The Majorana Experiment

1. The Majorana Experiment Experimental Design Background Reduction Recent results from Super-Kamiokande, SNO, KamLAND [1-3] and other experiments have demonstrated that neutrinos are massive and change flavor. Probing the absolute scale of neutrino masses requires additional experiments including direct mass measurements and neutrinoless double-beta decay searches. Τhe discovery of neutrinoless double-beta (0) decay would determine the absolute mass scale of the neutrino as well as establish the Majorana nature of the neutrino. The Majorana collaboration proposes to search for this process by employing high-purity, segmented germanium enriched to 86% 76Ge as both source and detector. Recent improvements in signal processing, detector design, and advances in controlling intrinsic and external backgrounds will augment this well-established technique [4-5]. In addition, the collaboration seeks to demonstrate backgrounds near 1 count/tonne/year in the 0 decay peak 4 keV region of interest in order to justify a possible future scaling to larger detector mass (1 tonne). Such low backgrounds would enable the Majorana experiment to achieve a sensitivity to the 76Ge 0 half-life of 5.51026 years for an exposure of 464 kg-years. [1] Y. Ashie et al. (Super-Kamiokande), Phys. Rev. D71, 112005 (2005), hep-ex/0501064. [2] S. N. Ahmed et al. (SNO), Phys. Rev. Lett. 92, 181301 (2004), nucl-ex/0309004. [3] T. Araki et al. (KamLAND), Phys. Rev. Lett. 94, 081801 (2005), hep-ex/0406035. [4] C. E. Aalseth et al., Physical Review D 65, 092007 (2002). [5] H. V. Klapdor-Kleingrothaus et al., European Physical Journal A 12, 147 (2001a). The Majorana Experiment will make use of the latest in background rejection techniques, material assay, and clean construction methods in an effort to reach our background goal of 1 count/tonne-year in the 4 keV region of interest around 2039 keV. Cu Vacuum Jacket LFEPs Multi-crystal granularity: The close-packed, 57-crystal array design increases the chance of rejection in a multiple crystal event. Simultaneous signals in two detectors cannot be 0. Cu Cap Cu Tube (0.178 mm wall) Ge (1.1 kg) (62mm x 70 mm) Segmentation allows for further discrimination of multi-site events within an individual crystal. Shown is a 2 x 3 segmentation scheme as in the Majorana Reference Design. Cu Contact Ring Cu Cold Finger Cu Thermal Shroud A cutaway view of the proposed 57-crystal cryostat module and an enlarged view of the 3-crystal string. Tray (Plastic, Si, etc) Pulse-shape analysis can distinguish between single-site (e.g. top of figure) and multi-site (e.g. bottom of figure) events. More advanced techniques can perform a full 3-D reconstruction of an event and potentially serve to reject surface activity. Double-Beta Decay Inner Cu Shield Lead Shield 57-crystal Modules Most even-even nuclei are energetically forbidden to undergo  decay. However, double-beta decay is possible for a number of these nuclei. This process (2-decay), involving 2 neutrinos and 2 s, has been observed, for example: Single-site time correlation: Looking forward or backward in time from an event in the ROI to find a signature of parent or daughter isotopes at the same location can lead to further background reduction (rejection of 68Ge shown). Removable Monolith 2-decay simplified Feynman diagram. Dewars A related Feynman diagram may be drawn describing a process when the two nuclei exchange a virtual neutrino and emit only two electrons (0 decay). 0 decay requires that neutrinos be their own anti-particles, or Majoranaparticles. Since this process violates lepton number conservation, its observation would provide an indication of physics beyond the standard model. Neutron Moderator Stripped-away views of the Majorana reference design with two modules. Clockwise from above: shown with inner copper shield; with lead shielding; and with neutron moderator (4 muon veto not shown). This use of removable monoliths enables servicing and adding modules without disturbing other modules. 0-decay simplified Feynman diagram. The largest background contributions for Majorana originate from impurities in copper. Advances in the electroforming of copper underground and in the development of ultra-sensitive material assay methods (ICPMS) lead to construction of ultra-pure materials. In 0 neutrinos do not carry away any kinetic energy so the decay endpoint energy (Q-value) deposited by the two s provides the signal of interest. The “cartoon” plot to the right shows the double-beta decay spectrum and position of the 0 peak as a function of the sum of the electrons’ energy normalized to the Q-value of the decay (2039 keV for 76Ge). The peak widths are given for a detector energy resolution of 5%. Transport Cart Sensitivity N2 sparse gas flow control Corrosive aerosol & gas removal Radium & particulate filtration and chemical scavenge Front view Side view A plot of Majorana’s sensitivity vs. exposure time for different backgrounds levels in the 4 keV Region of Interest (ROI) around the -decay Q-value (2039 keV). Sensitivity of KKDC [6] appears as a reference point. [6] H. V. Klapdor-Kleingrothaus et al., Physics Letters B 586, 198 (2004). S. R. Elliott and P. Vogel, Ann. Rev. Nucl. Part. Sci. 52, 115 (2002). Inner bath containment & Cu bus Cu sulfate bath with cover gas, mandrel, current Programmable power supply