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Register a new userAMoRE: A search for neutrinoless double-beta decay of 100Mo using low-temperature molybdenum-containing crystal detectors
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Moo Hyun Lee
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The AMoRE is an experiment to search for neutrinoless double-beta decay of 100Mo in molybdate crystal scintillators using a cryogenic detection technique. The crystals are equipped with metallic magnetic calorimeter sensors that detect both phonon and photon signals at temperatures of a few tens of mK. Simultaneous measurements of thermal and scintillation signals produced by particle interactions in the crystals by MMC sensors provide high energy resolution and efficient particle discrimination. AMoRE-Pilot, an R&D phase with six 48deplCa100MoO4 crystals and a total mass of ~1.9 kg in the final configuration, operated at the 700 m deep Yangyang underground laboratory (Y2L). After completion of the AMoRE-Pilot run at the end of 2018, AMoRE-I with a ~6 kg crystal array comprised of thirteen 48deplCa100MoO4 and five Li2100MoO4 crystals is currently being assembled and installed at Y2L. We have secured 110 kg of 100Mo-isotope-enriched MoO3 powder for the production of crystals for the AMoRE-II phase, which will have ~200 kg of molybdate crystals and operate at Yemilab, a new underground laboratory located ~1,100 m deep in the Handeok iron mine that is currently being excavated and with a scheduled completion date of December 2020. AMoRE-II is expected to improve the upper limit on the effective Majorana neutrino mass to cover the entire inverted hierarchy neutrino mass region: 20-50 meV, in the case when no such decays are observed. Results from AMoRE-Pilot and progress of the preparations for AMoRE-I and AMoRE-II are presented.
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The Advanced Molybdenum-based Rare process Experiment (AMoRE) searches for neutrino-less double-beta (0{ u}b{eta}b{eta}) decay of 100Mo in enriched molybdate crystals. The AMoRE crystals must have low levels of radioactive contamination to achieve low background signals with energies near the Q-value of the 100Mo 0{ u}b{eta}b{eta} decay. To produce low-activity crystals, radioactive contaminants in the raw materials used to form the crystals must be controlled and quantified. 100EnrMoO3 powder, which is enriched in the 100Mo isotope, is of particular interest as it is the source of 100Mo in the crystals. A high-purity germanium detector having 100% relative efficiency, named CC1, is being operated in the Yangyang underground laboratory. Using CC1, we collected a gamma spectrum from a 1.6-kg 100EnrMoO3 powder sample enriched to 96.4% in 100Mo. Activities were analyzed for the isotopes 228Ac, 228Th, 226Ra, and 40K. They are long-lived naturally occurring isotopes that can produce background signals in the region of interest for AMoRE. Activities of both 228Ac and 228Th were < 1.0 mBq/kg at 90% confidence level (C.L.). The activity of 226Ra was measured to be 5.1 pm 0.4 (stat) pm 2.2 (syst) mBq/kg. The 40K activity was found as < 16.4 mBq/kg at 90% C.L.
Environmental radioactivity is a dominant background for rare decay search experiments, and it is difficult to completely remove such an impurity from detector vessels. We propose a scintillation balloon as the active vessel of a liquid scintillator in order to identify this undesirable radioactivity. According to our feasibility studies, the scintillation balloon enables the bismuth--polonium sequential decay to be tagged with a 99.7% efficiency, assuming a KamLAND (Kamioka Liquid scintillator AntiNeutrino Detector)-type liquid scintillator detector. This tagging of sequential decay using alpha-ray from the polonium improves the sensitivity to neutrinoless double-beta decay with rejecting beta-ray background from the bismuth.
This paper reports on the development of a technology involving $^{100}$Mo-enriched scintillating bolometers, compatible with the goals of CUPID, a proposed next-generation bolometric experiment to search for neutrinoless double-beta decay. Large mass ($sim$1~kg), high optical quality, radiopure $^{100}$Mo-containing zinc and lithium molybdate crystals have been produced and used to develop high performance single detector modules based on 0.2--0.4~kg scintillating bolometers. In particular, the energy resolution of the lithium molybdate detectors near the $Q$-value of the double-beta transition of $^{100}$Mo (3034~keV) is 4--6~keV FWHM. The rejection of the $alpha$-induced dominant background above 2.6~MeV is better than 8$sigma$. Less than 10~$mu$Bq/kg activity of $^{232}$Th ($^{228}$Th) and $^{226}$Ra in the crystals is ensured by boule recrystallization. The potential of $^{100}$Mo-enriched scintillating bolometers to perform high sensitivity double-beta decay searches has been demonstrated with only 10~kg$times$d exposure: the two neutrino double-beta decay half-life of $^{100}$Mo has been measured with the up-to-date highest accuracy as $T_{1/2}$ = [6.90 $pm$ 0.15(stat.) $pm$ 0.37(syst.)] $times$ 10$^{18}$~yr. Both crystallization and detector technologies favor lithium molybdate, which has been selected for the ongoing construction of the CUPID-0/Mo demonstrator, containing several kg of $^{100}$Mo.
The {sc Majorana} collaboration is searching for neutrinoless double beta decay using $^{76}$Ge, which has been shown to have a number of advantages in terms of sensitivities and backgrounds. The observation of neutrinoless double-beta decay would show that lepton number is violated and that neutrinos are Majorana particles and would simultaneously provide information on neutrino mass. Attaining sensitivities for neutrino masses in the inverted hierarchy region, $15 - 50$ meV, will require large, tonne-scale detectors with extremely low backgrounds, at the level of $sim$1 count/t-y or lower in the region of the signal. The {sc Majorana} collaboration, with funding support from DOE Office of Nuclear Physics and NSF Particle Astrophysics, is constructing the {sc Demonstrator}, an array consisting of 40 kg of p-type point-contact high-purity germanium (HPGe) detectors, of which $sim$30 kg will be enriched to 87% in $^{76}$Ge. The {sc Demonstrator} is being constructed in a clean room laboratory facility at the 4850 level (4300 m.w.e.) of the Sanford Underground Research Facility (SURF) in Lead, SD. It utilizes a compact graded shield approach with the inner portion consisting of ultra-clean Cu that is being electroformed and machined underground. The primary aim of the {sc Demonstrator} is to show the feasibility of a future tonne-scale measurement in terms of backgrounds and scalability.
The MAJORANA Collaboration is constructing the MAJORANA DEMONSTRATOR, an ultra-low background, modular, HPGe detector array with a mass of 44.8-kg (29.7 kg enriched >88% in Ge-76) to search for neutrinoless double beta decay in Ge-76. The next generation of tonnescale Ge-based neutrinoless double beta decay searches will probe the neutrino mass scale in the inverted-hierarchy region. The MAJORANA DEMONSTRATOR is envisioned to demonstrate a path forward to achieve a background rate at or below 1 count/tonne/year in the 4 keV region of interest around the Q-value of 2039 keV. The MAJORANA DEMONSTRATOR follows a modular implementation to be easily scalable to the next generation experiment. First data taken with the DEMONSTRATOR are introduced here.
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