Neutrinoless Double Beta Decay Experiments Research Articles

First results from the AMoRE-Pilot neutrinoless double beta decay experiment

The advanced molybdenum-based rare process experiment (AMoRE) aims to search for neutrinoless double beta decay (0nu beta beta ) of ^{100}Mo with sim 100,hbox {kg} of ^{100}Mo-enriched molybdenum embedded in cryogenic detectors with a dual heat and light readout. At the current, pilot stage of the AMoRE project we employ six calcium molybdate crystals with a total mass of 1.9 kg, produced from ^{48}Ca-depleted calcium and ^{100}Mo-enriched molybdenum (^{48{{text {depl}}}}hbox {Ca}^{100}hbox {MoO}_{4}). The simultaneous detection of heat (phonon) and scintillation (photon) signals is realized with high resolution metallic magnetic calorimeter sensors that operate at milli-Kelvin temperatures. This stage of the project is carried out in the Yangyang underground laboratory at a depth of 700 m. We report first results from the AMoRE-Pilot 0nu beta beta search with a 111 kg day live exposure of ^{48{{text {depl}}}}hbox {Ca}^{100}hbox {MoO}_{4} crystals. No evidence for 0nu beta beta decay of ^{100}Mo is found, and a upper limit is set for the half-life of 0nu beta beta of ^{100}Mo of T^{0nu }_{1/2} > 9.5times 10^{22}~hbox {years} at 90% C.L. This limit corresponds to an effective Majorana neutrino mass limit in the range langle m_{beta beta }rangle le (1.2-2.1),hbox {eV}.

CUPID-0: A double-readout cryogenic detector for Double Beta Decay search

CUPID-0 is the first large mass neutrino-less double beta decay (0νββ) experiment based on cryogenic calorimeters with dual read-out of light and heat for background rejection. The detector assembly, consisting of 26 ZnSe crystals, 2 natural and 24 enriched at 95% in 82Se, all coupled with bolometric light detectors, has been constructed respecting very strict protocols and procedures, from the material selection during crystal growth to the new and innovative detector framework, in the attempt to achieve the best performance of the array. The successful construction of the detector lead to promising preliminary results, here presented. The array is in fact taking data underground at LNGS (Italy) since March 2017 and the particle identification, enabled by the light read-out, provides an unprecedented background level, for cryogenic calorimeters, of only 3.2 countskeV−1t−1yr−1 in the region of interest of the 0νββ search for 82Se, namely around 3 MeV.

Highly predictive and testable A4 flavor model within type-I and II seesaw framework and associated phenomenology

We investigate neutrino mass model based on A4 discrete flavor symmetry in type-I+II seesaw framework. The model has imperative predictions for neutrino masses, mixing and CP violation testable in the current and upcoming neutrino oscillation experiments. The important predictions of the model are: normal hierarchy for neutrino masses, a higher octant for atmospheric angle (θ23>45°) and near-maximal Dirac-type CP phase (δ≈π/2 or 3π/2) at 3σ C.L. These predictions are in consonance with the latest global-fit and results from Super-Kamiokande (SK), NOνA and T2K. Also, one of the important feature of the model is the existence of a lower bound on effective Majorana mass, |Mee|≥0.047 eV (at 3σ) which corresponds to the lower part of the degenerate spectrum and is within the sensitivity reach of the neutrinoless double beta decay (0νββ) experiments.

First test of a CdMoO4 scintillating bolometer for neutrinoless double beta decay experiments with 116Cd and 100Mo nuclides

A large cylindrical cadmium molybdate crystal with natural isotopic abundance has been used to fabricate a scintillating bolometer. The measurement was performed above ground at milli-Kelvin temperature, with simultaneous readout of the heat and the scintillation light. The energy resolution as FWHM has achieved from 5 keV (at 238 keV) to 13 keV (at 2615 keV). We present the results of the $\alpha$ versus $\beta$/$\gamma$ events discrimination. The low internal trace contamination of the $\mathrm{CdMoO_4}$ crystal was evaluated as well. The detector performance with preliminary positive indications proves that cadmium molybdate crystal is a promising absorber for neutrinoless double beta decay scintillating bolometric experiments with $\mathrm{{}^{116}Cd}$ and $\mathrm{{}^{100}Mo}$ nuclides in the next-generation technique.

