This proposal will fund the search for neutrino-less double beta decay. The neutrinos are fundamental particles that play key roles in the early universe, in cosmology and astrophysics, and in nuclear and particle physics. Results from neutrino oscillation experiments have provided compelling evidence that neutrinos have mass and give the first indication that the Standard Model of nuclear and particle physics is incomplete. A 1-tonne detector searching for neutrino-less double beta-decay will be about 100 times more sensitive than current measurements and will confirm or exclude the inverted mass hierarchy for Majorana neutrinos. To achieve the ultimately realizing 1-tonne scale detectors using Ge detectors, the Majorana and GERDA collaborations have established a formal cooperative agreement. The collaborations have agreed to share resources and knowledge in their development of detector designs to reach for a Majorana mass sensitivity below 50 meV. This proposal requests support for the design of a 1-tonne-scale Ge experiment. Work will be performed on the engineering, design, simulation, and risk analysis of an apparatus based on two different concepts, one developed by the Majorana collaboration, and the other by the GERDA collaboration.
The technology of larger, lower-background Ge arrays developed from a tonne-scale neutrino-less double beta decay experiment can be expected to enable a new generation of highly efficient, ultra-low-background gamma spectroscopy measurements. Among the fields that stand to benefit from this new generation of technology are: measurements of anthropogenic radiation in the environment; atmospheric, ocean, and groundwater environmental transport measured via natural isotopic tracers; methods of radioactive dating; reactor monitoring; bioassay for determining very low occupational exposures to radiation; and biological studies involving radiotracers at very low activities. Students and postdocs will be trained in deep underground physics and related disciplines, such as low-background techniques, detector technology, nuclear physics and neutrino physics. An education and outreach program with the Morehead Planetarium on the campus of University of North Carolina will bring this science to the public.
The 1TGe collaboration has completed a series of activities aimed at the development of a large-scale (~ ton) neutrinoless double-beta decay experiment using an isotope of germanium (76Ge). This research aims to provide insights into fundamental symmetries of nature: Is the total number of leptons (electrons, muons, and tau particles and their corresponding neutrinos) and anti-leptons in our universe a conserved quantity? Might neutrinos be their own anti-particle? How much do neutrinos weigh? Recently a collection of experiments around the world made the revolutionary discovery that neutrinos are not massless particles as espoused in the standard model of fundamental interactions. Instead the neutrinos have non-zero masses, are admixtures of neutrino mass eigenstates, and oscillate as they travel through space and matter. The fact that neutrinos have mass opens the door to searching for the exotic decay mode known as neutrinoless double beta decay (0νββ), first proposed by Ettore Majorana nearly eighty years ago. Majorana realized that if neutrinos were their own anti-particle, then in certain nuclei where nuclear beta decay is forbidden or disfavored, one might observe 0νββ-decay, where within the nucleus two neutrons change into two protons and emit two electrons (beta particles). Many theories predict that neutrinos must be such "Majorana" particles, and if so may offer an explanation of why neutrinos have such small masses when compared to the charged leptons and quarks. Searching for 0νββ decay is an extremely daunting challenge because of the exceeding long decay half-lives of greater than 1025 years! Any experiment that hopes to discover such a rare decay must both have sufficient mass (~ ton) and take unprecedented steps to eliminate or minimize potential backgrounds from naturally occurring uranium and thorium and their associated decay chain products. The measurement must also be situated deep underground to shield against cosmic rays on the earth’s surface that can create cosmogenic activity. The Majorana and GERDA collaborations are exploring the feasibility of a large-scale, 76Ge-based 0νββ-decay experiment. Germanium is an ideal isotope to search for this rare decay mode, since high-purity germanium (HPGe) detectors are commercially used as radiation detectors for a variety of applications and have properties well suited to searching for 0νββ-decay. Shield Concepts & Background Studies: Majorana and GERDA are exploring two radically different approaches to shield gamma rays originating from primordial radioactivity external to the Ge diodes. GERDA, located at the Gran Sasso National Laboratory in Italy, immerses Ge diodes in high purity liquid argon surrounded by a large water tank. The Majorana Demonstrator (MJD), being constructed at the 4850-foot level at the Sanford Underground Research Facility in SD, utilizes a layered shield design with ultra-pure electroformed copper (EFCu ) and lead. We completed R&D to determine how the expected background in the Demonstrator and GERDA scale for a ton-scale experiment. For GERDA, the degree of background shielding can be scaled easily, and if a scintillating cryogen (e.g. liquid Ar) is used, the scintillation would provide an additional tag for external background rejection. However, backgrounds from radioisotopes such as Rn and 42Ar must be understood. Majorana has demonstrated that the EFCu process can produce copper of sufficient purity. However, cosmic-muon induced backgrounds in the high-Z shielding materials must be well understood and the overburden required to meet the background goals determined. We completed a conceptual feasibility study of standard, hybrid and alternative shielding configurations. Cryostat & Detector Array Thermal & Mechanical Studies: Each of the MJD cryostats houses 20kg of HPGe detectors. For a ton-scale experiment, a module of at least three times the mass is envisioned. We identified and explored possible technical constraints for scaling the MJD cryostat and detector array, including those imposed by the EFCu process and the need to minimize thermal gradients across the array. We also estimated the heat load of a ton-scale module and assessed the feasibility of using either thermosyphon or pulse tube coolers to cool the detectors. Finally, we explored ways to minimize the mass of a ton-scale detector mount and automate the string and module assembly process. The 1TGe Laboratory: We produced a detailed underground facilities requirements document. Germanium Recycling R&D: A ton-scale experiment justifies an extensive effort to optimize the cost of detector production, the production schedule, and the physics reach of the detector design. R&D to improve the production yield of crystals and finished detectors is especially valuable because the single largest expense of a ton-scale experiment is expected to be the cost of the enriched material. As part of our effort we worked with a contractor to successfully develop a process for the recovery of enriched germanium material from the etch solutions generated during detector manufacture. Summary: This research has shown that germanium remains a favorable candidate for a large-scale 0νββ-decay experiment. The Majorana and GERDA collaborations are joining together to continue the pursuit of such a measurement.
