1. COBRA A new Approach to -Decay UK HEP Forum, Abingdon, May 11 th, 2003 Daniel Muenstermann University of Dortmund www.COBRA.physik.uni-dortmund.de COBRA

2. What is COBRA about? COBRA aims to study -decays of Cd, Zn and Te isotopes. deals with and evolves low-level physics. is currently based in the UK (Univ. of Oxford, K. Zuber, spokesperson) Germany (Univ. of Dortmund) Italy (LNGS)

3. COBRA Why is (0 ) -decay an opportunity? It can determine the nature of the neutrino (Majorana vs. Dirac particle). n n p p e e Leptoquarks, Rp violating SUSY It might help to disclose other modifications of weak processes like right-handed weak currents n n p p L=2, Eff. Majorana masses It can reveal the rest-mass of the electron neutrino on an absolute scale.

4. COBRA More on + + decays There are 3 possible modes: + +: (A,Z) (A,Z-2) + 2 e + (+2 e )(Q - 4 m e c 2 ) + /EC:e - + (A,Z) (A,Z-2) + e + (+2 e ) (Q - 2 m e c 2 ) EC/EC:2 e - + (A,Z) (A,Z-2) (+2 e ) (Q) (eV) + + -ellipsoid Heidelberg- Moscow evidence + + decays are dominated by right-handed weak currents ! EC/EC is preferred by phase space!

5. COBRA Why is CdZnTe a favourable material? 116 Cd:Q-Value at 2.8 MeV, above all naturally occuring -lines 130 Te:has large natural abundance (34 %) sits in compton gap of 208 Tl direct comparison to geochemical experiments 106 Cd: + + -decay at 2.7 MeV possible, but + /EC or EC/EC decays preferred by phase space It contains 9 -isotopes, among them potential for a background-free experiment where mass limits grow linear in measurement time!

6. COBRA Why are CdZnTe semiconductor detectors advantagous devices? good energy resolution (compared to scintillators) high density compared to gaseous ionisation detectors, large source masses no cooling needed with CdZnTe industrially available, commercial development in progress in our case: source = detector particle tracking is possible with pixel-devices (solid-state-TPC) operation in cryogenic mode possible

7. COBRA Reality strikes back: Current Limitations of CdZnTe Crystal growth:large quantities: 1 cm 3 cubes selected detectors: 16 cm 3 cubes Large hole trapping: Use selective electron readout Therefore: Instead of few large crystals (Heidelberg-Moscow = 1-3 kg) build an array of small detectors

8. COBRA Idea: Build an array of CdZnTe detectors. Characteristics of COBRA: minimal use of construction material close to the detectors necessary as no cooling is needed source = detector 9 -isotopes measured simultaneously

9. COBRA Early Achievements of COBRA First ad-hoc efforts with commercial 0.5 cm 3 detector and MCA-based readout: Veto and shield are working 2 publications with new and improved limits: Muenstermann and Zuber: An alternative Search for the Electron Capture of 123 Te, J. Phys. G: Nucl. Part. Phys. 29 (2003) B1-B4 Kiel, Muenstermann and Zuber: A Search for various Double Beta Decay Modes of Cd, Te and Zn Isotopes, nucl-ex/0301007, to be published in Nucl. Phys. A

10. COBRA Early Achievements of COBRA Calibration spectrum with 228 Th:

11. COBRA Low energy part of the spectrum: Early Achievements of COBRA Background at 2.8 MeV: ~ 5 ·10 -4 events/(keV g d) 208 Tl-peak at 2.6 MeV visible. Contamination in detector or surroundings? Build own low-level mounting Spectrum with and without shield:

12. COBRA The Status of COBRA There is a 2 x 2 array of 1 cm 3 detectors with custom-built preamplifier electronics (still) CAMAC-based event-by-event readout passive copper and lead shielding a plastic-scintillator based cosmic muon veto

13. COBRA Time schedule Next week: relocation of the setup into the Gran Sasso laboratory Autumn 2003:First results due, decision about funding of 4 x 4 prototype due Spring 2004:scalable 4 x 4 prototype and scalable readout electronics operational Winter 2004:proposal for a competitive 0 -decay based on CdZnTe

14. COBRA ad hoc: COBRA compared to An Ideal Experiment by Steve Elliot large mass:CdZnTe detectors are available with m < 100 g Array of detectors good source radiopurity:Work in progress, no serious cosmogenic background demonstrated technology:Again, work in progress natural isotopes:Yes source = detector: Yes energy resolution (< 3 %):Yes ease of operation:Yes, but ask us in autumn... large Q value:Yes event reconstruction: With pixel detectors (3D)

15. COBRA Summary and Outlook COBRA uses commercially available CdZnTe detectors to measure 9 isotopes simultaneously has already improved 6 limits with comparably little effort will move from the surface to the Gran Sasso underground lab for improved low-level measurements next week is up to now a rather small collaboration, so...

16. COBRA Last Words Join the party! Collaborators most welcome!