Transcript Slide 1
Search for double beta decay of zinc and tungsten with low background ZnWO
4
scintillators
NPA4
Frascati, June 12 - 2009 F. Cappella Univ. La Sapienza e INFN-Roma
DAMA/LXe DAMA/NaI DAMA/LIBRA DAMA/R&D low bckg DAMA/Ge for sampling meas.
http://people.roma2.infn.it/dama
DAMA/R&D set-up @ LNGS
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 + 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, CaF 2 (Eu) Astrop.Phys.7(1997)73 2
b
decay in 136 Ce and in 142 Ce Il Nuov.Cim.A110(1997)189 2EC2
n
40 Ca decay Astrop. Phys. 7(1999)73 2
b
decay in 46 Ca and in 40 Ca NPB563(1999)97 2
b
+ decay in 106 Cd 2
b
and
b
decay in 48 Ca 2EC2
n
2
b
+ 0
n
in 136 Ce, 138 Ce and
a
and EC
b
+ 0
n
decay in decay in 130 142 Ce Ba Astrop.Phys.10(1999)115 NPA705(2002)29 NIMA498(2003)352 NIMA525(2004)535 Cluster decay in 139 La Rare
a
Rare
b
2
b
+ decay on natural Europium decay of 113 Cd decay in 64 Zn NIMA555(2005)270 NPA789(2007)15 PRC76(2007)064603 PLB658(2008)193 2
b
decay in 108 Cd and 114 Cd EPJA36(2008)167 In progress: data taking with a new ZnWO 4 Radiopurity of the selected materials (95%C.L.):
Detector assembling
In press in Nucl. Phys. A.
DOI: 10.1016/j.nuclphysa.2009.05.139
Development of low background ZnWO 4 scintillation properties has been realized crystal scintillators with large volume and high 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 schema
PMT Light guide ZnWO 4 + HP oil 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).
Potentially 2
b
active nuclides present in ZnWO
4
crystals
The nucleus 64 Zn is one of the few exceptions among 2 b + isotopic abundance nuclei having big natural 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 m s (at room temperature); (vi) non-hygroscopic and chemically inert with melting point at 1200 C.
Contamination of ZnWO
4
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 184 W 16 O 3 molecule.
Energy calibration
Energy scale and resolution of the detectors measured with 22 Na , 133 Ba , 137 Cs , 228 Th and 241 Am g sources.
Dependence of energy resolution of the ZnWO 4 detectors on energy can be fitted by the function: FWHM γ a b E γ For instance, the values of the parameters for the detector ZWO-2a are: a = 190(40) keV 2 and b = 7.34(35) keV.
ZWO-2a detector (662 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).
Energy spectra and background identification
Measurements carried out in four runs The energy interval 0.01 - 1 MeV The energy interval 0.05 - 4 MeV was chosen to search for the 2 n 2EC 64 Zn.
to search for other possible 2 b processes in 64 Zn Energy spectra accumulated over Runs 2 and 3 with the ZnWO 4 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 ZnWO 4 detectors with: • time-amplitude analysis; • pulse-shape discrimination; • Monte Carlo simulations.
Time-amplitude analysis
Arrival time and energy of each event were used to select the fast decay chains in the 232 Th and 235 U families 220 Rn (Q α =6.405 MeV) → 216 Po (Q =6.907 MeV, T 1/2 =0.145 s) → 212 Pb 228 Th 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) m Bq/kg in the ZWO-1 2(1) m Bq/kg in the ZWO-2 219 Rn (Q α =6.946 MeV)
→
215 Po (Q =7.526 MeV, T 1/2 =1.78 ms)
→
211 Pb … with similar procedure ….
7 m Bq/kg in the ZWO-1 3 m Bq/kg in the ZWO-2 227 Ac
Pulse-shape discrimination between
b
(
g
) and
a
particles
Optimal filter technique
[E. Gatti e F. DeMartini, Nucl. Electronics 2 (IAEA,Vienna, 1962) 265]
SI
k f
k
(
t f k
)
P
(
t k
(
t k
) )
P
(
t
)
f
a
f
a (
t
) (
t
)
f
b (
t
)
f
b (
t
) Energy resolution for of the a / b a is considerably worse than that for g quanta due to dependence ratio on the direction of particles relative to the ZnWO 4 crystal axes.
Simulation of
b
(
g
) background
Several b active radionuclides could produce background in the ZnWO 4 detectors.
Contamination of the PMTs can also contribute.
Run 3 (2130 h) ZWO-2 65 Zn can be produced from 64 Zn by thermal neutrons or/and by cosmogenic activation.
Run 2 (2906 h) ZWO-1
Search for 2
e
and
eb
+
decay of
64
Zn
No clear peculiarities in the measured energy spectra can be interpreted as a signal for 2 b decays of Zinc or Tungsten Expected energy distributions isotopes.
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
run 2 + run 3
Search for 2
b
decay of
186
W and
70
Zn
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 70 b processes in Zn and the two neutrino mode of 2 b decay in 186 W are one order of magnitude higher than those set in previous experiments.
run 3 + run 4
Search for 2
e
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 n 2 e process in 180 W.
Limits obtained by the least squares fit of the spectrum in the 70-270 keV energy interval The best previous limits
Conclusions
In press in Nucl. Phys. A.
DOI: 10.1016/j.nuclphysa.2009.05.139
Low background experiments to search for 2 out over more than 10000 h scintillators b processes in 64 Zn , 70 Zn , 180 W , 186 W @ LNGS by using low background 0.1–0.7 kg ZnWO 4 were carried crystal 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 e , eb +, 2 b level of 10 + active isotopes were investigated at the 21 yr Most of the obtained T established in previous experiments 1/2 limits are near one order of magnitude higher than those Very radiopure ZnWO been developed 4 crystal scintillators have
excluded
Possible future potentiality for ZnWO an experiment involving 10 tons of non enriched crystals (9 T 2 1/2 n 2 e sensitivity 3 10 28 4 in the future: 10 27 nuclei of 64 Zn) could reach the yr (supposing zero background during 10 years); should be surely observed in accordance with theoretical expectations (10 25 -10 26 yr)