953 research outputs found
Large-Mass Ultra-Low Noise Germanium Detectors: Performance and Applications in Neutrino and Astroparticle Physics
A new type of radiation detector, a p-type modified electrode germanium
diode, is presented. The prototype displays, for the first time, a combination
of features (mass, energy threshold and background expectation) required for a
measurement of coherent neutrino-nucleus scattering in a nuclear reactor
experiment. The device hybridizes the mass and energy resolution of a
conventional HPGe coaxial gamma spectrometer with the low electronic noise and
threshold of a small x-ray semiconductor detector, also displaying an intrinsic
ability to distinguish multiple from single-site particle interactions. The
present performance of the prototype and possible further improvements are
discussed, as well as other applications for this new type of device in
neutrino and astroparticle physics (double-beta decay, neutrino magnetic moment
and WIMP searches).Comment: submitted to Phys. Rev.
Experimental constraints on a dark matter origin for the DAMA annual modulation effect
A claim for evidence of dark matter interactions in the DAMA experiment has
been recently reinforced. We employ a new type of germanium detector to
conclusively rule out a standard isothermal galactic halo of Weakly Interacting
Massive Particles (WIMPs) as the explanation for the annual modulation effect
leading to the claim. Bounds are similarly imposed on a suggestion that dark
pseudoscalars mightlead to the effect. We describe the sensitivity to light
dark matter particles achievable with our device, in particular to
Next-to-Minimal Supersymmetric Model candidates.Comment: v4: introduces recent results from arXiv:0807.3279 and
arXiv:0807.2926. Sensitivity to pseudoscalars is revised in light of the
first. Discussion on the subject adde
Detrital-Zircon Geochronology of the Metasedimentary Rocks of North-Western Graham Land
Metasedimentary rocks constitute an important but comparatively poorly understood part of the Antarctic Peninsula. Herein we report single-grain U-Pb detrital-zircon ages from samples of the Trinity Peninsula and Botany Bay Groups of north-western Graham Land. All studied samples are dominated by a large and narrowly defined population of late Palaeozoic zircons. Significant early–middle Palaeozoic and minor Neoproterozoic and Mesoproterozoic sub-populations constitute the majority of pre-Carboniferous grains. These detrital-zircon age populations are consistent with sediment derivation entirely from western Gondwana sources. Despite the clear Gondwana signatures, our data suggest that the Trinity Peninsula Group province was either a parautochthonous peri-Gondwanan terrane later accreted to the Antarctic Peninsula, or a significant topographic barrier precluded voluminous sediment contributions from the interior of Gondwana. Statistical comparisons with similar metasedimentary complexes of southern South America, the South Shetland Islands and eastern New Zealand indicate a diversity of sediment provenance not previously recognized, but may provide a means to better determine the pre-break-up configuration of western Gondwana. Although insufficient to definitively restore Antarctic Peninsula components adjacent to South American complexes, some Trinity Peninsula Group samples exhibit robust affinities to the Miers Bluff Formation in the South Shetland Islands and the Duque de York and Main Range Metamorphic Complexes of the Patagonian Andes
Results from a Search for Light-Mass Dark Matter with a P-type Point Contact Germanium Detector
We report on several features present in the energy spectrum from an ultra
low-noise germanium detector operated at 2,100 m.w.e. By implementing a new
technique able to reject surface events, a number of cosmogenic peaks can be
observed for the first time. We discuss several possible causes for an
irreducible excess of bulk-like events below 3 keVee, including a dark matter
candidate common to the DAMA/LIBRA annual modulation effect, the hint of a
signal in CDMS, and phenomenological predictions. Improved constraints are
placed on a cosmological origin for the DAMA/LIBRA effect.Comment: 4 pages, 4 figures. v2: submitted version. Minimal changes in
wording, one reference adde
Beta cell compensation for insulin resistance in Zucker fatty rats: increased lipolysis and fatty acid signalling.
