Search for rare processes with ZnWO 4 crystal scintillators

Radiopure ZnWO4 crystal scintillators with mass (0.1–0.7 kg) have been developed and put in measurement in the Gran Sasso National Laboratories of the INFN to search for rare processes. The radioactive contamination of the crystals have been estimated and the double beta decay of zinc and tungsten isotopes was searched for, reaching a sensitivity at the level of 10 18 − 1021 yr; in addition a new half-life limit on alpha transition of183W to the metastable excited level of 179Hf has also been obtained. The achieved radiopurity of the ZnWO4 crystals make them very promising detectors for ββ decay investigations while their anisotropic features make them very interesting detectors to investigate dark matter particles directionality. 1 ZnWO4 crystal scintillators In recent years ZnWO 4 crystal scintillators were considered in the search for rare processes [1–5]. In particular four clear, slightly colored ZnWO 4 crystal scintillators have been produced: i) crystals ZWO-1 (117 g) and ZWO-2 (699 g) were produced in the Institute for Scintillation Materials (ISMA, Kharkiv, Ukraine) from crystal ingots grown in platinum crucibles by the Czochralski method; ii) the crystal ZWO-3 (141 g) was obtained by recrystallization from the sample ZWO-2 at the ISMA; iii) the ZWO-4 (239 g) was produced in the Nikolaev Institute of Inorganic Chemistry (Novosibirsk, Russia) by the low-thermal gradient Czochralski technique also in platinum crucible. The radioactive contamination of these ZnWO4 crystal scintillators has been investigated deep underground in the Gran Sasso laboratory, by using the low background facility DAMA/R&D [6]. In the measurement the considered ZnWO 4 crystals were fixed inside a cavity of 49 × 59 mm in the central part of a polystyrene light-guide 66 mm in diameter and 312 mm in length. The cavity was filled up with high purity silicone oil. The light-guide was optically connected, on the opposite sides, to two low radioactive EMI9265B53 /FL 3 inches photomultipliers (PMT). The light-guide was wrapped by PTFE reflection tape. Some upgradings of the detector system have been performed in the di fferent periods of measurements; in particular, in the final stages of the experiment two polished quartz light-guides ( 66× 100 mm) were installed between the polystyrene light-guide and the PMTs to suppressγ ray background from the PMTs. The detector was surrounded by Cu bricks and sealed in a low radioactive air-tight Cu box continuously flushed with high purity nitrogen gas. The Cu box was surrounded by a passive shield made of 10 cm of high purity Cu, 15 cm of low radioactive lead, 1.5 mm of cadmium and 4 /10 cm polyethylene/paraffin to reduce the external background. The whole shield has been closed inside a Plexiglas box, also continuously flushed by high purity nitrogen gas. An event-by-event data acquisition system accumulates the amplitude, the arrival time, and the pulse shape of the events. The energy scale and the energy resolution of the ZnWO4 detectors have been measured with γ sources 22Na, 60Co, 133Ba, 137Cs, 228Th, and241Am. The energy resolution of the detectors (full width at half of maximum) was in the range of (8 . − 14.6)% for 662 keVγ line of 137Cs. As an example of the energy distribution accumulated in the measurements, in Fig. 1 the spectra collected in some runs are reported [4]. Some peaks in the spectra can be ascribed to γ lines of naturally occurring radionuclides such as40K, 214Bi (238U chain) and208Tl (232Th) from the materials of the set-up. A detailed investigation of the measured spectra has been performed by using various data analysis strategies and Monte Carlo simulations. In particular, the technique of the time-amplitude analysis (described in details in [7]) has been used in order to estimate the228Th and227Ac (235U family) activities. The pulse shape discrimination (PDS) capability of the ZnWO4 scintillators has been considered in order to study the measured α spectra and the so called BiPo events. The radioactive contamination of the ZnWO 4 crysDOI: 10.1051/ C © Owned by the authors, published by EDP Sciences, 2014 , / 01002 (2014) 201 65 epjconf EPJ Web of Conferences


