - background

13. 3.2 mm 2800 keV electrons 2800 keV electron pairs Pixel Detectors 0  3 MeV 

14. Acceptable Pixel Response Event not vetoed Unacceptable pixel response Event Vetoed Compton Scattering Alpha-decay

15. Total U-238 Th-232 K-40 Cs-137 U, Th chains, 40 K and 137 Cs. Remove events where more than 1 crystal has E>10keV. What are the main natural backgrounds?

16. For 200  m 130 Te and 116 Cd would hit >7 pixels 130 Te β 2β2β 0 1 1 70%

17. Identification of alpha-particles

18. 122 keV 136 keV Pixelisation - Running 256 pixel det with ASIC, 1.6mm pixel size crystal ASIC readout Single pixel 57 Co spectrum Additional 16 pixel detector with conventional readout running

19. Compton photo Compton E1 E2 154 Eu 60 Co

20. Anode Cathode Anode Cathode

21. Anode Background measurements 214 Po (7.68 MeV ) 218 Pb (6.00 MeV ) 218 Po T 1/2 =3.10 mins Uranium decay chain 238 U Cosmic rays

22. Scanning the 16 pixel - Liverpool 30  m precision 1 GBq 57 Co source Homogeneity scan Sliced scanning at side with Flash ADC z-information in risetime

23. Pixel detectors - next step 64 pixel detectors 2x2x0.5 cm 3 Pixel electrodes will be replaced by 200  m pixels, mask in hand

24. Summary COBRA collaboration is presently completing the installation of a 64 element CZT array in Gran Sasso. This will further improve half-life limits. Ultimate tool for double beta search would be a high resolution Pixel CZT detector – presently engaged in design of a 200 micon pixel detector TIMESCALE: prototype Pixel detector and readout by 2011 running experiment by 2013

25. Enrichment Crystal growing will be done at Material Research Centre Freiburg (Germany) Isotopical enrichment is crucial to keep experimental size under control and target mass high Crystal growing requires 7N input material quality Enriched material has to be purified for that, this has to be explored Goal: To produce an enriched CdZnTe detector, setup logistic

26. Background Cosmogenics neutrons 116 Cd (Q=2809 keV) 2  113 Cd (n th,  ) 114 Cd Measurement: E. Porras et al., NIM B 111, 325 (1996) Alphas, Betas, Gammas muon induced neutrons

27. Pixel pads 50  m pixel detector, produced at Washington Univ. at St. Louis Will require new readout electronics, ie ASIC

28. Pixel-detectors and readout Develop prototype pixel detector Full testing, simulation, optimisation Library of well defined events (ion implantation) Running pixel detector underground Develop and test prototype ASIC for pixel readout Evaluate responses, data rates and performance Demonstrate radiopurity

29. The 64 detector array Installed at LNGS in April 2006 Mass factor 16 higher, about 0.42 kg CdZnTe Crucial to identify background components long term stability, …energy measurement only Worldwide largest experiment of 1cm 3 CPG detectors Remaining 48 will be installed in two weeks

30. Underground at Boulby Pixel detectors will be installed in Boulby, readout totally different from LNGS setup. Link with Boulby already established

31. The strategy We believe that pixelated CZT detectors are THE step forward in background reduction with respect to pure passive methods Experimentally confirm the large background reduction of pixelated detectors, includes development of high resolution detectors and readout electronics Ultimate prove that such a system running underground is clean enough Setup logistic for enriched detector production and produce a running detector with good parameters We have a clear strategy and but we need manpower

32. New Results PRELIMINARY PRELIMINARY PRELIMINARY

33. New Results PRELIMINARY PRELIMINARY PRELIMINARY Some new world-best limits

34. Additional issues LNGS very supportive, offering more space at a different location New funding from German Research Society (DFG): 100 kEuro plus two 2 year PhD positions US grant submitted (DUSEL R&D opportunities): 600 k$

35. Shielding and Veto (WP4) Simulated LNGS neutron flux <1 neutron per year!<1 neutron per year! (in 64000 detectors) detectors Development of light sensors, optical simulation, test module construction Mechanical design, experimental test of shielding Ge-facility for material selection Full background model D. Stewart et al., accepted by Nucl. Inst. Meth. A COBRA rather compact experiment

36. Monte Carlo and Physics (WP6) Sophisticated MC based on GEANT4 exists Upgrade of simulation framework Development of detector response framework Development of data analysis framework

37. Latest Limits world best First COBRA Double beta results T. Bloxham et al., submitted C. Goessling et al., Phys. Rev. C 72, 064328 (2005) Rare 113 Cd Beta decay Based on 4 detectors only

38. Double beta decay Neutrino mass sensitivity F. Simkovic, ILIAS DBD meeting, Valencia 2006 Latest shell model calculations make Cd-116 most promising I isotope to search for ! E. Caurier et al., arXiv:0709.0277 QRPA:

39. Advantages Source = detector Semiconductor (Good energy resolution, clean) Room temperature (exp. handling) Tracking („Solid state TPC“) Modular design (Coincidences) Industrial development of CdZnTe detectors (medical physics, homeland security) 116 Cd Q-value above 2.614 MeV (no gamma-BG from natural decay chains)

40. Detector studies (WP1+WP5) Paint contribution at 2.8 MeV: about 0.2 counts/keV/kg/yr N2 atmosphere running 3D scanning Depth information via pulse shape New grid design Suitable passivation, contacting Photoluminescense Thermal neutron capture 113 Cd (n th,  ) 114 Cd Ion implantation (in-situ, ex-situ calibration) N2 atmosphere running Installation of a sophisticated slow control system (temperature, humiditiy, vibrations...) Long term behaviour study Improve shielding Coincidence studies Publish physics results based on 64 array Calibration New larger space at LNGS All activities focus on implications for a large scale experiment

41. Underground location Best location determined by high energy neutron background due to muon interactions in the rock

42. Examples: 16 pixel 4.4 MeV  from 12 C

43. 0  Any ∆L=2 process can contribute to 0  R p violating SUSY V+A interactions Leptoquarks Double charged Higgs bosons Compositeness Heavy Majorana neutrino exchange Light Majorana neutrino exchange... 1 / T 1/2 = PS * NME 2 *  2

44. The standard lore Measured quantity Phase space integral calculable Nuclear transition matrix element Quantity of interest Effective Majorana neutrino mass 1 / T 1/2 = PS * NME 2 * ( / m e ) 2 Light Majorana neutrino exchange

45. What are the goals? Step 1 : m=0.23 eVStep 2: m <50 meV H. V. Klapdor-Kleingrothaus et al. Mod. Phys. Lett. 2006

46. Half-life 45 mins The 480 hour spectrum. The box indicating the region used for the background analysis is shown.