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P. Belli
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A search for double beta processes in 190Pt and 198Pt was realized with the help of ultra-low background HP Ge 468 cm^3 gamma spectrometer in the underground Gran Sasso National Laboratories of the INFN (Italy). After 1815 h of data taking with 42.5 g platinum sample, T_{1/2} limits on 2beta processes in 190Pt (epsilonbeta^+ and 2epsilon) have been established on the level of 10^{14}-10^{16} yr, 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 times 10^{16} yr at 90% C.L. In addition, T_{1/2} limit on 2 beta^- decay of 198Pt (2 u+0 u) to the 2^+_1 excited level of 198Hg has been set at the first time: T_{1/2}>3.5 times 10^{18} yr. The radiopurity level of the used platinum sample is reported.
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A search for double $beta$ decay of dysprosium was realized for the first time with the help of an ultra low-background HP Ge $gamma$ detector. After 2512 h of data taking with a 322 g sample of dysprosium oxide limits on double beta processes in $^{156}$Dy and $^{158}$Dy have been established on the level of $T_{1/2}geq 10^{14}-10^{16}$ yr. Possible resonant double electron captures in $^{156}$Dy and $^{158}$Dy were restricted on a similar level. As a by-product of the experiment we have measured the radioactive contamination of the Dy$_2$O$_3$ sample and set limits on the $alpha$ decay of dysprosium isotopes to the excited levels of daughter nuclei as $T_{1/2}geq 10^{15} - 10^{17}$ yr.
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 2beta+/epsilon beta+/2epsilon 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 u 2beta+) > 1.4 10^{20} yr, T1/2(2 u epsilonbeta+) > 8.0 10^{19} yr and T1/2(0 u 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 u KL) > 2.0 10^{20} yr and T1/2(0 u 2L) > 3.6 10^{20} yr, respectively. Various two neutrino and neutrinoless 2beta half lives of 96Ru have been estimated in the framework of the QRPA approach. In addition, the T1/2 limit for 0 u 2beta- transitions of 104Ru to the first excited state of 104Pd has been set as > 6.5 10^{20} yr.
A search for double-beta decay of osmium has been realized for the first time with the help of an ultra-low background HPGe gamma detector at the underground Gran Sasso National Laboratories of the INFN (Italy). After 2741 h of data taking with a 173 g ultra-pure osmium sample limits on double-beta processes in 184Os have been established at the level of T_{1/2} about 10^{14}-10^{17} yr. Possible resonant double-electron captures in 184Os were searched for with a sensitivity T_{1/2} about 10^{16} yr. A half-life limit T_{1/2} > 5.3 10^{19} yr was set for the double-beta decay of 192Os to the first excited level of 192Pt. The radiopurity of the osmium sample has been investigated and radionuclides 137Cs, 185Os and 207Bi were detected in the sample, while activities of 40K, 60Co, 226Ra and 232Th were limited at the mBq/kg level.
The Standard Model of particle physics cannot explain the dominance of matter over anti-matter in our Universe. In many model extensions this is a very natural consequence of neutrinos being their own anti-particles (Majorana particles) which implies that a lepton number violating radioactive decay named neutrinoless double beta ($0 ubetabeta$) decay should exist. The detection of this extremely rare hypothetical process requires utmost suppression of any kind of backgrounds. The GERDA collaboration searches for $0 ubetabeta$ decay of $^{76}$Ge ($^{76}rm{Ge} rightarrow,^{76}rm{Se} + 2e^-$) by operating bare detectors made from germanium with enriched $^{76}$Ge fraction in liquid argon. Here, we report on first data of GERDA Phase II. A background level of $approx10^{-3}$ cts/(keV$cdot$kg$cdot$yr) has been achieved which is the world-best if weighted by the narrow energy-signal region of germanium detectors. Combining Phase I and II data we find no signal and deduce a new lower limit for the half-life of $5.3cdot10^{25}$ yr at 90 % C.L. Our sensitivity of $4.0cdot10^{25}$ yr is competitive with the one of experiments with significantly larger isotope mass. GERDA is the first $0 ubetabeta$ experiment that will be background-free up to its design exposure. This progress relies on a novel active veto system, the superior germanium detector energy resolution and the improved background recognition of our new detectors. The unique discovery potential of an essentially background-free search for $0 ubetabeta$ decay motivates a larger germanium experiment with higher sensitivity.
GERDA, the GERmanium Detector Array experiment, is a new double beta-decay experiment which is currently under construction in the INFN National Gran Sasso Laboratory (LNGS), Italy. It is implementing a new shielding concept by operating bare Ge diodes - enriched in Ge-76 - in high purity liquid argon supplemented by a water shield. The aim of GERDA is to verify or refute the recent claim of discovery, and, in a second phase, to achieve a two orders of magnitude lower background index than recent experiments. The paper discusses motivation, physics reach, design and status of construction of GERDA, and presents some R&D results.
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