Search

The ZnWO$_4$ is an anisotropic crystal scintillator; for its peculiar characteristics, it is a very promising detector to exploit the so-called directionality approach in the investigation of those Dark Matter (DM) candidates inducing nuclear recoils. Recently, in the framework of the ADAMO project, an R\&D to develop high quality and ultra-radiopure ZnWO$_4$ crystal scintillators has been carried out. In the present paper the measurements to study the anisotropic response of a ZnWO$_4$ to $��$ particles and to nuclear recoils induced by neutron scattering are reported. Monochromatic neutrons have been produced by a neutron generator at ENEA-CASACCIA. The quenching factor values for nuclear recoils along different crystallographic axes have been determined for three different nuclear recoils energies. These results open the possibility to realize a pioneer experiment to investigate the above mentioned DM candidates by means of the directionality., Proceedings of the IPRD 2019 Conference, 10 pages, 7 figures, accepted for publication in JINST

International audience; Final results of studies of 116Cd 2β decay with CdWO4 crystal scintillators, enriched in 116Cd to 82%, are presented. After few years (2011 – 2017) of measurements with radiopure 116CdWO4 scintillators (mass of 1.162 kg) at the Gran Sasso underground laboratory (Italy), the half-life of 116Cd relatively to 2ν2β decay to the ground state of 116Sn was measured with the highest up-to-date accuracy as T1/2=(2.63−0.12+0.11)×1019 yr. A new limit on 0ν2β decay of 116Cd was set as T1/2≥2.2×1023 yr at 90% C.L., which is the most stringent to-date restriction for this isotope. Limits for 0ν2β decay with majoron(s) emission, Lorentz-violating 2ν2β decay and 2β transitions to excited states of 116Sn were set at the level of T1/2≥1020−1022 yr.

An experiment to search for double beta processes in 106Cd is in progress with the help of 106CdWO4 crystal scintillator (enriched in 106Cd to 66%) in coincidence with two large volume CdWO4 crystal scintillators in close geometry at the Gran Sasso Underground Laboratory. The time resolution of the set-up was analyzed, and the methods of pulse-shape discrimination of α and γ(β) events was applied to estimate the radioactive contamination of the crystal and to reduce the background counting rate. The possible double beta decay processes and the background components have been simulated by a Monte Carlo method with GEANT4. The experimental sensitivity is approaching the theoretical predictions for the two-neutrino electron capture with positron emission in 106Cd: lim T1/2≥4×1021 yr at 90% C.L. (the best previous limit was established in the previous stage of the experiment as T1/2≥1.1× 1021 yr, while the theoretical predictions are in the region of T1/2∼1021−1022 yr).An experiment to search for double beta processes in 106Cd is in progress with the help of 106CdWO4 crystal scintillator (enriched in 106Cd to 66%) in coincidence with two large volume CdWO4 crystal scintillators in close geometry at the Gran Sasso Underground Laboratory. The time resolution of the set-up was analyzed, and the methods of pulse-shape discrimination of α and γ(β) events was applied to estimate the radioactive contamination of the crystal and to reduce the background counting rate. The possible double beta decay processes and the background components have been simulated by a Monte Carlo method with GEANT4. The experimental sensitivity is approaching the theoretical predictions for the two-neutrino electron capture with positron emission in 106Cd: lim T1/2≥4×1021 yr at 90% C.L. (the best previous limit was established in the previous stage of the experiment as T1/2≥1.1× 1021 yr, while the theoretical predictions are in the region of T1/2∼1021−1022 yr).

International audience; Recent developments, results, and perspectives arising from double beta decay experiments at the Gran Sasso National Laboratory (LNGS) of the INFN by using HPGe detectors and crystal scintillators and by exploiting various approaches and different isotopes are summarized. The measurements here presented have been performed in the experimental set-ups of the DAMA collaboration. These setups are optimized for low-background studies and operate deep underground at LNGS. The presented results are of significant value to the field, and the sensitivity achieved for some of the considered isotopes is one of the best available to date.

