1,981 research outputs found
Nulling interferometry: impact of exozodiacal clouds on the performance of future life-finding space missions
Earth-sized planets around nearby stars are being detected for the first time
by ground-based radial velocity and space-based transit surveys. This milestone
is opening the path towards the definition of missions able to directly detect
the light from these planets, with the identification of bio-signatures as one
of the main objectives. In that respect, both ESA and NASA have identified
nulling interferometry as one of the most promising techniques. The ability to
study distant planets will however depend on exozodiacal dust clouds
surrounding the target stars. In this paper, we assess the impact of
exozodiacal dust clouds on the performance of an infrared nulling
interferometer in the Emma X-array configuration. For the nominal mission
architecture with 2-m aperture telescopes, we found that point-symmetric
exozodiacal dust discs about 100 times denser than the solar zodiacal cloud can
be tolerated in order to survey at least 150 targets during the mission
lifetime. Considering modeled resonant structures created by an Earth-like
planet orbiting at 1 AU around a Sun-like star, we show that the tolerable dust
density for planet detection goes down to about 15 times the solar zodiacal
density for face-on systems and decreases with the disc inclination. The upper
limits on the tolerable exozodiacal dust density derived in this study must be
considered as rather pessimistic, but still give a realistic estimation of the
typical sensitivity that we will need to reach on exozodiacal discs in order to
prepare the scientific programme of future Earth-like planet characterisation
missions.Comment: 17 pages, accepted for publication in A&
The Audit Logic: Policy Compliance in Distributed Systems
We present a distributed framework where agents can share data along with usage policies. We use an expressive policy language including conditions, obligations and delegation. Our framework also supports the possibility to refine policies. Policies are not enforced a-priori. Instead policy compliance is checked using an a-posteriri auditing approach. Policy compliance is shown by a (logical) proof that the authority can systematically check for validity. Tools for automatically checking and generating proofs are also part of the framework.\u
Strong Quasiparticle Trapping In A 6x6 Array Of Vanadium-Aluminum Superconducting Tunnel Junctions
A 6x6 array of symmetrical V/Al/AlOx/Al/V Superconducting Tunnel Junctions
(STJs) was fabricated. The base electrode is a high quality epitaxial film with
a residual resistance ratio (RRR) of ~30. The top film is polycrystalline with
an RRR of ~10. The leakage currents of the 25x25 mm^2 junctions are of the
order of 0.5 pA/mm^2 at a bias voltage of 100 mV, which corresponds to a
dynamical resistance of ~ 3 10^5 ohms. When the array was illuminated by 6 keV
X-ray photons from a 55Fe radioactive source the single photon charge output
was found to be low and strongly dependent on the temperature of the devices.
This temperature dependence at X-ray energies can be explained by the existence
of a very large number of quasiparticle (QP) traps in the Vanadium. QPs are
confined in these traps, having a lower energy gap than the surrounding
material, and are therefore not available for tunneling. The number of traps
can be derived from the energy dependence of the responsivity of the devices
(charge output per electron volt of photon input energy).Comment: 4 pages. presented at Low Temperature Detectors-
Systematic UHV-AFM experiments on Na nano-particles and nano-structures in NaCl
Results of systematic AFM (atomic force microscopy) experiments on heavily and moderatly irradiated NaCl samples are presented. The sodium nanoparticles and structures of nanoparticles are poduced in sodium chloride during irradiation. The AFM images of the nanoparticles have been obtained in ultra high vacuum (UHV) in the non-contact mode with an Omicron UHV AFM/STM system. The sizes and arrangements of the observed particles depend on the irradiation conditions. The melting behaviour of the sodium nanoclusters has been studied for these samples by differential scanning calorimetry (DSC). A phenomenological model has been proposed which explains the observed behaviour by the existence of two different structural states in the population of sodium colloids. AFM measurements of heavily irradiated natural rock salt samples have revealed ring- and chain-like clusters (or agglomerates) of Na colloid particles.
AFM of metallic nano-particles and nano-structures in heavily irradiated NaCl
AFM investigations are reported for heavily, electron irradiated NaCl crystals in ultra high vacuum (UHV) in the non-contact mode with an UHV AFM/STM Omicron system. To avoid chemical reactions between the radiolytic Na and oxygen and water, the irradiated samples were cleaved and prepared for the experiments in UHV. At the surface of freshly cleaved samples, we have observed sodium nano-precipitates with shapes, which depend on the irradiation dose and the volume fraction of the radiolytic Na. It appears that the nano-structures consist of (i) isolated nano-particles, (ii) more or less random aggregates of these particles, (iii) fractally shaped networks and (iv) ‘‘fabrics’’ consisting of bundles of Quasi-1D arrays forming polymeric networks of nano-particles. Almost independent of the concentration of the metallic Na in the samples the size of the individual nano-particles is in the range 1–3 nm. Our new AFM results are fully in line with our CESR and previous Raman scattering results.
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