F. Cappella Univ. La Sapienza e INFN-Roma

1. Search for double beta decay of zinc and tungsten with low background ZnWO4 scintillators F. Cappella Univ. La Sapienza e INFN-Roma NPA4 Frascati, June 12 - 2009

2. DAMA: an observatory for rare processes @LNGS DAMA/R&D DAMA/LXe low bckg DAMA/Ge for sampling meas. DAMA/NaI DAMA/LIBRA http://people.roma2.infn.it/dama

3. Radiopurity of the selected materials (95%C.L.): DAMA/R&D set-up @ LNGS • Air-tight Cu box continuously flushed with HP N2 • 10 cm of high purity Cu • 15 cm of low radioactive lead • 1.5 mm of cadmium • 4/10 cm polyethylene/paraffin • The whole shield closed inside a Plexiglas box also continuously flushed with HP N2 + Automatized opening of the shield + Calibration facility to calibrate in the same running condition without any contact with the installation environment Some recent results on rare processes with DAMA/R&D: • Particle Dark Matter investigation with NPB563(1999)97, • CaF2(Eu) Astrop.Phys.7(1997)73 • 2b decay in 136Ce and in 142Ce Il Nuov.Cim.A110(1997)189 • 2EC2n40Ca decay Astrop. Phys. 7(1999)73 • 2b decay in 46Ca and in 40Ca NPB563(1999)97 • 2b+ decay in 106Cd Astrop.Phys.10(1999)115 • 2b and b decay in 48Ca NPA705(2002)29 • 2EC2n in 136Ce,138Ce and a decay in 142Ce NIMA498(2003)352 • 2b+ 0n and ECb+ 0n decay in 130Ba NIMA525(2004)535 • Cluster decay in 139La NIMA555(2005)270 • Rare a decay on natural Europium NPA789(2007)15 • Rare b decay of 113Cd PRC76(2007)064603 • 2b+ decay in 64Zn PLB658(2008)193 • 2b decay in 108Cd and 114Cd EPJA36(2008)167 • In progress: data taking with a new ZnWO4

4. Light guide + HP oil PMT PMT ZnWO4 In press in Nucl. Phys. A. DOI: 10.1016/j.nuclphysa.2009.05.139 Detector assembling • Development of low background ZnWO4 crystal scintillators with large volume and high scintillation properties has been realized • Three low background ZnWO4 crystal scintillators have been used in this experiment, produced from 2 crystal boules grown by the Czochralski method • Inside a cavity (filled up with high-pure silicon oil)  47 x 59 mm in central part of a polystyrene light-guide 66 mm in diameter and 312 mm in length. • Two Selected low background photomultiplier (PMT) EMI9265–B53/FL 3’’ diameter (All light guide wrapped by PTFE reflection tape) Detector schema An event-by-event DAQ accumulates the amplitude and the arrival time of the events. Sum of the PMTs signal recorded by transient digitizer (20 MS/s, time window: 100 ms).

5. Potentially 2b active nuclides present in ZnWO4 crystals The nucleus 64Zn is one of the few exceptions among 2b+ nuclei having big natural isotopic abundance ZnWO4 scintillators offer good potential in searching for double beta processes in Zinc and Tungsten isotopes. • Main ZnWO4 properties: • density = 7.8 g/cm3; • light yield  13% of that of NaI(Tl); • refractive index = 2.1-2.2; • emission maximum at 480 nm; • an effective average decay time of 24 ms (at room temperature); • non-hygroscopic and chemically inert with melting point at 1200 C.

6. Contamination of ZnWO4 crystal measured by ICP-MS analysis To estimate the presence of naturally occurring radioactive isotopes, as well as some other elements important for growing of the crystals MAM Measured atomic masses CE Concentrations of elements ARI Activities of radioactive isotopes The measurements have an estimated accuracy of 20-30%. The ICP-MS detection limit for Thorium is rather low due to interference with tungsten oxide 184W16O3 molecule.

7. (662 keV) ZWO-2a detector Energy calibration Energy scale and resolution of the detectors measured with 22Na, 133Ba, 137Cs, 228Th and 241Amg sources. Dependence of energy resolution of the ZnWO4 detectors on energy can be fitted by the function: For instance, the values of the parameters for the detector ZWO-2a are: a = 190(40) keV2 and b = 7.34(35) keV. In addition, the energy scale of the detectors was checked by using background lines at 609 keV of 214Bi, 1461 keV (40K), 1764 keV (214Bi) and 2615 keV (208Tl).

