346 research outputs found
Recent results in Dark Matter direct detection
Finding a solution to the Dark Matter problem is surely one of the main challenges of modern cosmology. The existence of both Dark Matter and Dark Energy has been formulated on the basis of strong observational evidences, and constitutes the main success of the most accredited cosmological models. Yet none of them has been directly detected. In this review the Dark Matter problem will be discussed and the approaches to directly detect it, in the form of a special category of particles, i.e. the WIMPs (Weakly Interacting Massive Particles), will be presented and discussed
Neutrino physics with multi-ton scale liquid xenon detectors
We study the sensitivity of large-scale xenon detectors to low-energy solar
neutrinos, to coherent neutrino-nucleus scattering and to neutrinoless double
beta decay. As a concrete example, we consider the xenon part of the proposed
DARWIN (Dark Matter WIMP Search with Noble Liquids) experiment. We perform
detailed Monte Carlo simulations of the expected backgrounds, considering
realistic energy resolutions and thresholds in the detector. In a low-energy
window of 2-30 keV, where the sensitivity to solar pp and Be-neutrinos is
highest, an integrated pp-neutrino rate of 5900 events can be reached in a
fiducial mass of 14 tons of natural xenon, after 5 years of data. The
pp-neutrino flux could thus be measured with a statistical uncertainty around
1%, reaching the precision of solar model predictions. These low-energy solar
neutrinos will be the limiting background to the dark matter search channel for
WIMP-nucleon cross sections below 210 cm and WIMP
masses around 50 GeVc, for an assumed 99.5% rejection of
electronic recoils due to elastic neutrino-electron scatters. Nuclear recoils
from coherent scattering of solar neutrinos will limit the sensitivity to WIMP
masses below 6 GeVc to cross sections above
410cm. DARWIN could reach a competitive half-life
sensitivity of 5.610 y to the neutrinoless double beta decay of
Xe after 5 years of data, using 6 tons of natural xenon in the central
detector region.Comment: 17 pages, 4 figure
Spatially uniform calibration of a liquid xenon detector at low energies using 83m-Kr
A difficult task with many particle detectors focusing on interactions below
~100 keV is to perform a calibration in the appropriate energy range that
adequately probes all regions of the detector. Because detector response can
vary greatly in various locations within the device, a spatially uniform
calibration is important. We present a new method for calibration of liquid
xenon (LXe) detectors, using the short-lived 83m-Kr. This source has
transitions at 9.4 and 32.1 keV, and as a noble gas like Xe, it disperses
uniformly in all regions of the detector. Even for low source activities, the
existence of the two transitions provides a method of identifying the decays
that is free of background. We find that at decreasing energies, the LXe light
yield increases, while the amount of electric field quenching is diminished.
Additionally, we show that if any long-lived radioactive backgrounds are
introduced by this method, they will present less than 67E-6 events/kg/day in
the next generation of LXe dark matter direct detection searchesComment: 9 pages, 9 figures. Accepted to Review of Scientific Instrument
Scintillation efficiency of liquid argon in low energy neutron-argon scattering
Experiments searching for weak interacting massive particles with noble gases
such as liquid argon require very low detection thresholds for nuclear recoils.
A determination of the scintillation efficiency is crucial to quantify the
response of the detector at low energy. We report the results obtained with a
small liquid argon cell using a monoenergetic neutron beam produced by a
deuterium-deuterium fusion source. The light yield relative to electrons was
measured for six argon recoil energies between 11 and 120 keV at zero electric
drift field.Comment: 21 pages, 19 figures, 4 table
A simple high-sensitivity technique for purity analysis of xenon gas
We report on the development and performance of a high-sensitivity
purity-analysis technique for gaseous xenon. The gas is sampled at macroscopic
pressure from the system of interest using a UHV leak valve. The xenon present
in the sample is removed with a liquid-nitrogen cold trap, and the remaining
impurities are observed with a standard vacuum mass-spectroscopy device. Using
calibrated samples of xenon gas spiked with known levels of impurities, we find
that the minimum detectable levels of N2, O2, and methane are 1 ppb, 160 ppt,
and 60 ppt respectively. This represents an improvement of about a factor of
10,000 compared to measurements performed without a coldtrap.Comment: 20 pages, 5 figure
A Grid-enabled web portal for NMR structure refinement with AMBER
Abstract
Motivation: The typical workflow for NMR structure determination involves collecting thousands of conformational restraints, calculating a bundle of 20–40 conformers in agreement with them and refining the energetics of these conformers. The structure calculation step employs simulated annealing based on molecular dynamics (MD) simulations with very simplified force fields. The value of refining the calculated conformers using restrained MD (rMD) simulations with state-of-art force fields is documented. This refinement however presents various subtleties, from the proper formatting of conformational restraints to the definition of suitable protocols.
