F. Cappella

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 _at_LNGS
DAMA/RD
DAMA/LXe
low bckg DAMA/Ge for sampling meas.
DAMA/NaI DAMA/LIBRA
http//people.roma2.infn.it/dama
3
DAMA/RD set-up _at_ 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/RD

• 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
• 2EC2n 40Ca 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
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)
  EMI9265B53/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).
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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.

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

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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 (Qa6.946 MeV) ? 215Po (Qa7.526 MeV,
T1/21.78 ms) ? 211Pb
235U
? 227Ac

• with similar procedure .
• ? 7 mBq/kg in the ZWO-1
• ? ? 3 mBq/kg in the ZWO-2

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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.
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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
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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
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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
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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
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 _at_ LNGS by using low
background 0.10.7 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)