ZnWO 4 crystal scintillators
In recent years ZnWO 4 crystal scintillators were considered in the search for rare processes [1][2][3][4][5].In particular four clear, slightly colored ZnWO 4 crystal scintillators have been produced: i) crystals ZWO-1 (117 g) and ZWO-2 (699 g) were produced in the Institute for Scintillation Materials (ISMA, Kharkiv, Ukraine) from crystal ingots grown in platinum crucibles by the Czochralski method; ii) the crystal ZWO-3 (141 g) was obtained by recrystallization from the sample ZWO-2 at the ISMA; iii) the ZWO-4 (239 g) was produced in the Nikolaev Institute of Inorganic Chemistry (Novosibirsk, Russia) by the low-thermal gradient Czochralski technique also in platinum crucible.The radioactive contamination of these ZnWO 4 crystal scintillators has been investigated deep underground in the Gran Sasso laboratory, by using the low background facility DAMA/R&D [6].In the measurement the considered ZnWO 4 crystals were fixed inside a cavity of 49 × 59 mm in the central part of a polystyrene light-guide 66 mm in diameter and 312 mm in length.The cavity was filled up with high purity silicone oil.The light-guide was optically connected, on the opposite sides, to two low radioactive EMI9265B53/FL 3 inches photomultipliers (PMT).The light-guide was wrapped by PTFE reflection tape.Some upgradings of the detector system have been performed in the different periods of measurements; in particular, in the final stages of the experiment two polished quartz light-guides ( 66 × 100 mm) were installed between the polystyrene light-guide and the PMTs to suppress γ ray background from the PMTs.The detector was surrounded by Cu bricks and sealed in a low ra-dioactive air-tight Cu box continuously flushed with high purity nitrogen gas.The Cu box was surrounded by a passive shield made of 10 cm of high purity Cu, 15 cm of low radioactive lead, 1.5 mm of cadmium and 4/10 cm polyethylene/paraffin to reduce the external background.The whole shield has been closed inside a Plexiglas box, also continuously flushed by high purity nitrogen gas.An event-by-event data acquisition system accumulates the amplitude, the arrival time, and the pulse shape of the events.
The energy scale and the energy resolution of the ZnWO 4 detectors have been measured with γ sources 22 Na, 60 Co, 133 Ba, 137 Cs, 228 Th, and 241 Am.The energy resolution of the detectors (full width at half of maximum) was in the range of (8.8 − 14.6)% for 662 keV γ line of 137 Cs.
As an example of the energy distribution accumulated in the measurements, in Fig. 1 the spectra collected in some runs are reported [4].Some peaks in the spectra can be ascribed to γ lines of naturally occurring radionuclides such as 40 K, 214 Bi ( 238 U chain) and 208 Tl ( 232 Th) from the materials of the set-up.A detailed investigation of the measured spectra has been performed by using various data analysis strategies and Monte Carlo simulations.In particular, the technique of the time-amplitude analysis (described in details in [7]) has been used in order to estimate the 228 Th and 227 Ac ( 235 U family) activities.The pulse shape discrimination (PDS) capability of the ZnWO 4 scintillators has been considered in order to study the measured α spectra and the so called BiPo events.The radioactive contamination of the tals is on the level of 0.002 − 0.025 mBq/kg; the total α activity is in the range 0.2 − 2 mBq/kg.Moreover, particular contaminations associated with the composition of ZnWO 4 detector were observed [4]: the EC active cosmogenic nuclide 65 Zn -that can be also produced by neutrons -(T 1/2 = 244.26d [8]) with activity 0.5 − 0.8 mBq/kg (depending on the ZnWO 4 sample) and the α active tungsten isotope 180 W (with half-life: T 1/2 ≈ 10 18 yr [4,[9][10][11], and energy of the decay: Q α = 2508(4) keV [12]) with activity 0.04 mBq/kg.In Fig. 2 it is shown the energy distribution of the β(γ) events (identified by the PSD) accumulated in the low background set-up with the ZWO-4 crystal scintillator over 4305 h (Run 5) together with the model of the background.The main components of the background are shown: spectra of internal 65 Zn, 90 Sr-90 Y, daughters of 238 U, and the contribution from the external γ quanta from PMTs and Cu box in these experimental conditions.A summary of the radioactive contamination of the ZnWO 4 crystal scintillators can be found in ref. [4].

