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G. Antona, V. Bocarovb, P. Cermakb, O. Civitaresec, J. Dursta, A. Faulerd

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G. Antona, V. Bocarovb, P. Cermakb, O. Civitaresec, J. Dursta, A. Faulerd COBRA G. Antona, V. Bocarovb, P. Cermakb, O. Civitaresec, J. Dursta, A. Faulerd. M. Fiederled, A. Garsone, D. Gehref, C. GÄo¼lingg, H. Gastrichg, C. Hagnerh, M. Heinef, N. Heidrichh, B. Januttaf, M. Junkeri, T. KÄottigg, S. Kietzmannh, H. Krawczynskie, V.K. Leee, Q. Lie, F. LÄucka, J. Martine, T. Michela, D. MÄunstermanng, T. Neddermanng, C. Oldorfh, S. Rajekg, O. Reineckef, W. Schmidt-Parzefallh, O. Schulzg, M. Schwenkef, F. Simkovicj, J. Suhonenk, I. Steklb, J. Timmh, W. Thurowf, B. Wonsakh, Y. Yine, R. Zimmermannh, K. Zuberf;¤ a UniversitÄatErlangen{NÄurnberg { Germany b Technical University of Prague { Czech Republic c University of La Plata { Argentina d Freiburg Materials Research Center { Germany e Washington University in St. Louis { USA f Technische UniversitÄatDresden { Germany g Technische UniversitÄatDortmund { Germany h UniversitÄatHamburg { Germany i Laboratori Nazionali del Gran Sasso { Italy j University of Bratislava { Slovakia k University of Jyvaskyla { Finnland (¤ Spokesperson) Abstract The COBRA collaboration searches for the neutrinoless double beta decay with a large array of Cadmium Zinc Telluride (CZT) semiconductor detectors. CZT o®ers the low radioactivity levels and the energy resolution needed for a rare decay search, with the advantage of operation at room temperature. It contains a number of double beta decay candidates, among which 116Cd is the most promising one. Its Q-value of 2.8 MeV is well above many of the possible background contributions from natural radioactivity. Besides a prototype apparatus consisting of 64 detectors is being established at LNGS, the ¯rst CZT pixel detector system was installed to investigate the major experimental issues of operating tracking CZT detectors in low background mode. Additional studies into the detector technology are proceeding in surface laboratories. 9 1 Introduction Over the last decade the fact that neutrinos oscillate between flavour states was well established. The oscillation is explained by assigning a non-vanishing rest mass to the neutrinos. The absolute mass scale can not be probed with oscillation experiments. Ad- ditionally to the unknown absolute mass scale of the neutrino, the fundamental nature of neutrinos, i.e. if the neutrino is a Majorana or Dirac particle, is still unknown. Both of these questions can be answered in neutrinoless double beta decay experiments. The COBRA experiment looks for the neutrinoless double beta decay of 116Cd isotope in a large array of Cadmium Zinc Telluride (CZT) semiconductor detectors. 1.1 Neutrinoless Double Beta Decay Considering the binding energies of nuclei, one will ¯nd that for 35 known nuclides the single beta decay is energetically forbidden, but the double beta decay is not. This process is a second order process in the Standard Model of particle physics which will read: ¡ (Z; A) ! (Z + 2;A) + 2e + 2¹ºe (2º¯¯{decay): (1) A neutrinoless decay would also occur, if the neutrino was its own antiparticle. In that case, all the energy of the decay would be carried away by the two emitted electrons: (Z; A) ! (Z + 2;A) + 2e¡ (0º¯¯{decay): (2) Based on the rate of this process the calculation of the e®ective Majorana neutrino mass is possible. The results from oscillation experiments suggest an absolute mass scale of the order of » 50 meV in the inverted mass scheme. Searches for the 0º¯¯ did not yet observe the predicted signals, yielding an upper limit on the neutrino mass of the order of 0.2- 0.5 eV. The observational evidence for 0º¯¯ of 76Ge[1], published in 2002, has resulted in a lot of discussion in the ¯eld. To verify this result, further large scale experiments are planned. In addition, any Ge-speci¯c backgrounds and the uncertainties deriving from the calculations of the nuclear matrix elements have to be ruled out, thus exploration of other nuclei is vital in this ¯eld. 2 COBRA The idea of COBRA is to use a large quantity of CZT semiconductor material, which contains a number of nuclides that are able to undergo double beta decay[2]. The main focus will be on 116Cd, which is well suited for a double beta search due to its high Q- value of 2809 keV. Calculations show that the theoretical rates of 0º¯¯{decay for 116Cd is favourably high. First shell model calculations for nuclei heavier than 48Ca make 116Cd the most sensitive isotope from the theoretical point of view [3]. COBRA follows the strategy that source and detector are identical, a method proven to be successful in various other double beta decay approaches. As a semiconductor, CZT crystals can be produced with good energy resolution and low levels of intrinsic 10 radioactivity. Due to a large bandgap, the detectors can be operated at