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The COBRA double beta decay experiment
Available online at www.sciencedirect.com
Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 488 www.elsevier.com/locate/npbps
The COBRA double beta decay experiment C. Oldorf on behalf of the COBRA Collaboration Universit¨at Hamburg, Institut f¨ur Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
Abstract The COBRA experiment is searching for double beta decay using CdZnTe semiconductor detectors. The main focus is on the isotope 116 Cd. In addition to pure energy measurements, pixelisation allows also for tracking capabilities. This kind of semiconductor tracker is unique in the field. The current status of the experiment is shown including the latest half-life limits. Keywords: Neutrinoless Double Beta Decay, Low Background Experiment, COBRA
The aim of COBRA is the search for neutrinoless double beta decay (0νββ) with a large number of CdZnTe semiconductor detectors [1]. Several candidate 0νββ isotopes are intrinsic to the CdZnTe detector material, among them two of the most promising isotopes, 116 Cd and 130 Te. 116 Cd is the most important isotope for COBRA due to its high Q-value of 2809 keV. This decay energy lies well above the highest naturally occurring γ–background (2614.5 keV from 208 Tl). 130 Te is interesting for β− β− decay due to its high natural abundance and also 106 Cd for positron decay in combination with electron capture. CdZnTe, like semiconductor detectors in general, provide good energy resolution and are very clean in respect to radioactive impurities. The Source = Detector approach of COBRA makes a large detector mass easily achievable. A big advantage of CdZnTe detectors is the operation at room temperature. Background studies for the COBRA experiment are performed at Laboratori Nazionali del Gran Sasso (LNGS). The current setup consists of Co–Planar Grid Detectors (CPG). Resolutions of less than 2% FWHM at 2614.5 keV have been achieved even with cost efficient low resolution detectors. The current test setup can house 64 of these detectors and is shielded by lead, high purity copper, boron loaded polyethylene, and a Faraday cage. This results in 0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.09.125
a background rate of about 8 counts/keV/kg/year in the main region of interest. With the LNGS test 0ν ≥ 9.4 · 1019 years for 116 Cd and setup, limits on T 1/2 0ν ≥ 5.0 · 1020 years for 130 Te were achieved. In adT 1/2 dition, new world best values for 0νβ+ EC decay to g.s. were measured for 120 Te [2]. For a large scale experiment, an array of 64000 CPG detectors with a total mass of 420 kg is proposed. This allows for coincidence studies to search for decays to excited states and to reduce background. For further background suppression, the operation of CdZnTe detectors in liquid scintillator is under investigation. The COBRA Collaboration is investigating different types of pixelated detector systems. Pixelated detectors add tracking capabilities to the pure energy measurement of CPG detectors and therefore combine the advantages of semiconductor detectors with the possiblity of particle identification. COBRA is the only experiment that allows for operation as a kind of solid state TPC. Simulations have shown that with pixel sizes of ∼200 μm, background events can be reduced by three orders of magnitude [3]. Three different systems already performed background measurements for several months at LNGS. First results will be published soon. [1] K. Zuber, Phys. Lett. B519 (2001) 1–7. [2] J. V. Dawson, et al., Phys. Rev. C80 (2009) 025502. [3] T. Bloxham, et al., Phys. Rev. C76 (2007) 025501.
Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 488 www.elsevier.com/locate/npbps
The COBRA double beta decay experiment C. Oldorf on behalf of the COBRA Collaboration Universit¨at Hamburg, Institut f¨ur Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
Abstract The COBRA experiment is searching for double beta decay using CdZnTe semiconductor detectors. The main focus is on the isotope 116 Cd. In addition to pure energy measurements, pixelisation allows also for tracking capabilities. This kind of semiconductor tracker is unique in the field. The current status of the experiment is shown including the latest half-life limits. Keywords: Neutrinoless Double Beta Decay, Low Background Experiment, COBRA
The aim of COBRA is the search for neutrinoless double beta decay (0νββ) with a large number of CdZnTe semiconductor detectors [1]. Several candidate 0νββ isotopes are intrinsic to the CdZnTe detector material, among them two of the most promising isotopes, 116 Cd and 130 Te. 116 Cd is the most important isotope for COBRA due to its high Q-value of 2809 keV. This decay energy lies well above the highest naturally occurring γ–background (2614.5 keV from 208 Tl). 130 Te is interesting for β− β− decay due to its high natural abundance and also 106 Cd for positron decay in combination with electron capture. CdZnTe, like semiconductor detectors in general, provide good energy resolution and are very clean in respect to radioactive impurities. The Source = Detector approach of COBRA makes a large detector mass easily achievable. A big advantage of CdZnTe detectors is the operation at room temperature. Background studies for the COBRA experiment are performed at Laboratori Nazionali del Gran Sasso (LNGS). The current setup consists of Co–Planar Grid Detectors (CPG). Resolutions of less than 2% FWHM at 2614.5 keV have been achieved even with cost efficient low resolution detectors. The current test setup can house 64 of these detectors and is shielded by lead, high purity copper, boron loaded polyethylene, and a Faraday cage. This results in 0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.09.125
a background rate of about 8 counts/keV/kg/year in the main region of interest. With the LNGS test 0ν ≥ 9.4 · 1019 years for 116 Cd and setup, limits on T 1/2 0ν ≥ 5.0 · 1020 years for 130 Te were achieved. In adT 1/2 dition, new world best values for 0νβ+ EC decay to g.s. were measured for 120 Te [2]. For a large scale experiment, an array of 64000 CPG detectors with a total mass of 420 kg is proposed. This allows for coincidence studies to search for decays to excited states and to reduce background. For further background suppression, the operation of CdZnTe detectors in liquid scintillator is under investigation. The COBRA Collaboration is investigating different types of pixelated detector systems. Pixelated detectors add tracking capabilities to the pure energy measurement of CPG detectors and therefore combine the advantages of semiconductor detectors with the possiblity of particle identification. COBRA is the only experiment that allows for operation as a kind of solid state TPC. Simulations have shown that with pixel sizes of ∼200 μm, background events can be reduced by three orders of magnitude [3]. Three different systems already performed background measurements for several months at LNGS. First results will be published soon. [1] K. Zuber, Phys. Lett. B519 (2001) 1–7. [2] J. V. Dawson, et al., Phys. Rev. C80 (2009) 025502. [3] T. Bloxham, et al., Phys. Rev. C76 (2007) 025501.