Investigation of β+β+ and β+/EC decay of 106Cd

A low background scintillation detector with a CdWO₄ crystal of 1.046 kg was used to search for β⁺β⁺ and β⁺/EC processes in ¹⁰⁶Cd. For the neutrinoless mode the limits T_1/2(0νβ⁺β⁺) ≥ 2.2 · 10¹⁹ y and T_1/2(0νβ⁺/EC) ≥ 5.5 · 10¹⁹ y were obtained with 90% C.L. For the possible two neutrino decay limits of T_1/2(2νβ⁺β⁺) ≥ 9.2 · 10¹⁷ y and _1/2(2νβ⁺/EC) ≥ 2.6 · 10¹⁷ y have been determined with 99% C.L.     

First search for double β decay of dysprosium

A search for double β decay of dysprosium was realized for the first time with the help of an ultra-low background HP Ge γ detector. After 2512 h of data taking with a 322 g sample of dysprosium oxide limits on double beta processes in ¹⁵⁶Dy and ¹⁵⁸Dy have been established on the level of T_1/2?¹⁰¹⁴–¹⁰¹⁶ yr$T_{1/2}?{10}^{14}\text{–}{10}^{16}\text{yr}$. Possible resonant double electron captures in ¹⁵⁶Dy and ¹⁵⁸Dy were restricted on a similar level. As a by-product of the experiment we have measured the radioactive contamination of the Dy₂O₃ sample and set limits on the α   decay of dysprosium isotopes to the excited levels of daughter nuclei as T_1/2?¹⁰¹⁵–¹⁰¹⁷ yr$T_{1/2}?{10}^{15}\text{–}{10}^{17}\text{yr}$.     

A multi-isotope $$0\nu 2\beta $$ bolometric experiment

There are valuable arguments to perform neutrinoless double beta (\(0\nu 2\beta \)) decay experiments with several nuclei: the uncertainty of nuclear-matrix-element calculations; the possibility to test these calculations by using the ratio of the measured lifetimes; the unpredictability of possible breakthroughs in the detection technique; the difficulty to foresee background in \(0\nu 2\beta \) decay searches; the limited amount of isotopically enriched materials. We propose therefore approaches to estimate the Majorana neutrino mass by combining experimental data collected with different \(0\nu 2\beta \) decay candidates. In particular, we apply our methods to a next-generation experiment based on scintillating and Cherenkov-radiation bolometers. Current results indicate that this technology can effectively study up to four different isotopes simultaneously (\(^{82}\)Se, \(^{100}\)Mo, \(^{116}\)Cd and \(^{130}\)Te), embedded in detectors which share the same concepts and environment. We show that the combined information on the Majorana neutrino mass extracted from a multi-candidate bolometric experiment is competitive with that achievable with a single isotope, once that the cryogenic experimental volume is fixed. The remarkable conceptual and technical advantages of a multi-isotope investigation are discussed. This approach can be naturally applied to the proposed CUPID project, follow-up of the CUORE experiment that is currently taking data in the Gran Sasso underground laboratory.     

Search for double- β decay processes in 108Cd and 114Cd with the help of the low-background CdWO 4 crystal scintillator

The lifetime of the recently discovered six-quasiparticle (6-qp) isomer in ¹⁴⁰Nd has been measured using the ¹²⁶Te(¹⁸O, 4n) reaction and the pulsed-beam technique at the Institut de Physique Nucléaire (IPN) Orsay. The deduced lifetime of the 6-qp isomer in ¹⁴⁰Nd of 1.23(7)μs supports the 20⁺ spin-parity assignment to the isomeric state which is based on a spherical configuration that coexists with the triaxial bands observed in this spin range. Evidence for delayed components for the transitions below the 19/2⁺ state in ¹³⁹Nd was observed, with an apparent half-life of 272(4)ns.     