The DAMA/LIBRA experiment ($\sim$ 250 kg of highly radio-pure NaI(Tl)) is running deep underground at the Gran Sasso National Laboratory (LNGS) of the I.N.F.N. Here we briefly recall the results obtained in its first phase of measurements (DAMA/LIBRA--phase1, total exposure: 1.04 ton $\times$ yr). DAMA/LIBRA--phase1 and the former DAMA/NaI (cumulative exposure: $1.33$ ton $\times$ yr) give evidence at 9.3 $\sigma$ C.L. for the presence of DM particles in the galactic halo by exploiting the model-independent DM annual modulation signature. No systematic or side reaction able to mimic the exploited DM signature has been found or suggested by anyone over more than a decade. At present DAMA/LIBRA--phase2 is running with increased sensitivity., Comment: Proceedings to the 19th Workshop "What Comes Beyond the Standard Models", Bled, July 11. -- 19., 2016

The application of very low background techniques is fundamental in deep underground experiments dedicated to the investigation of rare processes. In this paper, the case of NaI(Tl) detectors is discussed mainly considering their crucial role for direct Dark Matter (DM) investigation. In particular, the DAMA/LIBRA apparatus, as a result of a long-standing activity focused on the optimization of NaI(Tl) detectors, is underlined.

A search for the double beta decay of zinc and tungsten isotopes has been performed with the help of radiopure ZnWO4 crystal scintillators (0.1-0.7 kg) at the Gran Sasso National Laboratories of the INFN. The total exposure of the low background measurements is 0.529 kg yr. New improved half-life limits on the double beta decay modes of 64Zn, 70Zn, 180W, and 186W have been established at the level of 10^{18}-10^{21} yr. In particular, limits on double electron capture and electron capture with positron emission in 64Zn have been set: T_{1/2}(2\nu 2K) > 1.1 10^{19} yr, T_{1/2} (0\nu 2\epsilon) > 3.2 10^{20} yr, T_{1/2} (2\nu \epsilon \beta^+) > 9.4 10^{20} yr, and T_{1/2} (0\nu \epsilon \beta^+) > 8.5 10^{20} yr, all at 90% C.L. Resonant neutrinoless double electron capture in 180W has been restricted on the level of T_{1/2} (0\nu 2\epsilon) > 1.3 10^{18} yr. A new half-life limit on alpha transition of 183W to the metastable excited level 1/2^- 375 keV of 179Hf has been established: T_{1/2} > 6.7 10^{20} yr., Comment: This is an author-created, un-copyedited version of an article published in J. Phys. G: Nucl. Part. Phys. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The definitive publisher authenticated version is available online at doi: 10.1088/0954-3899/38/11/115107

The radioactive contamination of two 26 g samples of low background lithium iodide crystal scintillators doped by europium and enriched in 7 Li to 99.9% ( 7 LiI(Eu)) has been investigated by scintillation method at the sea level, and by ultra-low background HPGe γ spectrometry deep underground. No radioactive contamination was detected. In particular, the contamination of the crystal scintillators by 226 Ra and 228 Th does not exceed 1 mBq/kg, and the activity of 40 K is less than 0.5 Bq/kg.

Cadmium tungstate crystal scintillators enriched in 106Cd up to 66% (106CdWO4) and in 116Cd up to 82% (116CdWO4) have been developed. The low radioactive contamination of the crystals measured on the level of ≤ 1.5 mBq/kg (40K), ≤ 0.005 - 0.012 mBq/kg (226Ra), 0.04 - 0.07 mBq/kg (228Th) allows to carry out high sensitivity experiments to search for double beta processes in 106Cd and 116Cd.

In the anisotropic scintillators the light output and the pulse shape for heavy particles (p, α, nuclear recoils) depend on the direction with respect to the crystal axes; the response to γ/β radiation is isotropic instead. This feature offers the possibility to study the directionality approach, which is applicable in the particular case of those Dark Matter candidate particles inducing just nuclear recoils. Among the anisotropic scintillators, the ZnWO4 has unique features, which make it an excellent candidate for this type of research, and there is still plenty of room for the improvement of its performances. In this paper the possibility of a low background pioneer experiment (named ADAMO—Anisotropic detectors for DArk Matter Observation) to exploit deep underground the directionality approach by using anisotropic ZnWO4 scintillators is discussed.

A strontium iodide crystal doped by europium (SrI2(Eu)) was produced by using the Stockbarger growth technique. The crystal was subjected to a characterization that includes relative photoelectron output and energy resolution for gamma quanta. The intrinsic radioactivity of the SrI2(Eu) crystal scintillator was tested both by using it as scintillator at sea level and by ultra-low background HPGe gamma spectrometry deep underground. The response of the SrI2(Eu) detector to alpha particles (alpha/beta ratio and pulse shape) was estimated by analysing the 226Ra internal trace contamination of the crystal. We have measured: alpha/beta=0.55 at E_alpha=7.7 MeV, and no difference in the time decay of the scintillation pulses induced by alpha particles and gamma quanta. The application of the obtained results in the search for the double electron capture and electron capture with positron emission in 84Sr has been investigated at a level of sensitivity: T_1/2 \sim 10^{15}-10^{16} yr. The results of these studies demonstrate the potentiality of this material for a variety of scintillation applications, including low-level counting experiments., 24 pages, 12 figures, 4 tables, accepted for publication on Nucl. Instrum. Meth. A