Evaluation and lifetime study of a cryogenic SAR ADC for HPGe detectors in low-background physics experiments

Cryogenic ADCs can be a promising solution for ton-scale HPGe detectors for dark matter and neutrino-less double beta decay experiments for improved signal integrity and decreased cables and penetrations. This paper presents the characterization and lifetime study of a SAR ADC at liquid nitrogen temperature for CDEX and future LEGEND experiments. The chip is implemented in standard 65 nm CMOS process. It works well at liquid nitrogen temperature with proper biasing. The power consumptions of the SAR ADC are measured to be 6.1 mW and 6.5 mW at 300 K and 77 K respectively. The ENOB of the ADC is decreased from 8.94 bits at 300 K to 8.72 bits at 77 K at 100 MS/s sampling rate. For reasons of efficiency and economics, the lifetime of the cryogenic operation circuits must be in excess tens of years. Hot carrier effect (HCE) is the dominate mechanism that substantially affects the device lifetime at cryogenic temperature and the accelerated lifetime study has also been conducted. The predicted lifetime is ∼ 5.7 × 104 years at 1.24 V operation at 77 K, which means the ADC is cryogenic qualified to remain outside of the HCE degradation.

Charge-to-heat transducers exploiting the Neganov-Trofimov-Luke effect for light detection in rare-event searches

In this work we present how to fabricate large-area (15 cm2), ultra-low threshold germanium bolometric photo-detectors and how to operate them to detect few (optical) photons. These detectors work at temperatures as low as few tens of mK and exploit the Neganov-Trofimov-Luke (NTL) effect. They are operated as charge-to-heat transducers: the heat signal is linearly increased by simply changing a voltage bias applied to special metal electrodes, fabricated onto the germanium absorber, and read by a (NTD-Ge) thermal sensor. We fabricated a batch of five prototypes and ran them in different facilities with dilution refrigerators. We carefully studied how impinging spurious infrared radiation impacts the detector performances, by shining infrared photons via optical-fiber-guided LED signals, in a controlled manner, into the bolometers. We hence demonstrated how the radiation-tightness of the test environment tremendously enhances the detector performances, allowing to set electrode voltage bias up to 90 volts without any leakage current and signal-to-noise gain as large as a factor 12 (for visible photons). As consequence, for the first time we could operate large-area NTD-Ge-sensor-equipped NTL bolometric photo-detectors capable to reach sub 10-eV baseline noise (RMS). Such detectors open new frontiers for rare-event search experiments based on low light yield Ge-NTD equipped scintillating bolometers, such the CUPID neutrinoless double-beta decay experiment.

Development of methods for the preparation of radiopure 82Se sources for the SuperNEMO neutrinoless double-beta decay experiment

Abstract A radiochemical method for producing 82Se sources with an ultra-low level of contamination of natural radionuclides (40K, decay products of 232Th and 238U) has been developed based on cation-exchange chromatographic purification with reverse removal of impurities. It includes chromatographic separation (purification), reduction, conditioning (which includes decantation, centrifugation, washing, grinding, and drying), and 82Se foil production. The conditioning stage, during which highly dispersed elemental selenium is obtained by the reduction of purified selenious acid (H2SeO3) with sulfur dioxide (SO2) represents the crucial step in the preparation of radiopure 82Se samples. The natural selenium (600 g) was first produced in this procedure in order to refine the method. The technique developed was then used to produce 2.5 kg of radiopure enriched selenium (82Se). The produced 82Se samples were wrapped in polyethylene (12 μm thick) and radionuclides present in the sample were analyzed with the BiPo-3 detector. The radiopurity of the plastic materials (chromatographic column material and polypropylene chemical vessels), which were used at all stages, was determined by instrumental neutron activation analysis. The radiopurity of the 82Se foils was checked by measurements with the BiPo-3 spectrometer, which confirmed the high purity of the final product. The measured contamination level for 208Tl was 8–54 μBq/kg, and for 214Bi the detection limit of 600 μBq/kg has been reached.

