Search for 2\beta\ decays of 96Ru and 104Ru by ultra-low background HPGe gamma spectrometry at LNGS: final results

TL;DR

This study used ultra-low background gamma spectrometry at LNGS to search for double beta decay in 96Ru and 104Ru, setting new half-life limits and exploring decay mechanisms with improved sensitivity.

Contribution

It provides the most stringent half-life limits to date for various double beta decay modes in 96Ru and 104Ru, utilizing deep purification and extended measurement time.

Findings

Half-life limits for 96Ru double beta decay modes around 10^{20} years.

Resonant neutrinoless double electron capture limits in 96Ru.

New half-life limit for 104Ru to excited state > 6.5×10^{20} years.

Abstract

An experiment to search for double beta decay processes in 96Ru and 104Ru, which are accompanied by gamma rays, has been realized in the underground Gran Sasso National Laboratories of the I.N.F.N. (Italy). Ruthenium samples with masses of about (0.5-0.7) kg were measured with the help of ultra-low background high purity Ge gamma ray spectrometry. After 2162 h of data taking the samples were deeply purified to reduce the internal contamination of 40K. The last part of the data has been accumulated over 5479 h. New improved half life limits on 2\beta+/\epsilon \beta+/2\epsilon\ processes in 96Ru have been established on the level of 10^{20} yr, in particular for decays to the ground state of 96Mo: T1/2(2\nu 2\beta+) > 1.4 10^{20} yr, T1/2(2\nu \epsilon\beta+) > 8.0 10^{19} yr and T1/2(0\nu 2K) > 1.0 10^{21} yr (all limits are at 90% C.L.). The resonant neutrinoless double electron captures to the 2700.2 keV and 2712.7 keV excited states of 96Mo are restricted as: T1/2(0\nu KL) > 2.0 10^{20} yr and T1/2(0\nu 2L) > 3.6 10^{20} yr, respectively. Various two neutrino and neutrinoless 2\beta\ half lives of 96Ru have been estimated in the framework of the QRPA approach. In addition, the T1/2 limit for 0\nu 2\beta- transitions of 104Ru to the first excited state of 104Pd has been set as > 6.5 10^{20} yr.