A next-generation neutrinoless double beta decay experiment based on ZnMoO4 scintillating bolometers

The search for neutrinoless double β decay probes lepton number conservation with high sensitivity and investigates the neutrino nature and mass scale. Experiments presently in preparation will cover the quasi-degeneracy region of the neutrino mass pattern. Probing the inverted hierarchy region requires improved sensitivities and next-generation experiments, based either on large expansions of the present searches or on new ideas. We examine here a novel technology relying on ZnMoO₄ scintillating bolometers, which can provide an experiment with background close to zero in the ton × year exposure scale. The promising performance of a pilot detector is presented, both in terms of energy resolution and background control. A preliminary study of the sensitivities of future experiments shows that the inverted hierarchy region is within the reach of the technique here proposed. A realistic phased approach program towards a next-generation search is presented and briefly discussed.     

CAMEO project and discovery potential of the future 2<Emphasis Type="Italic">β</Emphasis>-decay experiments

The demands on the future supersensitivity 2β-decay experiments (aiming to observe neutrinoless 2β decay or to advance restrictions on the neutrino mass to m_ν≤0.01 eV) are considered and requirements for their discovery potential are formulated. The most realistic 2β projects are reviewed and the conclusion is obtained that only several of them with high energy resolution would completely satisfy these severe demands and requirements. At the same time, most of the recent projects (CAMEO, CUORE, DCBA, EXO, etc.) could certainly advance the limit on the neutrino mass up to m_ν≤0.05 eV.     

Random coincidence of 2<Emphasis Type="Italic">ν</Emphasis>2<Emphasis Type="Italic">β</Emphasis> decay events as a background source in bolometric 0<Emphasis Type="Italic">ν</Emphasis>2<Emphasis Type="Italic">β</Emphasis> decay experiments

Two-neutrino double β decay can create an irremovable background even in high energy resolution detectors searching for neutrinoless double β decay due to random coincidence of 2ν2β events in the case of poor time resolution. Some possibilities for suppressing this background in cryogenic scintillating bolometers are discussed. It is shown that the present bolometric detector technologies enable one to control this form of background at the level required to explore the inverted hierarchy of the neutrino mass pattern, including the case of bolometers searching for the neutrinoless double β decay of ¹⁰⁰Mo, which is characterized by a relatively short two-neutrino double β decay half-life.     

Search for Li solar axions using resonant absorption in LiF crystal: Final results

The resonance excitation of the ⁷Li nuclei in a LiF crystal with mass of 553 g by hypothetical axions emitted in the deexcitation of the ⁷Li nuclei in the Sun was searched for deep underground at the Gran Sasso National Laboratories (LNGS) of INFN (3600 m w.e.). The data collected with a low background HP Ge detector 244 cm³ during 4044h have allowed us to set the limit on the axion mass: _ma<8.6 keV$m_a<8.6\text{keV}$ (90% C.L.) which is the best one for the ⁷Li solar axions to-date.     

First search for double- \beta decay of platinum by ultra-low background HP Ge \gamma spectrometry

A search for double- $ \beta$ processes in ¹⁹⁰Pt and ¹⁹⁸Pt was realized with the help of ultra-low background HP Ge 468cm^3 $ \gamma$ spectrometer in the underground Gran Sasso National Laboratories of the INFN (Italy). After 1815 h of data taking with 42.5g platinum sample, T _1/2 limits on 2 $ \beta$ processes in ¹⁹⁰Pt ( $ \varepsilon$ $ \beta^{+}_{}$ and 2 $ \varepsilon$ have been established on the level of 10¹⁴-10¹⁶y, 3 to 4 orders of magnitude higher than those known previously. In particular, a possible resonant double-electron capture in ¹⁹⁰Pt was restricted on the level of 2.9×10¹⁶ y at 90% C.L. In addition, T _1/2 limit on 2 $ \beta^{-}_{}$ decay of ¹⁹⁸Pt (2 $ \nu$ +0 $ \nu$ ) to the 2⁺ ₁ excited level of ¹⁹⁸Hg has been set at the first time: T _1/2 > 3.5×10¹⁸ y. The radiopurity level of the used platinum sample is reported.