Results of the search for ββ decay in Zn and W isotopes and on rare α decay of W isotopes
The collected data have also been considered to search for double beta decay processes [5].  of two electrons from the K shell (E K = 65.4 keV), the decay energy is rather small (13 ±4) keV; such a coincidence could give a resonant enhancement of the 0ν double electron capture to the corresponding level of the daughter nucleus.
The response functions of the ZnWO 4 detectors for the 2β processes in Zn and W isotopes were simulated with the help of the GEANT4 package [13] with the Low Energy Electromagnetic extensions.The initial kinematics of the particles emitted in the decays was generated with the DECAY0 event generator [14].The background models included the internal contamination of the ZnWO 4 scintillators ( 40 K, 60 Co, 65 Zn, 87 Rb, 90 Sr-90 Y, 137 Cs), and the external γ rays from radioactive contamination of the PMTs and the copper box ( 40 K, 232 Th, 238 U).Comparing the simulated response functions with the measured energy spectra of the ZnWO 4 detectors, no clear peculiarities, which can be evidently attributed to double beta decay of zinc or tungsten isotopes, have been found.Therefore only lower half-life limits have been set.
As an example, in Fig. 3 the energy spectrum of the ZnWO 4 crystal scintillator 41 × 27 mm measured over 2798 h, corrected for the energy dependence of detection efficiency is reported, together with the 2ν2K peak of 64 Zn with T 2ν2K 1/2 = 1.1 × 10 19 yr excluded at 90% C.L. Several ββ decay modes have been studied and competitive limits have been set.The main results are shortly summarized.
The previous limits on the T 1/2 of the ββ decay modes of 64 Zn, 70 Zn, 180 W and 186 W have been improved up to 2 orders of magnitude.In particular, new improved halflife limits on double electron capture and electron capture with positron emission in 64   −0.5 × 10 18 yr.Also a new half-life limit on α transition of 183 W to the 1/2 − 375 keV metastable level of 179 Hf has been set as T 1/2 ≥ 6.7 × 10 20 yr [5].

ZnWO 4 scintillator to investigate Dark Matter particle directionality
The above mentioned measurements and the correlated R&D works have shown that the ZnWO 4 scintillators can offer suitable features to investigate some Dark Matter (DM) particle candidates by exploiting the directionality technique.The use of anisotropic scintillators to study the directionality signature was proposed for the first time in ref. [16] and revisited in [17].The directionality technique is effective only for those DM candidate particles able to induce just nuclear recoils.This approach studies the correlation between the arrival direction of the DM particles and the Earth motion in the galactic rest frame.In fact, the dynamics of the rotation of the Milky Way galactic disc through the halo of DM causes the Earth to experience a wind of DM particles apparently flowing along a direction opposite to that of the solar motion relative to the DM halo.
However, because of the Earth's rotation around its axis, their average direction with respect to an observer fixed on the Earth changes during the sidereal day.The possible nuclear recoils induced by the DM particles are expected to be strongly correlated with their impinging direction, while the background events are not; therefore, the study of the nuclear recoils direction can offer a way for pointing out the presence of the considered DM candidate particles.
The main advantages of ZnWO 4 scintillators to study the directionality are: i) high level of radio-purity reachable in future development, considering the very good results already achieved; ii) an energy threshold at level of few keV reachable (room for further improvement is possible); iii) light output of heavy particles (p, α, nuclear recoils) depending on the impinging direction of the particles with respect to the crystal axes, while the response to γ radiation being isotropic; iv) the scintillation decay time showing the same property.
The anisotropic effect has been ascribed to preferred directions of the excitons' propagation in the crystal lattice affecting the dynamics of the scintillation mechanism [18].The anisotropic features of the ZnWO 4 detectors can provide two independent ways to exploit the directionality approach.In particular, the presence of heavy ionizing particles with a preferred direction (like recoil nuclei induced by the DM candidates considered here) could be discriminated from the electromagnetic background by comparing the low energy distributions measured by using different orientations of the crystal axes along the day.Moreover, the directionality technique can also be explored at some extent studying the time behaviour of the induced nuclear recoil pulses.
Thus, in the case of the ZnWO 4 detector, the anisotropy of the light output for recoiling nuclei induced by DM candidates could be discriminated from the electromagnetic events because of the expected variation of their detected energy distribution during the day [17].In fact because of the Earth's daily rotation around its axis the preferential impinging direction of DM particle change during the sideral time and the expected counting rate in a defined energy interval is expected to have a diurnal variation.This peculiarity of the rate can be considered to investigate the presence DM particle candidate inducing just nuclear recoils in the Galactic halo.Detailed discussion can be found in ref. [19].

