Single Crystal Molybdates for Neutrinoless Double Beta Decay Experiments

75593B The Neutrinoless Double Beta Decay Underground Detector has been identified by the DOE as a high scientific priority for Nuclear Physics research; however, the current generation of proposed experiments to determine neutrino mass would require 500 kg of germanium (Ge), which would cost more than $100 million. Therefore, several alternate parent isotopes for neutrinoless double beta decay have been suggested as candidates. The 100Mo isotope of molybdenum is particularly promising because it has large decay energy, resulting in a signal above gamma ray backgrounds. The nuclear structure of this relatively light nuclide has been shown to be simple enough to allow reliable calculation of the parameters needed to extract the effective neutrino mass from the measured half-life. However, these advantages can only be exploited if a detector can be constructed with both high detection efficiency and excellent energy resolution. The only known method is to construct a bolometer of a single crystal from a compound that contains a significant fraction of molybdenum. Therefore, this project will develop technology for the bulk crystal growth of single crystal molybdate (a molybdenum oxide compound), so that large crystals can be routinely and cost-effectively grown. In Phase I, three molybdate compositions were investigated to determine: (1) their growability by the Czochralski method of bulk crystal growth; and (2) their potential use as cryogenic bolometer materials without any energy interferences, radiation effects, or other problems. One of the three materials was selected as a target composition, and independent bolometer tests indicated that the target composition would be an excellent host for 100Mo in bolometer applications. In Phase II, the target molybdate compound will be optimized for crystal growth and fabrication of cubes for bolometer applications. A large crystal enriched with 100Mo will be grown and fabricated for complete bolometer testing. Commercial Applications And Other Benefits as described by the awardee: The dominant application of the single crystal molybdates would be for cryogenic bolometers for neutrinoless double beta decay. Because the target molybdate compound is a good low-temperature scintillator, with luminescence arising from the molybdenum oxide complexes, secondary commercial applications are anticipated.