New sensitivity goal for neutrinoless double beta decay experiments

We study the implications of the Dark-LMA (DLMA) solution to the solar neutrino problem for neutrinoless double beta decay ($0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$). We show that, while the predictions for the effective mass governing $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$ remain unchanged for the inverted mass scheme, that for normal ordering becomes higher for the DLMA parameter space and moves into the ``desert region'' between the two. This sets a new goal for the sensitivity reach for the next-generation experiments if no signal is found for the inverted ordering by the future search programs.

High Pressure Gas Xenon TPCs for Double Beta Decay Searches

This article reviews the application of high pressure gas xenon (HPXe) time projection chambers to neutrinoless double beta decay experiments. First, the fundamentals of the technology and the historical development of the field are discussed. Then, the state of the art is presented, including the prospects for the next generation of experiments with masses in the ton scale.

Studies on β α transition in Sn and Sn-rich alloys for a cryogenic tin bolometer

Tin pest is a concern for applications involving tin at low temperatures. Efforts have been initiated for a neutrinoless double beta decay experiment in 124Sn—TIN.TIN (The INdia-based TIN detector). The experiment proposes to use cryogenic Sn bolometers. It is envisaged that the detector will be of large size (0.1–1 ton) and is expected to run for several years. Thus, methods of suppressing tin pest need to be investigated. In this paper, the stability of various Sn-rich alloys against tin pest and consequences for TIN.TIN have been studied. Additionally, an inconsistency in the literature data about the transition temperature in Sn has been resolved using Differential Scanning Calorimetry (DSC). The transition was also studied in the Sn–Cu alloy and its transition temperature was found to be 36.3 °C.

Vertical Bridgman growth and low temperature luminescence properties of NaY(MoO4)2 crystal

NaY(MoO4)2 crystal was investigated as a promising scintillator for neutrinoless double beta decay experiment. In this study, NaY(MoO4)2 crystal was grown by the Vertical Bridgman method using spontaneous crystallization. The as-grown NaY(MoO4)2 crystal appeared dark blue in color. The powder X-Ray Diffraction, optical transmission spectra at room temperature, luminescence properties and decay time from room temperature to 10 K excited by a 280 nm light emitting diode source were investigated. The transmittance of the annealed NaY(MoO4)2 crystal sample with the thickness of 3 mm was about 80%. The NaY(MoO4)2 crystal did not luminescence from room temperature to 200 K and the emission spectra were observed down from 175 K. With the temperature decreasing, the luminescence light yield increased and had maximum peak at 10 K. Compared to Li2MoO4 crystal, the luminescence light yield of NaY(MoO4)2 was much higher at 10 K. The longest average decay time was observed at 10 K (425 μs) and the shortest average decay time was observed at 175 K (88 μs). NaY(MoO4)2 crystal has a higher light yield at cryogenic temperature and no any heavy elements, so it is a promising candidate as a cryogenic detector for investigating the neutrinoless double-beta decay (0υββ) mechanism.

Fabrication and characterization of high-purity germanium detectors with amorphous germanium contacts

Large, high-purity, germanium (HPGe) detectors are needed for neutrinoless double-beta decay and dark matter experiments. Currently, large (> 4 inches in diameter) HPGe crystals can be grown at the University of South Dakota (USD). We verify that the quality of the grown crystals is sufficient for use in large detectors by fabricating and characterizing smaller HPGe detectors made from those crystals. We report the results from eight detectors fabricated over six months using crystals grown at USD. Amorphous germanium (a-Ge) contacts are used for blocking both electrons and holes. Two types of geometry were used to fabricate HPGe detectors. As a result, the fabrication process of small planar detectors at USD is discussed in great detail. The impact of the procedure and geometry on the detector performance was analyzed for eight detectors. We characterized the detectors by measuring the leakage current, capacitance, and energy resolution at 662 keV with a Cs-137 source. Four detectors show good performance, which indicates that crystals grown at USD are suitable for making HPGe detectors.