Conclusion
In the last years radiopure ZnWO 4 crystal scintillator have been realized and have been measured in the DAMA/R&D facility at the Gran Sasso National Laboratory over more than 19 thousands hours.The measurements allowed us to study in details the radioactive residual contaminations of these crystals.The reached very good level of radiopurity and the study of the procedures followed to grow these crystals give us confidence for future developments of new ZnWO 4 crystal scintillators with higher level of radiopurity.
A low background experiment to search for 2β processes in 64 Zn, 70 Zn, 180 W, and 186 W was also carried out International Workshop on Radiopure Scintillators RPSCINT 2013 01002-p.3 with a total exposure of 0.5295 kg × yr.New improved half-life limits on double beta decay modes in Zn and W isotopes in the range 10 19 yr to 10 21 yr have been set.The indication on the (2ν + 0ν)εβ + decay of 64 Zn with T 1/2 = (1.1 ± 0.9) ×10 19 yr suggested in [15] is not confirmed.Note that to date only four nuclides ( 40 Ca, 78 Kr, 112 Sn, and 120 Te) among 34 candidates to 2ε, εβ + , and 2β + processes were studied at 10 21 level of sensitivity in direct experiments.However, it is worth noting that the limits are still far from theoretical predictions.
A search for rare α decay in W isotopes have also been performed.The rare α decay of 180 W with a halflife T 1/2 = 1.3 +0.6  −0.5 × 10 18 yr has been observed and new half-life limit T 1/2 ≥ 6.7 × 10 20 yr on α transition of 183 W to the 1/2 − 375 keV metastable level of 179 Hf has been set.
The anisotropic features of ZnWO 4 crystals make them very interesting target to investigate dark matter particles directionality.
Thus the ZnWO 4 crystal scintillators can be very good detectors for future developments and experiments either to study ββ decay modes and to investigate the directionality for some dark matter candidate particles.

Figure 2 .
Figure 2. Energy distribution of the β(γ) events (identified by the PSD) accumulated in the low background set-up with the ZWO-4 crystal scintillator over 4305 h (Run 5) together with the model of the background.The main components of the background are shown: spectra of internal 65 Zn, 90 Sr-90 Y, daughters of 238 U, and the contribution from the external γ quanta from PMTs and Cu box in these experimental conditions.
Zn have been set in the range: EPJ Web of Conferences 01002-p.2

Figure 3 .
Figure 3.The energy spectrum of the ZnWO 4 crystal scintillator 41 × 27 mm measured over 2798 h, corrected for the energy dependence of detection efficiency, together with the 2ν2K peak of 64 Zn with T 2ν2K 1/2 = 1.1 × 10 19 yr excluded at 90% C.L.
Energy distributions of the ZnWO 4 scintillators measured in the low background set-up during Runs 1 (with ZWO-1 in 2906 h) (solid red line), 2 (with ZWO-2 in 2130 h) (dashed black line), and 4 (with ZWO-4 in 834 h) (dotted blue line).Energies of γ lines from residual contaminations are in keV.