The DAMA/LIBRA experiment, running at the Gran Sasso National Laboratory of the I.N.F.N. in Italy, has a sensitive mass of about 250 kg highly radiopure NaI(Tl). It is mainly devoted to the investigation of Dark Matter (DM) particles in the Galactic halo by exploiting the model independent DM annual modulation signature. The present DAMA/LIBRA experiment and the former DAMA/NaI one (the first generation experiment having an exposed mass of about 100 kg) have released so far results corresponding to a total exposure of 1.17 ton×yr over 13 annual cycles. They provide a model independent evidence of the presence of DM particles in the galactic halo at 8.9 σ C.L. on the basis of the investigated DM signature. Few aspects of the obtained results are shortly summarized and perspectives for the experiment are briefly addressed.

The similar or equal to 100 kg highly radiopure NaI(TI) set-up of the DAMA project (DAMA/NaI) has investigated the model- independent WIMP annual modulation signature over seven annual cycles for a total exposure of 107731 kg x day, obtaining a model- independent evidence for the presence of a dark matter particle component in the galactic halo at 6.3 sigma C.L.. Some of the many possible corollary model-dependent quests for the candidate particle have be m investigated with the total exposure as well.

As a result of about five years of new developments, the second-generation LIBRA (Large sodium Iodide Bulk for RAre processes) set-tip of the DAMA experiment has been realized and installed in the Gran Sasso National Laboratory of the I.N.F.M. It is made of similar or equal to250 kg low-background NaI(TI) detectors and has been operative since March 2003. It works as an observatory for rare processes and is mainly devoted to further investigation on the dark matter particle component in the galactic halo. New R&D efforts toward a possible NaI(TI) ton set-up, which we proposed in 1996, have been funded and started in 2003.

The DAMA liquid Xenon (LXe) experimental set up is running deep underground at the Gran Sasso National Laboratory (LNGS) of the I.N.F.N. (see Fig. 1) since several years and various upgradings to improve its performance and sensitivity have been carried out. During the 80’s preliminary efforts have been carried out by some of us in the framework of the ”Xelidon’’ experiment, developing various kinds of LXe detectors. Several prototype detectors were built and some related results published (see e.g. ref.l). In particular, these efforts allowed us to identify the best LXe detector for WIMP search; in fact,considering the required: i) low background features; ii) stability and reproducibility of the operating conditions; iii) relatively high duty cycle; iv) high global efficiency, a pure liquid Xenon scintillator employing the smallest number of (easily and highly radio-purifiable) materials was chosen. In addition, the use of ”Kr-free” gas is mandatory to avoid the relevant Kr content normally present in commercial Xenon as well as the suitable choice of the purifier materials. Moreover, in order to obtain a good light response, these preliminary experimental activities have also pointed out the relevance of: i) an initial vacuum (before filling) of order of mbar; ii) the highest initial purification level of the Xenon gas; iii) absence of significantly

The BPRS experiment is dedicated to particle Dark Matter search with low activity scintillators. Preliminary results on Weakly Interacting Massive Particles (WIMPs) have been already obtained with NaI(Tl) and CaF 2 (Eu) target-detectors. New detector developments are in progress. A new search for Strongly Interacting Massive Particles (SIMPs) with NaI(Tl) scintillators have been also performed.

DAMA is searching for rare processes by developing and using several kinds of radiopure scintillators: NaI(Tl), liquid Xenon and CaF2(Eu). Here, in particular, results achieved during four annual cycles with a model independent analysis and with model dependent ones in the investigation of the so-called WIMP annual modulation signature are summarized.

A development is in progress in order to detect dark matter particles with NaI crystals . This study is performed in parallel in three underground laboratories (Gran Sasso, Frejus and Mentogou) where the experimental set-ups are efficiently protected from cosmic radiation. Some data are presented on measurements of radioactive contaminants as well as preliminary results from the analysis of low energy event rates. The presently achieved sensitivity to axial coupling WIMPs is comparable to that obtained by germanium experiments. Systematic investigations are under way to further decrease the radioactive background in all elements of the installation.

Liquid xenon presents interesting features as scintillation medium. Some results are discussed paying attention to physical applications.