Detecting Cherenkov light from 1–2 MeV electrons in linear alkylbenzene

The FlatDot detector has been used to demonstrate the separation of Cherenkov and scintillation light for 1 to 2 MeV electrons in linear alkylbenzene (LAB). With an average PMT transit time spread (TTS) of 200 ps, the early light in each event is clearly dominated by the Cherenkov signal, which on average comprises 86+2−3% of the light collected in the first 4.1 ns of each event. The spatial distributions of the Cherenkov and scintillation light are found to match those predicted in Monte Carlo simulations. This is a key step towards demonstrating direction reconstruction of β decays, a technique that could reduce 8B solar neutrino backgrounds for neutrinoless double-beta decay experiments in liquid scintillator.

Isotope-selective laser photoionization of tin in supersonic atomic beam

The isotope 124Sn enriched to better than 50% is needed in substantial quantity as detector material for the proposed neutrinoless double beta decay (0νββ) experiment in India (TIN.TIN). Present work investigates isotope-selective laser photoionization scheme for 124Sn. Transition to the first excited state from the ground state lies at 286.3 nm having short lifetime (~ 5 ns) and isotope shift (IS), Δν(124–120) = 441 MHz. However, the availability of high repetition rate (kHz), narrowband tunable pulsed laser in UV with low temporal jitter is a technological impediment for achieving isotopic selectivity in this transition. An alternate three-color photoionization scheme is proposed for separation of 124Sn. Laser vaporization of tin followed by supersonic expansion in a molecular beam apparatus provided the tin atomic beam. Tin atoms were first excited to the 5p6s,3P1 state from the 5p2,3P0 ground state by a broadband pulsed dye laser (λ1 = 286.3 nm). Isotopic selectivity was achieved in the second excitation step at λ2 = 855.2 nm by a narrowband continuous wave laser. Subsequently, resonant photoionization at λ3 = 694.7 nm to a newly observed autoionizing state at 60,992.9 cm−1 provided efficient photoionization. The IS for all even and hyperfine splitting for odd isotopes have been measured for the 855.2 nm transition. A selectivity factor of 24 is achieved for 124Sn isotope. The absorption cross sections of the three excitation steps is reasonably high to have an efficient photoionization scheme.

Combining texture zeros with a remnant CP symmetry in the minimal type-I seesaw

In the framework of the two right-handed neutrino seesaw model, we consider maximally-restrictive texture-zero patterns for the lepton Yukawa coupling and mass matrices, together with the existence of a remnant CP symmetry. Under this premise, we find that several textures are compatible with the most recent data coming from neutrino oscillation and neutrinoless double beta decay experiments. It is shown that, the maximum number of allowed texture zeros in the Dirac Yukawa coupling matrix is two, for an inverted neutrino mass spectrum. In contrast, for Yukawa coupling matrices with just one texture zero, both normal and inverted orderings of neutrino masses are compatible with data. In all cases, the predictions for the low-energy Dirac and Majorana CP-violating phases, and for the effective mass parameter relevant in neutrinoless double-beta decay experiments, are presented and discussed. We also comment on the impact of future experimental improvements in scrutinising texture-zero patterns with a remnant CP symmetry, within the minimal version of the seesaw mechanism considered here.

Study on cosmogenic activation in germanium detectors for future tonne-scale CDEX experiment

A study on cosmogenic activation in germanium was carried out to evaluate the cosmogenic background level of natural and $^{70}$Ge depleted germanium detectors. The production rates of long-lived radionuclides were calculated with Geant4 and CRY. Results were validated by comparing the simulated and experimental spectra of CDEX-1B detector. Based on the validated codes, the cosmogenic background level was predicted for further tonne-scale CDEX experiment. The suppression of cosmogenic background level could be achieved by underground germanium crystal growth and high-purity germanium detector fabrication to reach the sensitivity requirement for direct detection of dark matter. With the low cosmogenic background, new physics channels, such as solar neutrino research and neutrinoless double-beta decay experiments, were opened and the corresponding simulations and evaluations were carried out.

