0ν2β Decay Research Articles

CAMEO/GEM program for future 2β decay and dark matter experiments

The sensitivity ( T 1/2 limit for 0ν2β decay) of CAMEO experiment with ≈100 kg of 116CdWO 4 crystals placed in the liquid scintillator of the BOREXINO CTF is ≈10 26 yr, which corresponds to a bound on the neutrino mass m ν, ≤ 0.06 eV. It would be improved by the GEM project with one ton of “naked” HP Ge detectors operating in HP liquid nitrogen. The latest is contained in the Cu cryostat placed in the CTF water tank. The sensitivity of the GEM-I phase with natural Ge crystals is ≈ 10 27 yr (or m ν ≤ 0.05 eV), while in the GEM-II stage with enriched (in 76Ge to 86%) detectors it is ≈ 10 28 yr (or m ν ≤ 0.015 eV). Besides, the GEM-I set up could advance the best limits on the existence of neutralinos - as dark matter candidates - by three order of magnitude, and at the same time would be able to identify the dark matter signal by detection of its seasonal modulation.

New results of 116Cd ββ decay experiment

The measurements with 0.33 kg low background 116CdWO 4 scintillators are in progress in the Solotvina Underground Laboratory. After 13254 h of data taking the new half-life limits for different modes of neutrinoless 2β decay of 116Cd are presented, in particular for 0ν2β decay of 116Cd to the ground state of 116Sn: T 1/2;(0ν2β) ≥ 1.3(1.8) × 10 23 yr at 90%(68%) C.L.

High sensitivity GEM experiment on 2β decay of76Ge

The GEM project is designed for the next-generation 2βdecay experiments with 76Ge. One ton of `naked' HP Ge detectors(natural at the first GEM-I phase and enriched in 76Ge to 86% at thesecond GEM-II stage) are operating in super-high-purity liquid nitrogencontained in a Cu vacuum cryostat (sphere of diameter 5 m). The latter isplaced in the water shield (of dimensions 11×11 m2). Monte Carlosimulation evidently shows that the sensitivity of the experiment (in termsof the T1/2 limit for 0ν2β decay) is ≈1027 yr with natural HP Ge crystals and ≈1028 yr with enrichedones. These bounds correspond to the restrictions on the neutrino massmν⩽0.05 eV and mν⩽0.015 eV with natural andenriched detectors, respectively. Besides, the GEM-I set-up could advancethe current best limits on the existence of neutralinos - as dark mattercandidates - by three orders of magnitude, and at the same time would beable to identify unambiguously the dark matter signal by detection of itsseasonal modulation.

New phase of the 116Cd 2β-decay experiment with 116CdWO4 scintillators

A new step of the 116Cd 2β-decay experiment is in progress at the Solotvina Underground Laboratory. The upgraded setup consists of four enriched 116CdWO4 crystal scintillators of total mass 339 g. As an active shield, 15 CdWO4 crystals (20.6 kg) are used. The background rate (in the energy interval 2.7–2.9 MeV) is 0.06 count/(yr kg keV), one order of magnitude lower than in the previous apparatus. Combined with results of preceding measurements, the half-life limit for the neutrinoless 2β decay of 116Cd to the ground state of 116Sn is determined as T1/2(0ν)≥5.1×1022 yr with 90% C.L. The limits on T1/2 for transitions to the excited 21+ and 01+ levels of 116Sn are 8.6×1021 and 4.1×1021 yr, respectively (at a 90% C.L.). For 0ν2β decay accompanied by the emission of one and two Majorons, the constraints on the half-life are T1/2(0νM1)≥1.4×1021 yr and T1/2 (0νM2)≥4.1×1020 yr (at a 90% C.L.). The corresponding constraints on the neutrino mass and the neutrino-Majoron coupling constant are 〈mv〉 ≤ 3.1 eV and 〈gM〉 ≤ 1.9 × 10−4, respectively.

Limits on Majoron modes of 116Cd neutrinoless 2β decay

Measuremennts of 116Cd double beta decay with the help of enriched 116CdWO 4 crystal scintillators are in progress in the Solotvina Underground Laboratory. The last part of the exposition with the background rate 0.7 counts/y·kg·keV (in the region of 116Cd 0ν2β decay 2.7–2.9 MeV) is 22826 hours. For neutrinoless modes with emission of one and two Majorons, the limits T 1 2 (0ν M1) ⩾ 1.2 × 10 21 y and T 1 2 (0ν M2) ⩾ 2.6 × 10 20 y (90% C.L.) are determined. The above mentioned values give the restriction on the Majoron-neutrino coupling constant of g M ⩽ 2.1 × 10 −4. To improve these results, an advanced set-up with four enriched 116CdWO 4 detectors shielded by fifteen large (1.5–2 kg) CdWO 4 crystals has been mounted.

The research of 2β decay of 116Cd with enriched 116CdWO 4 crystal scintillators

The experiment was performed at the Solotvina Underground Laboratory with CdWO 4 crystal scintillators enriched in 116Cd to 83%. The total data collection time was about 30000 hours. Limits of T 1 2 0ν ≥2.9·10 22 y (90% CL) and T 1 2 2ν ≥1.8·10 19 y (99% CL) were obtained for 0ν2β and 2ν2β decay of 116Cd. After subtraction of the spectrum obtained with a natural CdWO 4 scintillator a remaining yield was seen consistent with 2ν2β decay of 116Cd with T 1 2 2ν= {2.7 −0.4 +0.5( stat.) −0.6 +0.9( syst.)}·10 19 . However, the imitation of the effect by impurities of 90Sr (at the level of 2·10 −3 Bq/kg) cannot be excluded.

A remarkable soda spring

The CARVEL (CAlcium Research for VEry Low neutrino mass) experiment to search for the double β decay of 48Ca with the help of enriched 48CaWO4 crystal scintillators has been considered. Scintillation properties (energy resolution, α/β ratio, pulse-shape discrimination ability) and radiopurity of CaWO4 scintillators were studied. Despite rather high radioactive contaminations, the background rate of the CaWO4 detector in the energy region 3.6–5.4 MeV (energy window of the 48Ca neutrinoless 2β decay) was reduced down to 0.07 counts/(yr keV kg). With ≈100 kg array of the 48CaWO4 crystals the sensitivity of the CARVEL experiment (in terms of the half-life limit for the 0ν2β decay) is estimated as T1/20ν>1027 yr. This value corresponds to the neutrino mass constraint mν < (0.04–0.09) eV.