G. Gervasio

A scintillating bolometer for experiments on double beta decay

Abstract The scintillation yields of CaF 2 crystals with different doping concentration of Europium have been measured at low temperatures and their bolometric behavior has been investigated. After these studies we have constructed the first “scintillating bolometer” where the heat and scintillation pulses produced by charged particles are simultaneously recorded. With this method a strong suppression of the background from α -particles in the energy region of interest for searches on double beta decay of 48 Ca can be achieved.

A search for neutrinoless double beta decay of 130Te with a thermal detector

Abstract A 73.1 g TeO2 bolometric detector has been operating for more than two months at about 16 mK in a specially constructed low activity dilution refrigerator installed in the Gran Sasso Underground Laboratory and shielded against environmental radioactivity. Its energy resolution is comparable with that of a Ge diode. A spectrum of the thermal pulses collected in 1389 h of effective running time shows no evidence for neutrinoless double beta decay of 130Te. The corresponding lower limit of the lifetime is three orders of magnitude more stringent than those obtained for the some nucleus with conventional techniques. It also exceeds the value for the inclusive (two neutrino and neutrinoless) lifetime obtained by geochemical searches. Double beta decay of 130Te has therefore to be attributed mainly to the two neutrino channel.

Development of a thermal scintillating detector for double beta decay of 48Ca

Abstract Among double beta candidates 48Ca stands out for its 4.271 MeV transition energy, well above most of the contribution of natural γ and β radioactivity, but extremely near to the energy released in the α decay of 238U (4.274 MeV including nucleus recoil). A CaF2(Eu) detector with both thermal pulse and scintillation light readout would give very good discrimination against this very dangerous source of background. We tested CaF2 crystals with 0.01 to 0.07% Eu doping, in the range of temperature between 300 K and 20 mK. The result shows that detection of the scintillation light from alpha particles of 5.4 MeV with a silicon photodiode is possible down to 20 mK with high signal/noise ratio, and that such doping levels do not affect the performance of CaF2 as a thermal detector

A search for two neutrino and neutrinoless double beta decay of 136Xe in the Gran Sasso underground laboratory

Abstract 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.

Large mass bolometric detectors for double-beta decay experiments

TeO/sub 2/ bolometric detectors with masses of 5.5 g, 20.9 g, and 34 g have been developed. The latter have been operating for more than 440 h in the Gran Sasso underground laboratory, and a limit of 1.2*10/sup 20/ y for the half life of the neutrinoless double-beta decay of /sup 130/Te was reached. CaF/sub 2/ detectors for simultaneous detection of scintillating light and phonon signals are under development and will be used to search for double-beta decay of /sup 48/Ca. Scintillation of a CaF/sub 2/ crystal with 0.03% Eu doping was observed at 20 mK. A 2.1 g, 0.01% Eu doped crystal was used as a bolometer, and a resolution of 50 keV for the double escape peak of /sup 232/Th was obtained at 60 mK, showing a negligible contribution of the Eu atoms to the crystal heat capacity. Simultaneous detection of light and phonons will allow an efficient background rejection of the /sup 238/U to /sup 234/Th alpha decay from the /sup 238/U natural series occurring at almost the same energy of the /sup 48/Ca transition. Background rejection is necessary as even U contamination as low as 10/sup -10/ g/g would make the search impossible.<<ETX>>

A search for lepton number non-conservation in double beta decay of 136Xe

Abstract 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.

Multielement proportional chamber — a study of background in the Gran Sasso underground laboratory

We describe the counting and background characteristics of the multielement proportional chamber used for the search on two-neutrino and neutrinoless double beta decay of 136Xe in the Gran Sasso underground laboratory. Enriched xenon to 64% in the 136Xe isotope and natural xenon cleaned by ultracentrifugation were filled up to 9.5 bar and counted for 6210 and 5120 h, respectively. The reduction of chamber background using suitable software triggers is discussed in detail.

A thermal high resolution alpha and gamma-ray spectrometer

Abstract A thermal detector consisting of a 73 g TeO 2 crystal has been constructed and is operating at ∼ 15 mK in the underground Gran Sasso laboratory. It allows the performing of α and γ-ray spectroscopy with a resolution comparable to that of solid state detectors, and the determination of internal radioactive contaminations of 10 −3 Bqkg −1 or less.

Separation of rare gases from radioactive contaminants by means of an ultracentrifuge

Abstract A sample of xenon enriched in 136Xe has been obtained with the ultracentrifuge system of the United States Department of Energy in Oak Ridge, Tennessee. The same system has also been used for cleanup only, namely to eliminate undesired gas impurities. Measurements of radioactivity carried out with a multicell proportional chamber show that ultracentrifugation is indeed very effective in the elimination of krypton.

A multi-element proportional chamber used in an experiment on ββ-decay of 136Xe

The lower limits on the neutrinoless and two neutrino ββ-decay of 136Xe, obtained with a multi-element proportional chamber, are reported. The sensitivity of the experiment was limited by the level of the background contamination. The experimental apparatus, its performance and the background analysis are discussed in detail.