Displaced vertex signature of type-I seesaw model

A certain class of new physics models includes long-lived particles which are singlet under the Standard Model (SM) gauge group.A displaced vertex is a spectacular signature to probe such particles productions at the high energy colliders, with a negligible SM background. In the context of the minimal gauged $B-L$ extended SM, we consider a pair creation of Majorana right-handed neutrinos (RHNs) at the high energy colliders through the production of the SM and the $B-L$ Higgs bosons and their subsequent decays into RHNs. With parameters reproducing the neutrino oscillation data, we show that the RHNs are long-lived and their displaced vertex signature can be observed at the next generation displaced vertex search experiments, such as the HL-LHC, the MATHUSLA, the LHeC, and the FCC-eh.We find that the lifetime of the RHNs is controlled by the lightest light neutrino mass, which leads to a correlation between the displaced vertex search and the search limit of the future neutrinoless double beta-decay experiments.

Growth and Optical Properties of a Cs2Mo2O7 Single Crystal

A Cs2Mo2O7 single crystal, a molybdenum-based crystal as research and development scintillation material for future neutrinoless double beta decay experiments under milli-Kelvin temperatures, was grown by the Czochralski method. The grown crystal was prepared as a sample for measurements of its optical properties. The sample was cooled down from room temperature to 10 K, and temperature dependence of emission spectrum and decay time were measured. We used a light emitting diode as an excitation source for the emission spectra measurements and a pulsed-type laser for the decay time due to its high intensity. The intensity of emission spectrum and the decay time increased as the temperature decreased. We estimated the optical energy bandgap via a transmittance measurement at room temperature. As far as we know, it is the first time to grow a large Cs2Mo2O7 crystal and to study its luminescence properties. Moreover, we grew a well-studied Li2MoO4 crystal as a reference to compare the light output of the Cs2Mo2O7 at 10 K. Through these results, the Cs2Mo2O7 crystal found to be a candidate scintillator for the future neutrinoless double beta decay experiments.

Neutrino predictions from generalized CP symmetries of charged leptons

We study the implications of generalized CP transformations acting on the mass matrices of charged leptons in a model-independent way. Generalized e − μ, e − τ and μ−τ symmetries are considered in detail. In all cases the physical parameters of the lepton mixing matrix, three mixing angles and three CP phases can be expressed in terms of a restricted set of independent “theory parameters” that characterize a given choice of CP transformation. This leads to implications for neutrino oscillations as well as neutrinoless double beta decay experiments.

Mobility and clustering of barium ions and dications in high-pressure xenon gas

The clustering and drift properties of barium ions in xenon gas are explored theoretically, using density functional theory and computational ion mobility theory, with the goal of better understanding barium ion transport for neutrinoless double beta decay. We derive the equilibrium conformations, energies and entropies of molecular ions in the Ba$^{+}$-Xe and Ba$^{++}$-Xe systems, which yield a predictive model of cluster formation in high pressure gas. We calculate ion-neutral interaction potential curves for these species and use them to predict effective molecular ion mobilities. Our calculation consistently reproduces experimental data on effective mobility and molecular ion formation for the Ba$^+$ system, and predicts strong cluster formation in the Ba$^{++}$ system, dominated by stable [BaXe$_6$]$^{++}$,[BaXe$_7$]$^{++}$, [BaXe$_8$]$^{++}$ and [BaXe$_9$]$^{++}$ complexes in the range of interest. Some implications for barium tagging in gas-phase neutrinoless double beta decay experiments are discussed, and the first predictions of pressure-dependent mobility of the doubly charged Ba$^{++}$ species are presented.