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Register a new userDevelopment of $^{100}$Mo-containing scintillating bolometers for a high-sensitivity neutrinoless double-beta decay search
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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.
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NEXT-100 is an electroluminescent high-pressure xenon gas time projection chamber that will search for the neutrinoless double beta ($beta beta 0 u$) decay of Xe-136. The detector possesses two features of great value for $beta beta 0 u$ searches: energy resolution better than 1% FWHM at the $Q$ value of Xe-136 and track reconstruction for the discrimination of signal and background events. This combination results in excellent sensitivity, as discussed in this paper. Material-screening measurements and a detailed Monte Carlo detector simulation predict a background rate for NEXT-100 of at most $4times10^{-4}$ counts keV$^{-1}$ kg$^{-1}$ yr$^{-1}$. Accordingly, the detector will reach a sensitivity to the bbonu-decay half-life of $2.8times10^{25}$ years (90% CL) for an exposure of 100 $mathrm{kg}cdotmathrm{year}$, or $6.0times10^{25}$ years after a run of 3 effective years.
A high pressure xenon gas time projection chamber with electroluminescent amplification (EL HPGXe TPC) searching for the neutrinoless double beta ($0 ubetabeta$) decay offers: excellent energy resolution ($0.5-0.7%$ FWHM at the $Q_{betabeta}$), by amplifying the ionization signal with electroluminescent light, and tracking capabilities, as demonstrated by the NEXT collaboration using two kg-scale prototypes. The NEXT collaboration is building an EL HPGXe TPC capable of holding 100 kg (NEXT-100) of xenon isotopically enriched in ${{}^{136}rm Xe}$. The installation and commissioning of the NEXT-100 detector at the Laboratorio Subterraneo de Canfranc (LSC) is planned for 2018. The current estimated background level for the NEXT-100 detector is of $4times10^{-4}$ counts/keV-kg-yr or less in the energy region of interest. Assuming an energy resolution of 0.75$%$ FWHM at the $Q_{betabeta}$ and a $0 ubetabeta$ signal efficiency of about 28$%$, this gives an expected sensitivity (at 90$%$ CL) to the $0 ubetabeta$ decay half life of $T^{0 u}_{1/2}>6.0times10^{25}$ yr for an exposure of 275 kg yr. A first phase of the NEXT experiment, called NEW, is currently being commissioned at the LSC. The NEW detector is a scale 1:2 in size (1:10 in mass) of the NEXT-100 detector using the same materials and photosensors and will be used to perform a characterization of the $0 ubetabeta$ backgrounds and a measurement of the standard double beta decay with neutrinos (${2 ubetabeta}$). An 8 sigma significance for the ${2 ubetabeta}$ signal in the NEW detector has been estimated for a 100-day run.
We report the results of a search for the neutrinoless double-$beta$ decay (0$ ubetabeta$) of $^{100}$Mo, using the NEMO-3 detector to reconstruct the full topology of the final state events. With an exposure of 34.7 kg.y, no evidence for the 0$ ubetabeta$ signal has been found, yielding a limit for the light Majorana neutrino mass mechanism of $T_{1/2}(0 ubetabeta)>1.1 times 10^{24}$ years (90% C.L.) once both statistical and systematic uncertainties are taken into account. Depending on the Nuclear Matrix Elements this corresponds to an upper limit on the Majorana effective neutrino mass of $< m_{ u} > < 0.3-0.9$ eV (90% C.L.). Constraints on other lepton number violating mechanisms of 0$ ubetabeta$ decays are also given. Searching for high-energy double electron events in all suitable sources of the detector, no event in the energy region [3.2-10] MeV is observed for an exposure of 47 kg.y.
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.
The Neutrino Experiment with a Xenon TPC (NEXT) searches for the neutrinoless double-beta decay of Xe-136 using high-pressure xenon gas TPCs with electroluminescent amplification. A scaled-up version of this technology with about 1 tonne of enriched xenon could reach in less than 5 years of operation a sensitivity to the half-life of neutrinoless double-beta decay decay better than 1E27 years, improving the current limits by at least one order of magnitude. This prediction is based on a well-understood background model dominated by radiogenic sources. The detector concept presented here represents a first step on a compelling path towards sensitivity to the parameter space defined by the inverted ordering of neutrino masses, and beyond.
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