ZnWO4 scintillators for cryogenic dark matter experiments

The scintillation properties of a zinc tungstate crystal, shaped as a hexagonal prism (height 40 mm, diagonal 40 mm) were determined. An energy resolution of 10.7% for the 662 keV γ-line of 137Cs was measured with the scintillator placed in a light collection setup similar to that used by the CRESST dark matter search. The light output and decay kinetics of ZnWO4 were examined over the temperature range 7-300 K and confirmed to be competitive with those of CaWO4. The radioactive contaminations of the ZnWO4 scintillator measured in the Solotvina Underground Laboratory do not exceed 0.1-10 mBq/kg (depending on radionuclide). Our study highlights the excellent feasibility of this ZnWO4 scintillator for a cryogenic dark matter experiment. © 2008 Elsevier B.V. All rights reserved

Radioactive contamination of BaF2 crystal scintillator

Barium fluoride (BaF2) crystal scintillators are promising detectors to search for double beta decay processes in 130Ba (Q2β = 2619(3) keV) and 132Ba (Q2β = 844(1) keV). The 130Ba isotope is of particular interest because of the indications on 2β decay found in two geochemical experiments. The radioactive contamination of BaF2 scintillation crystal with mass of 1.714 kg was measured over 113.4 hours in a low-background DAMA/R&D set-up deep underground (3600 m w.e.) at the Gran Sasso National Laboratories of INFN (LNGS, Italy). The half-life of 212Po (present in the crystal scintillator due to contamination by radium) was estimated as T1/2 = 298.8 ± 0.8(stat.) ± 1.4(syst.) ns by analysis of the events pulse profiles

First search for double beta decay of platinum by ultra-low background HP Ge spectrometry,

A search for double- processes in 190Pt and 198Pt was realized with the help of ultra-low background HP Ge 468cm^3 spectrometer in the underground Gran Sasso National Laboratories of the INFN (Italy). After 1815 h of data taking with 42.5g platinum sample, T 1/2 limits on 2 processes in 190Pt ( + and 2 have been established on the level of 1014-1016y, 3 to 4 orders of magnitude higher than those known previously. In particular, a possible resonant double-electron capture in 190Pt was restricted on the level of 2.9×1016 y at 90% C.L. In addition, T 1/2 limit on 2− decay of 198Pt (2 +0) to the 2+ 1 excited level of 198Hg has been set at the first time: T 1/2 > 3.5×1018 y. The radiopurity level of the used platinum sample is reported

New limit on two neutrino electron capture with positron emission in 106Cd

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≥4×10^21 yr at 90% C.L. (the best previous limit was established in the previous stage of the experiment as T1/2≥1.1× 10^21 yr, while the theoretical predictions are in the region of T1/2∼10^21−10^22 yr)

MgWO4-A new crystal scintillator

Magnesium tungstate (MgWO4) crystals of ~1 cm3 volume were obtained for the first time using a flux growth technique. The crystal was subjected to comprehensive characterisation that included room-temperature measurements of the transmittance, X-ray luminescence spectra, afterglow under X-ray excitation, relative photoelectron output, energy resolution, non-proportionality of scintillation response to γ-quanta, response to α-particles, and pulse shape for γ-quanta and α-particles. The light output and decay kinetics of MgWO4 were studied over the temperature range 7-305 K. Under X-ray excitation the crystal exhibits an intense luminescence band peaking at a wavelength of 470 nm; the intensity of afterglow after 20 ms is 0.035%. An energy resolution of 9.1% for 662 keV γ-quanta of 137Cs was measured with a small (≈0.9 g) sample of the MgWO4 crystal. The photoelectron output of the MgWO4 crystal scintillator is 35% that of CdWO4 and 27% that of NaI(Tl). The detector showed pulse-shape discrimination ability in measurements with α-particles and γ-quanta, which enabled us to assess the radioactive contamination of the scintillator. The results of these studies demonstrate the prospect of this material for a variety of scintillation applications, including rare event searches. © 2008 Elsevier B.V

Crystal scintillators for the dark matter directionality approach

Low background anisotropic detectors can offer a unique way to study those dark matter (DM) candidate particles able to induce nuclear recoils through the directionality technique. This approach is based on studying the correlation between the nuclear recoils' direction and the Earth motion in the galactic rest frame. Thanks to the anisotropic features of such detectors, a signal, induced by the DM candidates, is expected to change with a particular behaviour as a function of the sidereal time. The ZnWO4 has unique features and it is an excellent candidate for the purposes. Both the light output and the scintillation pulse shape depend on the impinging direction of heavy particles (p, alpha, nuclear recoils, etc.) with respect to the crystal axes and can supply two independent modes to study the directionality and to discriminate the gamma/beta radiation (that does not give rise to any anisotropic effects). In this work, the measurements to study the anisotropic response of a ZnWO4 scintillation detector to a particles and to nuclear recoils, induced by neutron scattering, are briefly summarised

First search for 2ε, εβ+ decay of 162Er and new limit on 2β- decay of 170Er to the first excited level of 170Y

The first search for double electron capture (2ε) and electron capture with positron emission (εβ+) of 162Er to the ground state and to several excited levels of 162Dy was realized with 326 g of highly purified erbium oxide. The sample was measured over 1934 h by the ultra-low background HP Ge γ spectrometer GeCris (465 cm3) at the Gran Sasso underground laboratory. No effect was observed, the half-life limits were estimated at the level of limT_1/2 ~ 10^{15} - 10^{18} yr. A possible resonant 0ν KL1 capture in 162Er to the 2^+_1 782.7 keV excited state of 162Dy is restricted as T_1/2 > 5.0×10^{17} yr at 90% C.L. A new improved half-life limit T_1/2 > 4.1×10^{17} yr was set on the 2β− decay of 170Er to the 2^+ 84.3 keV first excited state of 170Yb

Induction of virus-specific serum antibodies in 2-week-old chickens in response to oculo-nasal infection with rLaSota and rMex mutant viruses.

<p>Chickens were inoculated with each virus (indicated on the x-axis) by the oculo-nasal route, mimicking natural infection. Sera were collected at 3 weeks post-infection. Virus-specific antibodies were measured by a hemagglutination inhibition assay using rLaSota or rMex/Las Fc virus and chicken erythrocytes.</p

ZnWO4 anisotropic scintillator for Dark Matter investigation with the directionality technique

The ZnWO4 crystal scintillator has unique features that make it very promising to realize a pioneering experiment to pursue Dark Matter investigation with the directionality technique. In particular in this detector the light output and the scintillation pulse shape for heavy particles (p, α, nuclear recoils) depend on the direction of the impinging particle with respect to the crystal axes, while the response to γ/β radiation is isotropic. The anisotropy of the light output can be considered to point out the presence in the diurnal counting rate of a Dark Matter signal produced by candidate particle inducing just nuclear recoils. In addition this crystal detector has also other important characteristics for a Dark Matter experiment: high light output, high level of radiopurity. In this paper the present performances of the developed ZnWO4 crystal scintillator will be summarized together with the possible future improvements. Some reachable sensitivities – under given assumptions – in the investigation of DM candidate particles with the directionality technique will also be addressed