room temperature avoiding extensive cooling operations. The COBRA experiment is still in the R&D phase and there are two competing concepts for the layout of the experiment. One would be an array of 40 £ 40 £ 40 1 cm3 coplanar grid (CPG) CZT detectors, the other would be a huge solid state time projection chamber (TPC) consisting of CZT pixel detectors. Both concepts add up to about 400 kg of CZT, enriched to 90% in 116Cd, to achieve a neutrino mass sensivity of 50 meV and below. The modular design of the COBRA experiment in both cases would lead to a signi¯cant background reduction, as for example high energetic gamma rays are likely to interact in multiple detectors, while the energy deposited in a double beta decay is most probably contained in a single crystal. In a ¯ne grained pixelated detector, it would be possible to reconstruct tracks of throughgoing particles. Two electrons emerging from a pixel would result in a line shaped track, while for example alpha particles of comparable energy would only hit very few pixels arranged within a circle, thus a pixelated solution would reduce the background even further. 3 R&D Activities Since there are two di®erent approaches for a COBRA experiment, namely an array of CPG CZT detectors or a solid state TPC consisting of pixelated CZT detector systems, the R&D activities have to be divided into a couple of subcategories. First of all, there are activities concerning the CPG detector option, which concern handling and operation of the CPG detectors as well as operating a CPG test setup at the LNGS laboratory. Second we have a number of institutes working on the pixel detector option. There are a number of di®erent pixelated CZT detector systems available to the collaboration, which have to be characterized to ¯gure out which best ¯ts our needs. So far CZT pixel systems have not been operated underground and for the ¯rst time we managed to install and operate a pixel detector system underground at LNGS in 2009. Common to both approaches and all low background experiments is the issue of shielding. There are a variety of di®erent choices concerning the shielding of the COBRA experiment. These vary from a purely passive shielding to an active shielding realized by a big tank of liquid scintillator. To optimize the shield geometry we have to run detailed Monte Carlo studies as well as experimental cross checks. Another part of the R&D activities is the growing of CZT crystals, in particular to be independent from commercial suppliers. Furthermore, a lot of e®ort has been put into building a material screening facility at TU Dortmund. The details on the general purpose and the progress made in the di®erent ¯elds in 2009 are given in the following. 3.1 CPG activities The COBRA collaboration has been operating a prototype setup for CPG detectors at the LNGS laboratory for a couple of years. This setup has been used to provide information on the background levels of the various setup components and thereby helped to reduce the background level step by step. Furthermore the setup will provide valuable information 11 about methods of instrumentation, long term stability and construction applicable to a larger array. The setup in its current status can house up to 64 1 cm3 detectors in the innermost volume, the nest. These are arranged in 4 layers, each housing 4 £ 4 detectors. The nest, measuring 10 £ 10 £ 10 cm3, is surrounded by 10 cm of pure copper, again surrounded by 20 cm of low radioactivity lead. These metal layers, called castle, are a passive shield against exterior gamma radiation. This castle is embedded in a copper faraday cage which is located within a neutron shield, consisting of 7 cm thick slabs of borated polyethylen. Low diameter coaxial cables and copper traces on Kapton foils are used to supply the bias voltage and read out the signals. These are feed through the shielding. In 2008 the ¯rst layer was installed into the prototype setup and with the data taken we were able to identify two of the major contributions to the background, namely the red color in the passivation lacquer of the crystals and the air within the setup. This was a±rmed by operating colorless passivated detectors provided by the manufacturer and installing a liquid nitrogen flushing system. The spectrum can be seen in Figure 1. With Sum spectrum of colourless detectors, 2.8 kg days Sumspectrum of red detectors, 18.0 kg days 2 10 113 β- Cd Expected 116Cd peak 10 214Bi (609 keV) Counts/keV/kg/day (2809 keV) 1 10-1 10-2 500 1000 1500 2000 2500 3000 Energy (keV) Figure 1: Sum spectrum of the ¯rst layer with an exposure of 18.0 kg¢days and a layer with colourless passivation and nitrogen flushing with an exposure of 2.8 kg¢days. The background level could be decreaded by an order of magnitude. this signi¯cant reduction of the background level, the 2º¯¯ decay of 116Cd is nearly in reach to be measured.
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