Results: We describe a web interface to set up and run calculations with the AMBER package, which we called AMPS-NMR (AMBER-based Portal Server for NMR structures). The interface allows the refinement of NMR structures through rMD. Some predefined protocols are provided for this purpose, which can be personalized; it is also possible to create an entirely new protocol. AMPS-NMR can handle various restraint types. Standard rMD refinement in explicit water of the structures of three different proteins are shown as examples. AMPS-NMR additionally includes a workspace for the user to store different calculations. As an ancillary service, a web interface to AnteChamber is available, enabling the calculation of force field parameters for organic molecules such as ligands in protein–ligand adducts.
Availability and Implementation: AMPS-NMR is embedded within the NMR services of the WeNMR project and is available at http://py-enmr.cerm.unifi.it/access/index/amps-nmr; its use requires registration with a digital certificate.
Contact: [email protected]
Supplementary information: Supplementary data are available at Bioinformatics online
Gator: a low-background counting facility at the Gran Sasso Underground Laboratory
A low-background germanium spectrometer has been installed and is being
operated in an ultra-low background shield (the Gator facility) at the Gran
Sasso underground laboratory in Italy (LNGS). With an integrated rate of ~0.16
events/min in the energy range between 100-2700 keV, the background is
comparable to those of the world's most sensitive germanium detectors. After a
detailed description of the facility, its background sources as well as the
calibration and efficiency measurements are introduced. Two independent
analysis methods are described and compared using examples from selected sample
measurements. The Gator facility is used to screen materials for XENON, GERDA,
and in the context of next-generation astroparticle physics facilities such as
DARWIN.Comment: 14 pages, 6 figures, published versio
Study of nuclear recoils in liquid argon with monoenergetic neutrons
For the development of liquid argon dark matter detectors we assembled a
setup in the laboratory to scatter neutrons on a small liquid argon target. The
neutrons are produced mono-energetically (E_kin=2.45 MeV) by nuclear fusion in
a deuterium plasma and are collimated onto a 3" liquid argon cell operating in
single-phase mode (zero electric field). Organic liquid scintillators are used
to tag scattered neutrons and to provide a time-of-flight measurement. The
setup is designed to study light pulse shapes and scintillation yields from
nuclear and electronic recoils as well as from {\alpha}-particles at working
points relevant to dark matter searches. Liquid argon offers the possibility to
scrutinise scintillation yields in noble liquids with respect to the
populations of the two fundamental excimer states. Here we present experimental
methods and first results from recent data towards such studies.Comment: 9 pages, 8 figures, proceedings of TAUP 2011, to be published in
Journal of Physics: Conference Series (JCPS
Effect of Crude Oil Vanadyl Porphyrin Content and Blending on Heat Exchanger Fouling
Fouling in the pre-heat train of crude oil distillation unit is a normal occurrence in oil refineries. With the increasing trend of heavy crude oil processing, blending of oils becomes a norm practice in a refinery. Fouling could be enhanced if incompatible oils were blended together. Also, heavy oils typically contain high trace metals composition and their effect towards fouling is yet to be explored. This study has been looking into the effect of changes in the original composition and inter-molecular stability of crude oils towards fouling. A batch system was designed and developed to produce a deposit sample from 3 crude oils of industrial interest. The reactor was successfully commissioned and an operating procedure was established.
In the first part of the work, various concentrations of Vanadium were added to crude oils and subjected to high temperature in the test section. The resultant deposits and liquid products were analysed for their molecular structure and weight distribution. Analysis by Inductively Coupled Plasma Mass Spectrometer (ICP-MS) shows the added Vanadium are largely concentrated in the deposits. The amount of deposits produced increases with the concentration of Vanadium in the original samples. Analyses by Size Exclusion Chromatogram (SEC), Gas Chromatography (GC) and UV-Fluorescence Spectroscopy (UVF) show larger and more complex molecules in deposits from oils with high concentration of Vanadium.
In the second part, the compatibility regions of crude oil blends were established prior to analysis in the reactor. More deposits were produced from incompatible oil blends though these were lighter and less complex than that of compatible blends. Also, oils that were blended from the incompatible region produced deposits of bigger molecular weight distributions and more complex structures. Results of Thermo-Gravimetric Analysis (TGA) confirmed those of SEC’s and UVF’s of which more volatile components were recorded in active blended deposit samples.Open Acces
Design of a mobile neutron spectrometer for the Laboratori Nazionali del Gran Sasso (LNGS)
Environmental neutrons are a source of background for rare event searches (e.g., dark matter direct detection and neutrinoless double beta decay experiments) taking place in deep underground laboratories. The overwhelming majority of these neutrons are produced in the cavern walls by means of intrinsic radioactivity of the rock and concrete. Their flux and spectrum depend on time and location. Precise knowledge of this background is necessary to devise sufficient shielding and veto mechanisms, improving the sensitivity of the neutron-susceptible underground experiments. In this report, we present the design and the expected performance of a mobile neutron detector for the LNGS underground laboratory. The detector is based on capture-gated spectroscopy technique and comprises essentially a stack of plastic scintillator bars wrapped with gadolinium foils. The extensive simulation studies demonstrate that the detector will be capable of measuring ambient neutrons at low flux levels (~ n/cm/s) at LNGS, where the ambient gamma flux is by about 5 orders of magnitude larger
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