EDELWEISS is a direct dark matter search experiment situated in the low radioactivity environment of the Modane Underground Laboratory. The experiment uses Ge detectors at very low temperature in order to identify eventual rare nuclear recoils induced by elastic scattering of WIMPs from our Galactic halo. We present results of the commissioning of the second phase of the experiment, involving more than 7 kg of Ge, that has been completed in 2007. We describe two new types of detectors with active rejection of events due to surface contamination. This active rejection is required in order to achieve the physics goals of 10-8 pb cross-section measurement for the current phase.
Deep Dive into Status of the EDELWEISS-II experiment.
EDELWEISS is a direct dark matter search experiment situated in the low radioactivity environment of the Modane Underground Laboratory. The experiment uses Ge detectors at very low temperature in order to identify eventual rare nuclear recoils induced by elastic scattering of WIMPs from our Galactic halo. We present results of the commissioning of the second phase of the experiment, involving more than 7 kg of Ge, that has been completed in 2007. We describe two new types of detectors with active rejection of events due to surface contamination. This active rejection is required in order to achieve the physics goals of 10-8 pb cross-section measurement for the current phase.
Recent cosmological observations of the CMB show that the main part of the matter in our Universe is dark and non baryonic (Hinshaw et al. 2009). If non baryonic Dark Matter is made of particles, they must be stable, neutral and massive : WIMPs (Weakly Interactive Massive Particles). In the MSSM (Minimal Supersymmetric Standard Model) framework, the WIMP could be the LSP (Lightest Supersymmetric Particle) called neutralino. It has a mass between few tens and few hundreds of GeV/c 2 , and a scattering cross-section with a nucleon below 10 -6 pb. The EDELWEISS (Expérience pour Détecter les WIMPs en Site Souterrain) experiment is dedicated to the direct detection of WIMPs. The direct detection principle (used also by other experiments like CDMS (Akerib et al. 2006), CRESST (Angloher et al. 2005) and XENON (Angle et al. 2008)) consists in the measurement of the energy released by nuclear recoils produced in an ordinary matter target by the elastic collision of a WIMP from the galactic halo. The main challenge is the expected extremely low event rate (≤ 1 evt/kg/year) due to the very small interaction cross-section of WIMP with nucleons. An other constraint is the relatively small deposited energy (≤ 100 keV).
The EDELWEISS experiment is located under ∼1700 m of rock (∼4800 mwe) in the Modane Underground Laboratory (LSM) in the highway Fréjus tunnel connecting France and Italy In the laboratory, the muon flux is reduced down to 4 µ/m 2 /d, a factor 10 6 times less than at the surface. The EDELWEISS-II experiment installation (see Fig. 1) was completed end of 2005. Specific improvements have been made in order to reduce the possible background sources that have limited the sensitivity of the previous experiment EDELWEISS-I (Fiorucci et al. 2007 ;Sanglard et al. 2005 ). To reduce the ra-Fig. 1. General scheme of the EDELWEISS-II experiment. The outer shell is the muon veto system, followed by the neutron polyethylen shield and the inner lead shield. The upper part can be open to access the bolometers, while the cryogenic systems are located under the detectors.
dioactive background in the cryostat all the materials were tested for radiopurity in a HPGe dedicated detector with very low radon level. The experiment is installed in a class 10 000 clean room and the cryostat environment is submitted to a permanent flow of deradonized air. The gamma background is screened by a 20 cm thick lead shield. Concerning the low energy neutron background, due to the radioactive surrounding rock, it is attenuated by more than three orders of magnitude thanks to a 50 cm polyethylene shield. In addition, a muon veto wraps the experiment though muons interacting in the lead shield are tagged. The dilution cryostat is of inverted design, with the experimental detector volume on the top. The large volume, 50 ℓ, allows for the installation up to 120 identical detectors in a compact arrangement, that will improve the possibility of detecting multiple interactions of neutrons and hence reject them. Simulations show a nuclear recoil rate above 10 keV estimated to be less than 10 -3 evt/kg/d, corresponding to a WIMP-nucleon cross-section sensitivity of 10 -8 pb for a WIMP mass of ∼ 100 GeV/c 2 corresponding to an improvement of a factor 100 compared to EDELWEISS-I. The detectors used in the experiment are high purity germanium crystals with measurement of phonon and ionization signals, cooled at a temperature of ∼ 20 mK. The simultaneous measurement of both heat and ionization signals provides an excellent event by event discrimination between nuclear recoils (induced by WIMP or neutron scattering) and electron recoils (induced by α, β or γ-radioactivity).
The ratio of the ionization to the heat signals depends on the recoiling particle, since a nucleus produces less ionization than an electron does. One important limitation for the EDELWEISS-I sensitivity was the presence of surface events, namely interactions occurring near electrodes. Because of diffusion, trapping and recombination, the charge induced by surface events is miscollected and can mimic nuclear recoils. To reach expected WIMP-nucleon cross-sections, it is necessary to have an active rejection in identifying the surface events. For this purpose, the experiment is running with three types of detectors : classic EDELWEISS-I design 320 g Ge/NTD equipped with new Teflon holders, 400 g Ge/NbSi equipped with two NbSi sensors sensitive to athermal phonons (Juillard et al. 2006 ;Marnieros et al. 2008) and 400 g Ge/NTD/ID crystal equipped with interdigitized electrode scheme (Broniatowski et al. 2008 ;Defay et al. 2008) (described in the next section). Also, an integration test is underway of a scintillation-phonon detector in EDEL-WEISS-II (Di Stefano et al. 2008). Such detectors could offer additional target materials, and assist understanding of backgrounds and demonstration of a signal.
Comparing to EDELWEISS-I, the EDELWEISS-II experiment is completely new with new cryostat,
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