Search

The first results are reported on the limit for neutrinoless double decay of 130Te obtained with the new bolometric experiment CUORICINO. The set-up consists of 44 cubic crystals of natural TeO2, 5 cm on the side and 18 crystals of 3×3×6 cm3. Four of these latter crystals are made with isotopically enriched materials: two in 128Te and two others in 130Te. With a sensitive mass of ∼40 kg, our array is by far the most massive running cryogenic detector to search for rare events. The array is operated at a temperature of ∼10 mK in a dilution refrigerator under a heavy shield in the Gran Sasso Underground Laboratory at a depth of about 3500 m.w.e. The counting rate in the region of neutrinoless double beta decay is ∼0.2 counts keV−1kg−1y−1, among the lowest in this type of experiment. No evidence for neutrinoless double beta decay is found with the present statistics obtained in about three months with a live time of 72%. The corresponding lower limit for the lifetime of this process is of 5.5×1023 years at 90% C.L. The corresponding limit for the effective neutrino mass ranges between 0.37 to 1.9 eV depending on the theoretically calculated nuclear matrix elements used. This constraint is the most restrictive one except those obtained with Ge diodes, and is comparable to them.

Crystals of silver perrhenate were used for the first time in the bolometric measurement of the spectrum of 187 Re with the aim of determining the electron antineutrino mass. The temperature rise was measured by means of neutron transmutation doped germanium and P-implanted silicon thermistors. The energy scale was calibrated using the K lines of Mn and the fluorescence L lines of Sn. Bolometers made from crystals of masses ranging from ∼250 to ∼500 μg were run with FWHM resolutions on the Sn lines ranging from 25 to 60 eV. The end point of the spectrum was located at 2460±5 stat ±10 syst eV. The measured half lifetime was 43±4 stat ±3 syst Gyr, which is in good agreement with the values obtained with mass spectroscopy from the build-up of 187 Os in rhenium samples. The sensitivity of the measurement on a non-zero neutrino mass achievable with these bolometers is discussed.

Preliminary results on double beta decay of 130 Te obtained in the first run of an array of twenty cryogenic detectors are presented. The set-up is made with crystals of TeO of 340 grams each corresponding to the largest presently operating 2 cryogenic mass. It was run under a heavy shield in the Gran Sasso Underground Laboratory at a depth of about 3500 m.w.e. By recording the pulses of each detector in anticoincidence with the others a 90% c.l. lower limit of 5.6 = 10 22 years has been obtained on the lifetime for neutrinoless double beta decay of 130 Te in a preliminary test run, corresponding to about one week of effective running time. No evidence is also found for double beta decay to the first excited 2 q state of 130 Xe with a 90% c.l. lower limit of 1.7 = 10 22 years on that lifetime. Some consequences of the present results in the

During a Double Beta Decay experiment performed at Laboratori Nazionali del Gran Sasso, a 1548 hours background spectrum was collected with a 340 g TeO2 thermal detector. An analysis of this spectrum has been carried out to search for possible WIMP signals. The values for parameters which are essential in the search for WIMPs, like energy resolution (2 keV), energy threshold (13 keV) and nuclear recoil quenching factor (≥ 0.93) have been experimentally determined and are discussed in detail. The spectrum of recoils induced by α decays has been directly observed for the first time in coincidence with the α particle pulse. Preliminary limits on the spin-independent cross sections of WIMPs on Te and O nuclei have been obtained.

A 334 g TeO2 crystal has been operated at a temperature of around 10 mK for more than one year in a low intrinsic radioactivity dilution refrigerator installed in the Gran Sasso Underground Laboratory. From the spectrum collected in 9234 h of effective running time we improve our limit on neutrinoless double beta decay of 130Te by an order of magnitude with respect to our previous experiment. It is the most stringent on this nucleus and excludes a large contribution of the neutrinoless mode to the rate of double beta decay found in geochemical experiments. Upper limits on the effective neutrino mass and on the contributions of right banded currents are given.

We have searched for two neutrino and neutrinoless double beta decay of 136 Xe with a multicell proportional counter in the Gran Sasso underground laboratory. The search for the two neutrino channel has been based on the comparison of spectra obtained in runs with natural and enriched xenon and with xenon which had been specifically cleaned with an ultracentrifuge. Special dedicated runs have been carried out with the various gases to evaluate the background from the rate of α-particles. No evidence has been found for two neutrino or neutrinoless double beta decay. The corresponding lower limits on the lifetime are evaluated with a new statistical procedure.

We have searched for neutrinoless double beta decay of 136Xe with a multielement proportional chamber installed in the Gran Sasso Underground Laboratory. The chamber has been operated with natural xenon and with xenon enriched to 64% in the isotope 136. No evidence has been found in a preliminary analysis for neutrinoless double beta decay with a lower limit on the halflife of 1.4×1022 yr at 68% confidence level.

New results are presented from an experiment on double beta decay of 76 Ge carried out with two Ge Li detectors in the Mont Blanc tunnel. No evidence has been found for neutrinoless double beta decay, and the obtained lower limit of the half lifetime (1.2 × 10 23 yr) is discussed in terms of the mass of Majorana neutrinos and of the possible presence of right-handed currents.

A new experiment on neutrinoless double beta decay of 76Ge to theground and first excited state of 76Se has been carried out in the Mont Blanc Laboratory at a depth of 5000 m.w.e. of rock. Lower limits of 2 × 1022 and 0.8 × 1022 yr have been set for the (0+ − 0+) and (0+ − 2+) transitions, respectively. The corresponding limits on the existence of massive neutrinos and of right handed currents are discussed.

A new technique is applied to the search for neutrinoless double β decay. A Ge(Li) crystal is used both as source and as detector of the 76Ge → 76Se transition. Our negative result ( τ 1 2 > 3 × 10 20 y ) is consistent with the lepton conservation law.