Aims/hypothesis The aim of this study was to determine the role of fatty acid signalling in islet beta cell compensation for insulin resistance in the Zucker fatty fa/fa (ZF) rat, a genetic model of severe obesity, hyperlipidaemia and insulin resistance that does not develop diabetes. Materials and methods NEFA augmentation of insulin secretion and fatty acid metabolism were studied in isolated islets from ZF and Zucker lean (ZL) control rats. Results Exogenous palmitate markedly potentiated glucose-stimulated insulin secretion (GSIS) in ZF islets, allowing robust secretion at physiological glucose levels (5-8 mmol/l). Exogenous palmitate also synergised with glucagon-like peptide-1 and the cyclic AMP-raising agent forskolin to enhance GSIS in ZF islets only. In assessing islet fatty acid metabolism, we found increased glucose-responsive palmitate esterification and lipolysis processes in ZF islets, suggestive of enhanced triglyceride-fatty acid cycling. Interruption of glucose-stimulated lipolysis by the lipase inhibitor Orlistat (tetrahydrolipstatin) blunted palmitate-augmented GSIS in ZF islets. Fatty acid oxidation was also higher at intermediate glucose levels in ZF islets and steatotic triglyceride accumulation was absent. Conclusions/interpreation The results highlight the potential importance of NEFA and glucoincretin enhancement of insulin secretion in beta cell compensation for insulin resistance. We propose that coordinated glucose-responsive fatty acid esterification and lipolysis processes, suggestive of triglyceride-fatty acid cycling, play a role in the coupling mechanisms of glucose-induced insulin secretion as well as in beta cell compensation and the hypersecretion of insulin in obesity
A new analytic model for the cognitive radio jump-stay algorithm
In cognitive radio networks, primary users have priority over the regulated radio spectrum. Secondary users may use residual air time. We focus on the problem of meeting on a common channel by a group of secondary users. The goal is to make the users rendezvous on a common channel in a minimum amount of time. The jump-stay algorithm has been created by Lin et al. to solve this problem. We construct a new analytic model for the two-user expected time to rendezvous in the jump-stay algorithm that better reflects its performance. For the sake of comparison, we also evaluate the performance of the jump-stay algorithm through simulation
Mitigation of topology control traffic attacks in OLSR networks
The core of the Optimized Link State Routing (OLSR) protocol is the selection of Multipoint Relays (MPRs) as a flooding mechanism for distributing control traffic messages. A node in an OLSR network, selects its MPR set such that all two-hop neighbors are reachable through, at least, one MPR. However, if an MPR misbehaves during the execution of the protocol, the connectivity of the network is compromised. Additional coverage in the selection of the MPRs helps to mitigate the effect of control traffic attacks. RFC3626 defines the selection of MPRs with additional coverage. Nevertheless, the overhead of the network increases due to the added number of control traffic messages. In this paper, we propose an improved MPR selection with additional coverage. Every node selects, if it is possible, k + 1 disjoint MPR sets. The union of those sets, is a k-robust-MPR set. Thus, given a node, alternative paths are created to reach any destination two-hops away. We test both approaches against two kinds of adversaries misbehaving during the execution of the protocol. Our proposed MPR selection with additional coverage mitigates the effect of control traffic attacks by offering equivalent protectio
Preventing the cluster formation attack against the hierarchical OLSR protocol
The Hierarchical Optimized Link State Routing (HOLSR) protocol enhances the scalability and heterogeneity of traditional OLSR-based Mobile Ad-Hoc Networks (MANETs). It organizes the network in logical levels and nodes in clusters. In every cluster, it implements the mechanisms and algorithms of the original OLSR to generate and to distribute control traffic information. However, the HOLSR protocol was designed with no security in mind. Indeed, it both inherits, from OLSR, and adds new security threats. For instance, the existence of misbehaving nodes can highly affect important HOLSR operations, such as the cluster formation. Cluster IDentification (CID) messages are implemented to organize a HOLSR network in clusters. In every message, the hop count field indicates to the receiver the distance in hops to the originator. An attacker may maliciously alter the hop count field. As a consequence, a receiver node may join a cluster head farther away than it appears. Then, the scalability properties in a HOLSR network is affected by an unbalanced distribution of nodes per cluster. We present a s
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