F. CappellaF. CappellaUniv. La SapienzaUniv. La Sapienza

e INFN-Romae INFN-Roma

NPA4NPA4Frascati, June 12 - 2009Frascati, June 12 - 2009

NPA4NPA4Frascati, June 12 - 2009Frascati, June 12 - 2009

Search for double beta Search for double beta decay of zinc and decay of zinc and tungsten with low tungsten with low background ZnWObackground ZnWO44

scintillatorsscintillators

Search for double beta Search for double beta decay of zinc and decay of zinc and tungsten with low tungsten with low background ZnWObackground ZnWO44

scintillatorsscintillators

DAMA/R&DDAMA/LXe low bckg DAMA/Ge

for sampling meas.

DAMA/NaI

DAMA/LIBRA

http://people.roma2.infn.it/dama

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

DAMA/R&D set-up @ LNGS

Radiopurity of the selected materials (95%C.L.):

• Particle Dark Matter investigation with NPB563(1999)97, CaF2(Eu) Astrop.Phys.7(1997)73• 2 decay in 136Ce and in 142Ce Il Nuov.Cim.A110(1997)189• 2EC2 40Ca decay Astrop. Phys. 7(1999)73 • 2 decay in 46Ca and in 40Ca NPB563(1999)97• 2+ decay in 106Cd Astrop.Phys.10(1999)115• 2 and decay in 48Ca NPA705(2002)29• 2EC2 in 136Ce,138Ce and decay in 142Ce NIMA498(2003)352• 2+ 0 and EC+ 0 decay in 130Ba NIMA525(2004)535• Cluster decay in 139La NIMA555(2005)270• Rare decay on natural Europium NPA789(2007)15• Rare decay of 113Cd PRC76(2007)064603• 2+ decay in 64Zn PLB658(2008)193• 2 decay in 108Cd and 114Cd EPJA36(2008)167• In progress: data taking with a new ZnWO4

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

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 assembling

Detector schema

Light guide

+ HP oil

PMT ZnWO4PMT

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

Potentially 2 active nuclides present in ZnWO4 crystals

The nucleus 64Zn is one of the few exceptions among 2+ 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:

(i) density = 7.8 g/cm3; (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.

Contamination of ZnWO4 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 184W16O3 molecule.

To estimate the presence of naturally occurring radioactive isotopes, as well as some other elements important for growing of the crystals

MAMMeasured atomic masses

CEConcentrations of elements

ARIActivities of radioactive isotopes

Energy calibration

Energy scale and resolution of the detectors measured with 22Na, 133Ba, 137Cs, 228Th and 241Am sources.

(662 keV)

ZWO-2a detector

Dependence of energy resolution of the ZnWO4 detectors on energy can be fitted by the function:

keVEbakeVFWHM γγ

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).

Energy spectra and background identification

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.

Energy spectra accumulated over Runs 2 and 3 with the ZnWO4 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 22EC 64Zn. The energy interval 0.05 - 4 MeV to search for other possible 2 processes in 64Zn

Time-amplitude analysis

220Rn (Qα=6.405 MeV)

→ 216Po (Q=6.907 MeV, T1/2=0.145

s)

→ 212Pb

228Th

23

22

32Th

Th

219Rn (Qα=6.946 MeV)

→ 215Po (Q=7.526 MeV, T1/2=1.78

ms)

→ 211Pb

227Ac

23

52

35UU

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) 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

Pulse-shape discrimination between () and particles

- Optimal filter technique [E. Gatti e F. DeMartini, Nucl. Electronics 2 (IAEA,Vienna, 1962) 265]

kk

kkk

tf

tPtfSI

)(

)()(

)()(

)()()(

tftf

tftftP

Energy resolution for is considerably worse than that for quanta due to dependence of the / ratio on the direction of particles relative to the ZnWO4 crystal axes.

Simulation of () backgroundSeveral active radionuclides could produce background in the ZnWO4 detectors.

Contamination of the PMTs can also contribute.

Run 2 (2906 h) ZWO-1

Run 3 (2130 h) ZWO-2

65Zn can be produced from 64Zn by thermal neutrons or/and by cosmogenic activation.

Search for 2 and + decay of 64Zn

Expected energy distributionsNo 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 65Zn

Search for 2 decay of 186W and 70Zn

run 3 + run 4

Expected energy distributions for

186WEnergy 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 70Zn and the two neutrino mode of 2 decay in 186W are one order of magnitude higher than those set in previous experiments.

Search for 2 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 02 process in 180W.

The best previous

limitsLimits obtained by the least squares fit of the spectrum in the 70-270 keV energy interval

Conclusions

Possible future potentiality for ZnWO4 in the future:an experiment involving 10 tons of non enriched crystals (91027 nuclei of 64Zn) could reach the T1/2 sensitivity 31028 yr (supposing zero background during 10 years);

22 should be surely observed in accordance with theoretical expectations (1025-1026 yr)

Low background experiments to search for 2 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 2, +, 2+ 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

In press in Nucl. Phys. A.DOI: 10.1016/j.nuclphysa.2009.05.139

excluded