25 research outputs found
Effect of annealing on the superconducting properties of a-Nb(x)Si(1-x) thin films
a-Nb(x)Si(1-x) thin films with thicknesses down to 25 {\AA} have been
structurally characterized by TEM (Transmission Electron Microscopy)
measurements. As-deposited or annealed films are shown to be continuous and
homogeneous in composition and thickness, up to an annealing temperature of
500{\deg}C. We have carried out low temperature transport measurements on these
films close to the superconductor-to-insulator transition (SIT), and shown a
qualitative difference between the effect of annealing or composition, and a
reduction of the film thickness on the superconducting properties of a-NbSi.
These results question the pertinence of the sheet resistance R_square as the
relevant parameter to describe the SIT.Comment: 9 pages, 12 figure
Identification of backgrounds in the EDELWEISS-I dark matter search experiment
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 localisation of the background sources. In
light of this interpretation, we draw conclusions regarding the background
suppression scheme for the EDELWEISS-II phase
Measurement of the response of heat-and-ionization germanium detectors to nuclear recoils
The heat quenching factor Q' (the ratio of the heat signals produced by
nuclear and electron recoils of equal energy) of the heat-and-ionization
germanium bolometers used by the EDELWEISS collaboration has been measured. It
is explained how this factor affects the energy scale and the effective
quenching factor observed in calibrations with neutron sources. This effective
quenching effect is found to be equal to Q/Q', where Q is the quenching factor
of the ionization yield. To measure Q', a precise EDELWEISS measurement of Q/Q'
is combined with values of Q obtained from a review of all available
measurements of this quantity in tagged neutron beam experiments. The
systematic uncertainties associated with this method to evaluate Q' are
discussed in detail. For recoil energies between 20 and 100 keV, the resulting
heat quenching factor is Q' = 0.91+-0.03+-0.04, where the two errors are the
contributions from the Q and Q/Q' measurements, respectively. The present
compilation of Q values and evaluation of Q' represent one of the most precise
determinations of the absolute energy scale for any detector used in direct
searches for dark matter.Comment: 28 pages, 7 figures. Submitted to Phys. Rev.
The silicon micro-strip detector plane for the LOFT/Wide Field Monitor
The main objective of the Wide Field Monitor (WFM) on the LOFT mission is to
provide unambiguous detection of the high-energy sources in a large field of
view, in order to support science operations of the LOFT primary instrument,
the LAD. The monitor will also provide by itself a large number of results on
the timing and spectral behaviour of hundreds of galactic compact objects,
Active Galactic Nuclei and Gamma-Ray Bursts. The WFM is based on the coded
aperture concept where a position sensitive detector records the shadow of a
mask projected by the celestial sources. The proposed WFM detector plane, based
on Double Sided micro-Strip Silicon Detectors (DSSD), will allow proper
2-dimensional recording of the projected shadows. Indeed the positioning of the
photon interaction in the detector with equivalent fine resolution in both
directions insures the best imaging capability compatible with the allocated
budgets for this telescope on LOFT. We will describe here the overall
configuration of this 2D-WFM and the design and characteristics of the DSSD
detector plane including its imaging and spectral performances. We will also
present a number of simulated results discussing the advantages that this
configuration offers to LOFT. A DSSD-based WFM will in particular reduce
significantly the source confusion experienced by the WFM in crowded regions of
the sky like the Galactic Center and will in general increase the observatory
science capability of the mission.Comment: Proceedings of SPIE, Vol. 8443, Paper No. 8443-89, 201
Axion searches with the EDELWEISS-II experiment
We present new constraints on the couplings of axions and more generic
axion-like particles using data from the EDELWEISS-II experiment. The EDELWEISS
experiment, located at the Underground Laboratory of Modane, primarily aims at
the direct detection of WIMPs using germanium bolometers. It is also sensitive
to the low-energy electron recoils that would be induced by solar or dark
matter axions. Using a total exposure of up to 448 kg.d, we searched for
axion-induced electron recoils down to 2.5 keV within four scenarios involving
different hypotheses on the origin and couplings of axions. We set a 95% CL
limit on the coupling to photons GeV in
a mass range not fully covered by axion helioscopes. We also constrain the
coupling to electrons, , similar to the more
indirect solar neutrino bound. Finally we place a limit on , where is the
effective axion-nucleon coupling for Fe. Combining these results we
fully exclude the mass range keV for DFSZ axions and
keV for KSVZ axions
Final results of the EDELWEISS-I dark matter search with cryogenic heat-and-ionization Ge detectors
The final results of the EDELWEISS-I dark matter search using cryogenic
heat-and-ionization Ge detectors are presented. The final data sample
corresponds to an increase by a factor five in exposure relative to the
previously published results. A recoil energy threshold of 13 keV or better was
achieved with three 320g detectors working simultaneously over four months of
stable operation. Limits on the spin-independent cross-section for the
scattering of a WIMP on a nucleon are derived from an accumulated fiducial
exposure of 62 kg.d.Comment: 42 pages, 16 figures, submitted to Physical Review D, 1 figure and 2
references added, some little changes in the tex
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.Comment: v2: new references and figures added, other minor changes. Accepted
for publication in Physics Letters
Superconductor-Insulator Transitions in Pure Polycrystalline Nb Thin Films
International audienceWe report on a study of the transport properties of Nb thin films. By varying the thickness of the films from 263 angstrom to 25 angstrom, we observed a depression of the superconductivity. Magnetic field was also applied up to 6 T, inducing the disappearance of the superconductivity and the onset of an insulating behavior. The results were compared to those we have already obtained on a highly disordered system, a-NbxSi1-x, to understand whether the same mechanisms for the disappearance of the superconductivity could be at play in pure metallic thin films and in highly disordered systems
Thick silicon microstrip detectors simulation for PACT: Pair and Compton Telescope
International audiencePACT is a space borne Pair and Compton Telescope that aims to make a sensitive survey of the gamma-ray sky between 100 keV and 100 MeV. It is based upon two main components: a silicon-based gamma-ray tracker and a crystal-based calorimeter. In this paper we will explain the imaging technique of PACT as a Multi-layered Compton telescope (0.1–10 MeV) and its major improvements over its predecessor COMPTEL. Then we will present a simulation study to optimize the silicon tracker of PACT. This tracker is formed of thousands of identical silicon double sided strip detectors (DSSDs). We have developed a simulation model (using SILVACO) to simulate the DSSD performance while varying its thickness, impurity concentration of the bulk material, electrode pitch, and electrode width. We will present a comprehensive overview of the impact of each varied parameter on the DSSD performance, in view of the application to PACT. The considered DSSD parameters are its depletion voltage, capacitance, and leakage current. After the selection of the PACT DSSD, we will present a simulation of the performance of the PACT telescope in the 0.1–10 MeV range