8. Energy spectra and background identification Measurements carried out in four runs The energy interval 0.01 - 1 MeV was chosen to search for the 2n2EC 64Zn. The energy interval 0.05 - 4 MeV to search for other possible 2b processes in 64Zn Energy spectra accumulated over Runs 2 and 3 with the ZnWO4 detectors normalized to the mass of the crystals and time of measurements • Reconstruction of the background spectra and estimation of the radioactive contamination of the ZnWO4 detectors with: • time-amplitude analysis; • pulse-shape discrimination; • Monte Carlo simulations.

9. 220Rn (Qα=6.405 MeV) → 216Po (Qα=6.907 MeV, T1/2=0.145 s) → 212Pb 232Th  228Th Time-amplitude analysis Arrival time and energy of each event were used to select the fast decay chains in the 232Th and 235U families • All events within 0.75 – 1.75 MeV were used as triggers, while a time interval 0.025 – 0.3 s (65% of 216Po decays) and the same energy window were set for the second events. • Four and seven events of the fast chain 220Rn  216Po  212Pb were found in the data of Run 2 and Run 3, respectively: • 5(3) mBq/kg in the ZWO-1 •  2(1) mBq/kg in the ZWO-2 219Rn (Qα=6.946 MeV) → 215Po (Qα=7.526 MeV, T1/2=1.78 ms) → 211Pb 235U  227Ac • … with similar procedure …. •  7 mBq/kg in the ZWO-1 •   3 mBq/kg in the ZWO-2

10. Pulse-shape discrimination between b(g) and a particles - Optimal filter technique [E. Gatti e F. DeMartini, Nucl. Electronics 2 (IAEA,Vienna, 1962) 265] Energy resolution for a is considerably worse than that for g quanta due to dependence of the a/b ratio on the direction of particles relative to the ZnWO4 crystal axes.

11. Simulation of b(g) background Several b active radionuclides could produce background in the ZnWO4 detectors. Contamination of the PMTs can also contribute. Run 3 (2130 h) ZWO-2 65Zn can be produced from 64Zn by thermal neutrons or/and by cosmogenic activation. Run 2 (2906 h) ZWO-1

12. Peak@1133±8 keV in the spectrum of Run 3 can be ascribed to 65Zn Search for 2e and eb+ decay of 64Zn Expected energy distributions No clear peculiarities in the measured energy spectra can be interpreted as a signal for 2b decays of Zinc or Tungsten isotopes. Energy spectrum fitted by model functions sum of the background model + the expected energy distribution. run 2 + run 3

13. Search for 2b decay of 186W and 70Zn Expected energy distributions for 186W Energy spectra collected in Runs 1-4 have been fitted (using background model and the expected signal) to determine the best limit for the searched decay modes The half-life limits on the 2b processes in 70Zn and the two neutrino mode of 2b decay in 186W are one order of magnitude higher than those set in previous experiments. run 3 + run 4

14. Search for 2e capture in 180W The sum of the background spectra of the ZnWO4 detectors accumulated in all four Runs was used to set limits on the 0n2e process in 180W. The best previous limits Limits obtained by the least squares fit of the spectrum in the 70-270 keV energy interval

15. excluded In press in Nucl. Phys. A. DOI: 10.1016/j.nuclphysa.2009.05.139 Conclusions Low background experiments to search for 2b processes in 64Zn, 70Zn, 180W, 186W were carried out over more than 10000 h @ LNGS by using low background 0.1–0.7 kg ZnWO4 crystal scintillators Obtained T1/2 limits for 64Zn on the level of 1020-1021 yr. Only two other nuclei (40Ca and 78Kr) among potentially 2e, eb+, 2b+ active isotopes were investigated at the level of 1021 yr Most of the obtained T1/2 limits are near one order of magnitude higher than those established in previous experiments Very radiopure ZnWO4 crystal scintillators have been developed Possible future potentiality for ZnWO4 in the future: an experiment involving 10 tons of non enriched crystals (91027 nuclei of 64Zn) could reach the T1/2 sensitivity 31028 yr (supposing zero background during 10 years); 2n2e should be surely observed in accordance with theoretical expectations (1025-1026 yr)