1. F. Cappella Univ. La Sapienza e INFN-Roma NPA4 Frascati, June 12 - 2009 NPA4 Search for double beta decay of zinc and tungsten with low background ZnWO 4 scintillators

2. DAMA/R&D DAMA/LXe low bckg DAMA/Ge for sampling meas. DAMA/NaI DAMA/LIBRA http://people.roma2.infn.it/dama

3.  Air-tight Cu box continuously flushed with HP N 2  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 N 2 DAMA/R&D set-up @ LNGS Radiopurity of the selected materials (95%C.L.): Particle Dark Matter investigation with NPB563(1999)97, CaF 2 (Eu) Astrop.Phys.7(1997)73 2  decay in 136 Ce and in 142 Ce Il Nuov.Cim.A110(1997)189 2EC2 40 Ca decay Astrop. Phys. 7(1999)73 2  decay in 46 Ca and in 40 Ca NPB563(1999)97 2  + decay in 106 Cd Astrop.Phys.10(1999)115 2  and  decay in 48 Ca NPA705(2002)29 2EC2 in 136 Ce, 138 Ce and  decay in 142 Ce NIMA498(2003)352 2  + 0 and EC  + 0 decay in 130 Ba NIMA525(2004)535 Cluster decay in 139 La NIMA555(2005)270 Rare  decay on natural Europium NPA789(2007)15 Rare  decay of 113 Cd PRC76(2007)064603 2  + decay in 64 Zn PLB658(2008)193 2  decay in 108 Cd and 114 Cd EPJA36(2008)167 In progress: data taking with a new ZnWO 4 Some recent results on rare processes with DAMA/R&D: + Automatized opening of the shield + Calibration facility to calibrate in the same running condition without any contact with the installation environment

4.  Development of low background ZnWO 4 crystal scintillators with large volume and high scintillation properties has been realized  Three low background ZnWO 4 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 assembling Detector schema Light guide + HP oil PMT ZnWO 4 PMT 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). In press in Nucl. Phys. A. DOI: 10.1016/j.nuclphysa.2009.05.139

5. Potentially 2  active nuclides present in ZnWO 4 crystals The nucleus 64 Zn is one of the few exceptions among 2  + nuclei having big natural isotopic abundance ZnWO 4 scintillators offer good potential in searching for double beta processes in Zinc and Tungsten isotopes. Main ZnWO 4 properties: (i)density = 7.8 g/cm 3 ; (ii)light yield  13% of that of NaI(Tl); (iii)refractive index = 2.1-2.2; (iv)emission maximum at 480 nm; (v)an effective average decay time of 24  s (at room temperature); (vi)non-hygroscopic and chemically inert with melting point at 1200 C.

6. Contamination of ZnWO 4 crystal measured by ICP-MS analysis 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 184 W 16 O 3 molecule. 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

7. Energy calibration Energy scale and resolution of the detectors measured with 22 Na, 133 Ba, 137 Cs, 228 Th and 241 Am  sources. (662 keV) ZWO-2a detector Dependence of energy resolution of the ZnWO 4 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) keV 2 and b = 7.34(35) keV. In addition, the energy scale of the detectors was checked by using background lines at 609 keV of 214 Bi, 1461 keV ( 40 K), 1764 keV ( 214 Bi) and 2615 keV ( 208 Tl).

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

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

10. Pulse-shape discrimination between  (  ) and  particles - Optimal filter technique [E. Gatti e F. DeMartini, Nucl. Electronics 2 (IAEA,Vienna, 1962) 265] Energy resolution for  is considerably worse than that for  quanta due to dependence of the  /  ratio on the direction of particles relative to the ZnWO 4 crystal axes.

11. Simulation of  (  ) background Several  active radionuclides could produce background in the ZnWO 4 detectors. Contamination of the PMTs can also contribute. Run 2 (2906 h) ZWO-1 Run 3 (2130 h) ZWO-2 65 Zn can be produced from 64 Zn by thermal neutrons or/and by cosmogenic activation.

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

13. Search for 2  decay of 186 W and 70 Zn run 3 + run 4 Expected energy distributions for 186 W 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 2  processes in 70 Zn and the two neutrino mode of 2  decay in 186 W are one order of magnitude higher than those set in previous experiments.

14. Search for 2  capture in 180 W The sum of the background spectra of the ZnWO 4 detectors accumulated in all four Runs was used to set limits on the 0 2  process in 180 W. The best previous limits Limits obtained by the least squares fit of the spectrum in the 70-270 keV energy interval

15. Conclusions Possible future potentiality for ZnWO 4 in the future: an experiment involving 10 tons of non enriched crystals (9  10 27 nuclei of 64 Zn) could reach the T 1/2 sensitivity 3  10 28 yr (supposing zero background during 10 years); 2 2  should be surely observed in accordance with theoretical expectations (10 25 -10 26 yr) Low background experiments to search for 2  processes in 64 Zn, 70 Zn, 180 W, 186 W were carried out over more than 10000 h @ LNGS by using low background 0.1–0.7 kg ZnWO 4 crystal scintillators Obtained T 1/2 limits for 64 Zn on the level of 10 20 -10 21 yr. Only two other nuclei ( 40 Ca and 78 Kr) among potentially 2 ,  +, 2  + active isotopes were investigated at the level of 10 21 yr Most of the obtained T 1/2 limits are near one order of magnitude higher than those established in previous experiments Very radiopure ZnWO 4 crystal scintillators have been developed In press in Nucl. Phys. A. DOI: 10.1016/j.nuclphysa.2009.05.139 excluded