The Majorana Project at UW

1. The Majorana Project at UW Double-Beta Decay The Majorana Experiment Majorana at UW Recent experiments with atmospheric and solar neutrinos have shown that neutrinos have mass, exhibit mixing, and make up a significant portion of the matter in the universe. Neutrinos have provided the first new physics beyond the standard model in nearly four decades. Many questions about neutrinos remain, including: What is their absolute mass scale? Are neutrinos Majorana or Dirac particles? These questions are of importance to the physics community, and a neutrinoless double-beta decay (0) experiment provides a possible avenue to answer them. The Majorana reference design consists of stacks of segmented, high-purity 76Ge crystals in a close packed configuration. The design consists of 1.1-kg crystals arranged in 19 columns of three-crystal stacks to form a 57-crystal array with 60 kg of Ge. Each individually removable stack contains three vertically aligned crystals as well as associated contacts, wiring, and support structure. The array is surrounded by a thin, high-purity copper shroud, which is in turn surrounded by a thicker high-purity copper vacuum jacket. Current University of Washington involvement in the Majorana Project includes the simulation of alpha and neutron backgrounds, research and development efforts involving the LArGe detector, the study of excited state decays, R&D on the array design, and development of the DAQ system. The Liquid Argon Germanium detector, LArGe, is a PNNL/UW R&D project exploring the use of liquid Ar as an active shield. The test mount is appropriate for both vacuum and direct cryogenic immersion applications and was successfully demonstrated in a liquid argon cryostat with the collection of initial spectra. The mounting concept of LArGe may be similar to that used in the Majorana 57-crystal module. Most even-even nuclei are energetically forbidden to undergo  decay. However, double-beta decay is possible for a number of these nuclei. For example: 76Ge 76Se+e-+e+e-+e. A three-crystal stack within a 57-crystal module. The 57-crystal modules are housed within bulk lead, an ultra-low background shield, and a 4πveto detector. They are arranged in such a manner so as to enable the addition of new modules allowing a scalable approach. The LArGe detector outside of the dewar. Double-beta decay Feynman diagram. The Multiple-Element Gamma Assay, or MEGA, is a toroid of large, p-type germanium detectors housed underground at WIPP. It is currently being assembled at PNNL/WIPP with the help of LANL and UW. MEGA will serve to test and evaluate ideas about the design and construction of a concentrated grouping of germanium detectors. The similar size of the cryostat approximates cryogenic and electronic challenges facing the construction of the multi-crystal modules in Majorana. The more interesting case is when the two neutrons exchange a virtual neutrino such that no neutrinos are emitted. Neutrinoless double-beta decay requires that neutrinos be their own anti-particles, or Majorana particles. This process violates lepton number and is an example of physics beyond the standard model. A schematic showing a monolith with detector ready for insertion. To achieve maximum sensitivity, it is necessary to understand and mitigate backgrounds. Ultra-pure materials such as electroformed copper will be manufactured underground to minimize cosmogenic activation. To further reduce backgrounds, Majorana will be situated deep underground. In addition to the granularity afforded by the crystal array, each crystal will be segmented to aid in discrimination between single- and multi-site events. 0 Feynman diagram. Since no kinetic energy is carried away by the neutrinos in 0, the signal of interest is the endpoint energy deposited by the two s. The difficulty in observing this process arises from the fact that it has a half life of at least ~1026 years. The decay signature is dwarfed by many backgrounds and so a detector with a very fine energy resolution is needed. Alpha emanations from the natural decay chains present on the surfaces as well as in the bulk of germanium detectors generate an important background. Another pertinent background involves the neutron flux arising from (, n) reactions, from natural fission in rock and, to a lesser extent, from hard neutrons originating in muon spallation in rock and shielding. Geant4 simulations created at UW are furthering the collaboration’s understanding of alpha backgrounds and the background signal produced by neutrons. Joint PNNL/UW projects, including Majorana, are taking advantage of these simulations. Sample Pulse Shapes. A single-site deposition (top) and a multi-site deposition (bottom) Information from the pulse shape will be used to distinguish 0 from a large fraction of -ray events by comparison of the pulse with known single-site event pulses (double humped). Finally, it will be possible to look forward and backward in time from an event in the ROI to find signatures of parent or progeny isotopes. The double-beta decay spectrum and the 0 peak. The University of Washington expects to play an important role in the future of the Majorana experiment. Many of our activities are being carried out in close collaboration with our colleagues at Pacific Northwest National Laboratory. Majorana sensitivity at 90% C.L for differing background models. In three years of running, a 60-kg module would reach an effective Majorana-neutrino mass sensitivity of ~200 meV, which will either conclusively establish the KKDC claim of double-beta decay, or will significantly improve lifetime limits from the current level of about 2 x 1025 years to approximately 2 x 1026 years. • Enriched 76Ge acts as both source and detector, and offers an excellent combination of capabilities and sensitivities: • Excellent energy resolution (0.16% at 2.039 MeV at the endpoint energy) • Enrichment to 86% maximizes source isotope to total mass ratio • Well understood technology • Powerful background rejection (segmentation, granularity, timing, pulse shape discrimination) In an anticipated 0 decay, 76Ge will decay primarily to the ground state of 76Se through the emission of two s. However, decays to excited states of 76Se are possible as well. In these excited state decays, s are emitted in addition to the s. Preliminary research is being conducted to determine detector sensitivity to excited state decays. An excited state decay scheme.