M. Chapellier

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.

Dark matter search with a low temperature sapphire bolometer

Abstract A dark matter detection experiment using a low temperature 24 g sapphire bolometer is presented. The low radioactive background cryogenic facility, installed in a deep underground site, is described, as well as the low-noise read-out electronics and the data analysis. From the energy spectrum, measured down to 4 keV, exclusion plots are derived for WIMPs having coherent vector coupling or axial coupling to ordinary matter.

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

Abstract This paper presents our interpretation and understanding of the different backgrounds in the EDELWEISS-I data sets. We analyze in detail the several populations observed, which include gammas, alphas, neutrons, thermal sensor events and surface events, and try to combine all data sets to provide a coherent picture of the nature and localization of the background sources. In light of this interpretation, we draw conclusions regarding the background suppression scheme for the EDELWEISS-II phase.

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.

A novel large-volume spherical detector with proportional amplification read-out

A new type of radiation detector based on a spherical geometry is presented. The detector consists of a large spherical gas volume with a central electrode forming a radial electric field. Charges deposited in the conversion volume drift to the central sensor where they are amplified and collected. We introduce a small spherical sensor located at the center acting as a proportional amplification structure. It allows high gas gains to be reached and operates in a wide range of gas pressures. Signal development and the absolute amplitude of the response are consistent with predictions. Sub-keV energy threshold with good energy resolution is achieved. This new concept has been proven to operate in a simple and robust way and allows reading large volumes with a single read-out channel. The detector performance presently achieved is already close to fulfill the demands of many challenging projects from low energy neutrino physics to dark matter detection with applications in neutron, alpha and gamma spectroscopy.

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.

Sensitivity of the EDELWEISS WIMP search to spin-dependent interactions

The EDELWEISS collaboration is searching for WIMP dark matter using natural Ge cryogenic detectors. The whole data set of the first phase of the experiment contains a fiducial exposure of 4.8 kg.day on Ge-73, the naturally present (7.8%), high-spin Ge isotope. The sensitivity of the experiment to the spin-dependent WIMP-nucleon interactions is evaluated using the model-independent framework proposed by Tovey et al.

Event categories in the EDELWEISS WIMP search experiment

Four categories of events have been identified in the EDELWEISS-I dark matter experiment using germanium cryogenic detectors measuring simultaneously charge and heat signals. These categories of events are interpreted as electron and nuclear interactions occurring in the volume of the detector, and electron and nuclear interactions occurring close to the surface of the detectors (within ≈10–20 μm of the surface). We discuss the hypothesis that low energy surface nuclear recoils, which seem to have been unnoticed by previous WIMP searches, may provide an interpretation of the anomalous events recorded by the UKDMC and Saclay NaI experiments. The present analysis points to the necessity of taking into account surface nuclear and electron recoil interactions for a reliable estimate of background rejection factors.

Dark matter search in the Fréjus Underground Laboratory EDELWEISS experiment

Abstract The status of the EDELWEISS experiment (Experience pour DEtecter Les Wimps En SIte Souterrain) in the Frejus Underground Laboratory is reported. The cryostat is described with the main lines of low radioactivity design and readout system. The first results using bolometer detectors together with the measurement of the internal radioactive background using a 100 cm 3 classical Ge crystal are reported. The future program of the experiment will be outlined.

Systematic study of massive germanium pin diode detectors at 20 mK

Abstract To investigate the properties and improve the performances of bolometers with simultaneous measurement of ionization for dark matter search, we have realized 6 g and 70 g germanium PIN diodes with thin (35–80 nm) B and P implanted faces on 4 and 10 mm thick ultrapure germanium plates. The faces were characterized by LT sheet resistance measurements, ellipsometry, SIMS and RBS, and the bulk by carriers lifetime and capacitance measurements. We found a diode behaviour at 20 mK and 4.2 K. We made a systematic study of the response to photons from 5 to 105 keV at 0.02, 0.08 and 4.2 K, for polarization voltages from 0 to 10 V. Our main purpose was to study the slow time evolutions of the pulse heights which characterize the behaviour of such detectors at very low temperatures. We obtained a threshold of 4 keV, a resolution of 0.76 keV (rms), and a stability better than 1 hour, with a polarization voltage V = 0.2 V on a 4 mm thick diode. Similar resolutions were obtained with a 10 mm thick diode, but the time stability was worse. We found that most of the carriers are drifting across the 4 mm diode for V = 0. A possible contribution of a built-in potential to the pulses time evolution is evoked. We also studied the incomplete carriers collection close to the edges.