J. Mallet

Pulse shape discrimination and dark matter search with NaI(Tl) scintillator

Abstract An extensive study of NaI(Tl) as a Dark Matter particle detector is presented. Emphasis is put on the response of the detector, both in energy and pulse shape, to all particles interacting in the detector, namely high energy (MeV) photons, low energy photons (X-rays), betas from external radioactivity, and neutrons, which induce nuclear recoils. The initial hope that the shorter decay times of nuclear recoils induced by WIMPs could be statistically separated from Compton interactions is weakened by the fact that low energy X-rays and betas exhibit pulse shapes similar to recoils. As a consequence, any indication of shorter decay time pulses leads to an ambiguous interpretation. Underground data for the WIMP search were obtained in a low activity environment at the Laboratoire Souterrain de Modane (LSM), with a 10 kg crystal having high photoelectron yield and 2 keV energy threshold. The data contain pulse shapes with decay times shorter than for Compton interactions and are not compatible with calibration reference shapes or a mixture of these. The effect is shown to come from a population with decay times even shorter than nuclear recoils but its origin was not identified. The Compton rejection efficiency is limited to factors ranging from 3 at 5 keV to 8 at 20 keV (electron equivalent energy). It is shown that the sensitivity of NaI(Tl) detectors to the cross section of Spin Independent coupling WIMPs is only slightly improved by the pulse shape analysis, while it is mostly determined by the differential energy rate at threshold. The sensitivity to the cross section of Spin Dependent coupling WIMPs is improved by about an order of magnitude by the pulse shape analysis.

First results of the EDELWEISS WIMP search using a 320 g heat-and-ionization Ge detector

The EDELWEISS collaboration has performed a direct search for WIMP dark matter using a 320 g heat-and-ionization cryogenic Ge detector operated in a low-background environment in the Laboratoire Souterrain de Modane. No nuclear recoils are observed in the fiducial volume in the 30-200 keV energy range during an effective exposure of 4.53 kg.days. Limits for the cross-section for the spin-independent interaction of WIMPs and nucleons are set in the framework of the Minimal Supersymmetric Standard Model (MSSM). The central value of the signal reported by the experiment DAMA is excluded at 90% CL.

WIMPs search with low activity NaI crystals: Preliminary results

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.

Charge-transfer luminescence and spectroscopic properties of Yb3+ in aluminium and gallium garnets

Luminescence of Yb 3 - from the charge-transfer state with broad emission bands and short radiative lifetimes (few to tens of nanoseconds depending on the host lattice and the temperature) is attractive for the development of fast scintillators capable of discriminating very short events. The most important currently considered application is that in solar neutrino (v e ) real-time spectroscopy, since the v e capture by 1 7 6 Yb is followed by a specific emission signature which can accordingly excite the Yb 3 + fluorescence. Studies on scintillation and luminescence in aluminium garnets containing Yb 3 + have shown that these materials meet some of the required properties for such scintillators. In defining our priorities, the best compromise between host crystal, Yb 3 + concentration, production method, post-growth treatment and performance is to be considered based on the studies of charge-transfer luminescence and quenching mechanisms. The experiments have been extended to a large number of compounds: YAG:Yb-YbAG, YGG:Yb-YbGG, YAP:Yb-YbAP, LaYbO 3 in the form of single crystals and/or powders. In garnets, the temperature-dependent fluorescence intensity and decay time under X-ray and VUV excitations decrease at low temperatures (T<100 K) and demonstrate the important role played by the traps. The thermoluminescence peaks show a strong dependence on the crystal history, composition and impurities introduced intentionally. The fluorescence intensity and decay time are also dependent on Yb 3 + concentration and the presence of Yb 2 + . The results trace the major directions to optimised scintillators in terms of their efficiency and lifetime.

Dark matter search with calcium fluoride crystals

Abstract A first result on dark matter direct search with calcium fluoride scintillators is presented. The low and high energy spectra are discussed together with the measurements of the fluorine and calcium recoil scintillation efficiencies. Exclusion plots for axial vector coupled WIMPs are derived and compared with previous measurements with NaI and Ge detectors.

Search for neutralino dark matter with NaI detectors

Theoretical predictions for neutralino dark matter in the framework of the minimal SUSY standard model are compared with recent results of a direct search for dark matter with NaI detectors. Perspectives for a future investigation by this kind of detectors are also discussed.

Calibration of a CsI(Tl) crystal with nuclear recoils and pulse shape measurements for dark matter detection

Abstract The quenching factor of cesium and iodine nuclei recoiling in a CsI(Tl) scintillator is measured by scattering of 3 to 6 MeV neutrons. This factor increases when recoil energy decreases, from 7% at 150 keV to 15% at 25 keV. This relatively high efficiency could be useful in experiments dealing with very low recoil energies like the WIMP direct detection. These values are well explained by the Birks model. Pulse shape discrimination between electron and nuclei recoils is also investigated. Results are sufficiently good to allow a significant statistical rejection of radioactive background. This rejection capability is shown to be better than for NaI(Tl), at the same electron equivalent energy.

Calibration of the EDELWEISS cryogenic heat-and-ionization germanium detectors for dark matter search

Several aspects of the analysis of the data obtained with the cryogenic heat-and-ionization Ge detectors used by the EDELWEISS dark matter search experiment are presented. Their calibration, the determination of their energy threshold, fiducial volume and nuclear recoil acceptance are detailed.

Ytterbium-based scintillators, a new class of inorganic scintillators for solar neutrino spectroscopy

Abstract The observed deficit of the solar neutrino flux is now well established. This puzzling problem of todays particle physics could be resolved soon. The most likely explanation would be the vacuum neutrino oscillation phenomenon, indirectly proving the non-zero mass of these fleeting particles. Following the proposition of Raghavan of using 176 Yb as a target for low-energy solar neutrino spectroscopy, an intense R&D work has started a few years ago to define a suitable scintillator incorporating a large amount of ytterbium. Recently, the observation of UV scintillation in mixed yttrium/ytterbium aluminium garnets opened the field of investigation to a new class of scintillating crystals with interesting luminescence properties, very attractive not only for neutrino physics but also for radiation detection, in general. Their luminescence properties present some peculiarities that make them interesting by themselves.

Calibration of a Ge crystal with nuclear recoils for the development of a dark matter detector

Abstract The ionization deposited in a Ge crystal by the scattering of ≈ 1 MeV neutrons on Ge nuclei is measured and its lowenergy behavior is investigated down to recoil energies of 3 keV. This calibration study is fundamental for the discrimination of Weakly Interacting Massive Particles (WIMPs) from the radioactive background. Experimental results are compared with theoretical predictions.