G. Gerbier

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.

A new high-background-rejection dark matter Ge cryogenic detector

A new design of a cryogenic germanium detector for dark matter search is presented, taking advantage of the coplanar grid technique of event localisation for improved background discrimination. Experiments performed with prototype devices in the EDELWEISS II setup at the Modane underground facility demonstrate the remarkably high efficiency of these devices for the rejection of low-energy

WIMPs search with low activity NaI crystals: Preliminary results

\beta

Low energy neutron propagation in MCNPX and GEANT4

, approaching 10

First results of the EDELWEISS-II WIMP search using Ge cryogenic detectors with interleaved electrodes

^5

Dark matter search with calcium fluoride crystals

. This opens the road to investigate the range beyond 10

Search for neutralino dark matter with NaI detectors

^{-8}

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

pb in the WIMP-nucleon collision cross-sections, as proposed in the EURECA project of a one-ton cryogenic detector mass.

Identification of backgrounds in the EDELWEISS-I dark matter search experiment

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.