Search

A first search for rare decays of gadolinium isotopes was performed with an ultra-low background high-purity germanium detector at Gran Sasso Underground Laboratory (Italy). A 198 g Gd 2 O 3 powder sample was measured for 63.8 d with a total Gd exposure of 12.6 kg × d. 152 Gd is the most promising isotope for resonant enhanced neutrinoless double electron capture which could significantly increase the decay rate over other neutrinoless double beta decay processes. The half-life for this decay was constrained to > 4.2 × 10 12 year (90% credibility). This limit is still orders of magnitude away from theoretical predictions but it is the first established limit on the transition paving the way for future experiments. In addition, other rare alpha and double beta decay modes were investigated in 152 Gd, 154 Gd, and 160 Gd with half-life limits in the range of 10 17 - 20 year. [ABSTRACT FROM AUTHOR]

180 m Ta is the longest-lived metastable state presently known. Its decay has not been observed yet. In this work, we report a new result on the decay of 180 m Ta obtained with a 2015.12-g tantalum sample measured for 527.7 d with an ultra-low background HPGe detector in the STELLA laboratory of the Laboratori Nazionali del Gran Sasso (LNGS), in Italy. Before the measurement, the sample has been stored deep-underground for ten years, resulting in subdominant background contributions from cosmogenically activated 182 Ta . We observe no signal in the regions of interest and set half-life limits on the process for the two channels EC and β - : T 1 / 2 , EC > 1.6 × 10 18 year and T 1 / 2 , β - > 1.1 × 10 18 year (90% C. I.), respectively. We also set the limit on the γ de-excitation / IC channel: T 1 / 2 , IC > 4.1 × 10 15 year (90% C. I.). These are, as of now, the most stringent bounds on the decay of 180 m Ta worldwide. Finally, we test the hypothetical scenarios of de-excitation of 180 m Ta by cosmological Dark Matter and constrain new parameter space for strongly-interacting dark-matter particle with mass up to 10 5 GeV. [ABSTRACT FROM AUTHOR]

A new study of double beta decay processes in natural Zr isotopes using Zr metal samples and an ultra-low-background high-purity germanium detectors was performed at Laboratori Nazionali del Gran Sasso of INFN (Italy). Two data sets were collected with a 736.35 g and a 129.94 g Zr metal over 59.7 days and 37.3 days, respectively, and were used for a detailed analysis. A new limit on the double beta decay of 94 Zr to the 1st excited level of 94 Mo was set as T 1 / 2 β β > 2.1 × 10 20 year , which is a factor of three better than the previous result. It is shown that the current radiopurity of Zr metal (tens of Bq/kg), produced via two methods, do not comply with the radiopurity requirements for low-background experiments. Hence, we propose to use a purified ZrCl 4 powder in an optimized geometry as a new approach for future experiments with extended sensitivity focused on studies of rare nuclear processes in Zr isotopes. [ABSTRACT FROM AUTHOR]

A 510 day long-term measurement of a 45.3 g platinum foil acting as the sample and high voltage contact in an ultra-low-background high purity germanium detector was performed at Laboratori Nazionali del Gran Sasso (Italy). The data was used for a detailed study of double beta decay modes in natural platinum isotopes. Limits are set in the range O (10 14 - 10 19) years (90% C.L.) for several double beta decay transitions to excited states confirming, and partially extending existing limits. The highest sensitivity of the measurement, greater than 10 19 years, was achieved for the two neutrino and neutrinoless double beta decay modes of the isotope 198 Pt. Additionally, novel limits for inelastic dark matter scattering on 195 Pt are placed up to mass splittings of approximately 500 keV. We analyze several techniques to extend the sensitivity and propose a few approaches for future medium-scale experiments with platinum-group elements. [ABSTRACT FROM AUTHOR]

A search for alpha and double beta decays of ytterbium isotopes was performed with an ultra low-background high purity germanium detector at Gran Sasso Underground Laboratory (Italy). A 194.7 g Yb 2 (C 2 O 4 ) 3 powder sample was measured for 11.3 days with a total Yb exposure of 1.25 kg × day. Half-life limits for α -decay modes of 168 Y b , 170 Y b , 171 Y b , 172 Y b , 173 Y b , 174 Y b and 176 Y b into the first excited states have been obtained between 6 × 10 14 years and 2 × 10 16 years. These are the first experimental constraints of these decay modes. Double electron capture of 168 Y b and double beta decay of 176 Y b into the first excited 2 + and 0 + states could be excluded with limits between 1 × 10 14 years to 8 × 10 16 years. This improves the experimental information on some of the decay modes compared to previous constraints. [ABSTRACT FROM AUTHOR]

As one of the primordial radioactive isotopes, 232 Th mainly undergoes α -decay with a half-life of 1.402 · 10 10 years. However, it is also one of 35 double beta decay candidates in which the single β -decay is forbidden or strongly suppressed. 181 mg of thorium contained in a gas mantle were measured in a HPGe well-detector at the Gran Sasso Underground Laboratory with a total exposure of 3.25 g × d. We obtain half-life limits on all double beta decay modes of 232 Th to excited states of 232 U on the order of 10 11 - 15 years. For the most likely transition into the 0 1 + state we find a lower half-life limit of 6.7 · 10 14 years (90% C.I.). These are the first constraints on double beta decay excited state transition in 232 Th . [ABSTRACT FROM AUTHOR]