[en] EDELWEISS is a direct non-baryonic cold dark matter detection experiment in the form of weakly interacting massive particles (also known as WIMPs), which currently constitute the most popular candidates to account for the missing mass in the Universe. To this purpose, EDELWEISS uses germanium bolometers at cryogenic temperature (20 mK approximately) in the Underground Laboratory of Modane (LSM) at the French-Italian border. Since 2008, a new type of detector is operated, equipped with concentric electrodes to optimize the rejection of surface events (coplanar-grid detectors). This thesis work is divided into several research orientations. First, we carried out measurements concerning charge collection in the crystals. The velocity laws of the carriers (electrons and holes) have been determined in germanium at 20 mK in the <100> orientation, and a complete study of charge sharing has been done, including an evaluation of the transport anisotropy and of the straggling of the carriers. These results lead to a better understanding of the inner properties of the EDELWEISS detectors. Then, studies relating to the improvement of the performances were carried out. In particular, we have optimized the space-charge cancellation procedure in the crystals and improved the passive rejection of surface events (β). The fiducial volume of the detectors has been evaluated using two X-ray lines from cosmically activated radionuclides: ⁶⁸Ge and ⁶⁵Zn. Finally, an exhaustive study of the low energy spectra has been carried out, which makes it possible to develop a systematic analysis method for the search of low-mass WIMPs in EDELWEISS. (author)

[en] Due to their insoluble nature in nuclear fuels, gaseous fission products (Kr, Xe) cause unfavourable evolutions of fuel properties (microstructure, thermal conductivity) that impair fuel performances and constrain operational safety margins (through the risk of fuel pin bursting in case of gas release from the fuel). The study of Kr and Xe behaviour in UO₂ and derived compositions has thus generated a considerable amount of experimental and computational research since the dawn of nuclear power; however, there is still some relevant experimental conditions that needs to be better understood. This notably encompass experimental diffusion in very low concentrations (allowing to reach defect free interstitial lattice diffusion constants) or in over stoichiometric compositions (UO_2+x, relevant as the high burn-ups UO₂ fuels can reach over stoichiometry of circa x = 0.001). During the last decade, we have tried to fill these gaps by investigating the separate effects of several parameters on fission gas diffusion in Kr or Xe ion-implanted UO₂ pellets: Xe vs. Kr, temperature, fluence, UO₂ bulk and surface microstructure, and oxygen over stoichiometry (either from surface oxidation during storage or by oxygen controlled additions during annealings). Xe and Kr diffusion kinetics and behaviours were measured using Thermo-Desorption Spectrometry, a technique that allowed us to work with fluences down to few 1010 ions.cm^-2. We thus propose to present at NuFuel a summary of these works and an outlook of what should be done next (application to MOX or irradiated fuels, to alternative fuels etc.). (authors)

[en] For most applications of laser driven ion beams, a well-characterized high repetition rate intense ion beam with low divergence and a controllable energy spectrum is needed. High power laser-solid targets interactions are usually used, in which the main acceleration mechanism is the so-called Target Normal Sheath Acceleration (TNSA). Changing solid targets for overcritical gas jet targets has given interesting results in theoretical simulations and these later have several technical advantages for high repetition rate lasers. In this work protons and helium ions are accelerated from a near-critical supersonic gas jet. The production of such targets is very challenging for near infrared lasers. We present recent results concerning the design and characterization of supersonic gas nozzles able to deliver such high densities and the first results obtained during the first experiment on PICO2000 facility at LULI. We succeeded to accelerate ions up to several MeV with a H-2 and He gas jet target. The number of accelerated ions is comparable to the one usually obtained with solid targets. (authors)

[en] Scintillating bolometers of LiF and Al₂O₃ can monitor the fast neutrons flux in WIMPs searches. With both materials we merge the traditional fast neutron detection methods of induced reactions and scattering. The ROSEBUD collaboration devoted an underground run in the old Canfranc laboratory to study the response of LiF and Al₂O₃ to fast neutrons from ²⁵²Cf. Both bolometers were used simultaneously in a common experimental set-up resembling those of current WIMPs searches, which could give valuable insights into future WIMPs searches with cryogenic detectors as EURECA.

[en] For most laser-driven ion acceleration applications, a well-characterized intense ion beam with a low divergence and a controllable energy spectrum produced at a high repetition rate is needed. Gas-jet targets have given promising results in simulations, and they have several technical advantages for high-repetition-rate lasers. In this work, we report on proton acceleration to energies up to 6 MeV using a supersonic H₂ gas-jet target at the LULI PICO2000 laser facility. The experimental results are compared with the plasma hydrodynamics and the particle-in-cell simulations to identify the acceleration mechanisms at play. (authors)

[en] 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 β, approaching 10⁵. This opens the road to investigate the range beyond 10^-8 pb in the WIMP-nucleon collision cross-sections, as proposed in the EURECA project of a one-ton cryogenic detector mass.

[en] Muon-induced neutrons constitute a prominent background component in a number of low count rate experiments, namely direct searches for dark matter. In this work we describe a neutron detector to measure this background in an underground laboratory, the Laboratoire Souterrain de Modane. The system is based on 1 m³ of Gd-loaded scintillator and it is linked with the muon veto of the EDELWEISS-II experiment for coincident muon detection. The system was installed in autumn 2008 and passed since then a number of commissioning tests proving its full functionality. The data-taking is continuously ongoing and a count rate of the order of 1 muon-induced neutron per day has been achieved. (authors)

[en] The EDELWEISS-II Collaboration has completed a direct search for WIMP dark matter with an array of ten 400-g cryogenic germanium detectors in operation at the Laboratoire Souterrain de Modane. The combined use of thermal phonon sensors and charge collection electrodes with an interleaved geometry enables the efficient rejection of γ-induced radioactivity as well as near-surface interactions. A total effective exposure of 384 kg d has been achieved, mostly coming from fourteen months of continuous operation. Five nuclear recoil candidates are observed above 20 keV, while the estimated background is 3.0 events. The result is interpreted in terms of limits on the cross-section of spin-independent interactions of WIMPs and nucleons. A cross-section of 4.4x10^-8 pb is excluded at 90%CL for a WIMP mass of 85 GeV. New constraints are also set on models where the WIMP-nucleon scattering is inelastic.

[en] The EDELWEISS-II Collaboration has performed a direct search for WIMP dark matter with an array of ten 400 g heat-and-ionization cryogenic detectors equipped with interleaved electrodes for the rejection of near-surface events. Six months of continuous operation at the Laboratoire Souterrain de Modane have been achieved. The observation of one nuclear recoil candidate above 20 keV in an effective exposure of 144 kg d is interpreted in terms of limits on the cross-section of spin-independent interactions of WIMPs and nucleons. A cross-section of 1.0x10^-7 pb is excluded at 90% CL for a WIMP mass of 80 GeV/c². This result demonstrates for the first time the very high background rejection capabilities of these simple and robust detectors in an actual WIMP search experiment.