185 research outputs found
Neutron background at the Canfranc Underground Laboratory and its contribution to the IGEX-DM dark matter experiment
A quantitative study of the neutron environment in the Canfranc Underground
Laboratory has been performed. The analysis is based on a complete set of
simulations and, particularly, it is focused on the IGEX-DM dark matter
experiment. The simulations are compared to the IGEX-DM low energy data
obtained with different shielding conditions. The results of the study allow us
to conclude, with respect to the IGEX-DM background, that the main neutron
population, coming from radioactivity from the surrounding rock, is practically
eliminated after the implementation of a suitable neutron shielding. The
remaining neutron background (muon-induced neutrons in the shielding and in the
rock) is substantially below the present background level thanks to the muon
veto system. In addition, the present analysis gives us a further insight on
the effect of neutrons in other current and future experiments at the Canfranc
Underground Laboratory. The comparison of simulations with the body of data
available has allowed to set the flux of neutrons from radioactivity of the
Canfranc rock, (3.82 +- 0.44) x 10^{-6} cm^{-2} s^{-1}, as well as the flux of
muon-induced neutrons in the rock, (1.73 +- 0.22(stat) \+- 0.69(syst)) x
10^{-9} cm^{-2} s^{-1}, or the rate of neutron production by muons in the lead
shielding, (4.8 +- 0.6 (stat) +- 1.9 (syst)) x 10^{-9} cm^{-3} s^{-1}.Comment: 17 pages, 8 figures, elsart document class; final version to appear
in Astroparticle Physic
Modelling the behaviour of microbulk Micromegas in Xenon/trimethylamine gas
We model the response of a state of the art micro-hole single-stage charge
amplication device (`microbulk' Micromegas) in a gaseous atmosphere consisting
of Xenon/trimethylamine at various concentrations and pressures. The amplifying
structure, made with photo-lithographic techniques similar to those followed in
the fabrication of gas electron multipliers (GEMs), consisted of a 100 um-side
equilateral-triangle pattern with 50 um-diameter holes placed at its vertexes.
Once the primary electrons are guided into the holes by virtue of an optimized
field configuration, avalanches develop along the 50 um-height channels etched
out of the original doubly copper-clad polyimide foil. In order to properly
account for the strong field gradients at the holes' entrance as well as for
the fluctuations of the avalanche process (that ultimately determine the
achievable energy resolution), we abandoned the hydrodynamic framework,
resorting to a purely microscopic description of the electron trajectories as
obtained from elementary cross-sections. We show that achieving a satisfactory
description needs additional assumptions about atom-molecule (Penning) transfer
reactions and charge recombination to be made
Spin Chirality in a Molecular Dysprosium Triangle: the Archetype of the Non-Collinear Ising Model
Single crystal magnetic studies combined with a theoretical analysis show
that cancellation of the magnetic moments in the trinuclear Dy3+ cluster
[Dy3(OH)2L3Cl(H2O)5]Cl3, resulting in a non-magnetic ground doublet, originates
from the non-collinearity of the single ion easy axes of magnetization of the
Dy3+ ions that lie in the plane of the triangle at 120 (deg.) one from each
other. This gives rise to a peculiar chiral nature of the ground non-magnetic
doublet and to slow relaxation of the magnetization with abrupt accelerations
at the crossings of the discrete energy levels.Comment: 4 pages and 5 figure
Magnetic interactions in thiazyl-based magnets: The role of the charge and spin densities
The crystal structure of the organic radical p-O2NC6F4CNSSN was
determined at 20 K through a single-crystal neutron-diffraction
experiment. It crystallises in the tetragonal space group P41212,
unchanged from a previous single-crystal X-ray diffraction
experiment at 220 K although there are some changes in molecular
geometry and intermolecular contacts arising from the contraction
of the unit cell. Polarized neutron diffraction at 1.5 K revealed that
the spin distribution is predominantly localised on the N and S
atoms of the heterocyclic ring with a small negative spin density
on the heterocyclic C atom. Spin populations determined using a
multipolar analysis were -0.06, +0.25 and +0.28 on the C, N and S
sites, respectively. These spin populations are in excellent agreement
with both ab-initio DFT calculations (spin populations on the C, N
and S sites of -0.07, 0.22 and 0.31, respectively) and cw-EPR studies
which estimated the spin population on the N site as 0.24. The DFT
calculated spin density revealed less than 1% spin delocalisation
onto the perfluoroaryl ring, several orders of magnitude lower than
the density on the heterocyclic ring. cw-ENDOR studies at both
X-band (9 GHz) and Q-band (34 GHz) frequencies probed the spin
populations at the two chemically distinct F atoms. These spin
populations on the F atoms ortho and meta to the dithiadiazolyl
ring are of magnitude 10-3 and 10-4 respectively. Additional high-resolution single-crystal X-ray diffraction studies at 100 K analysed
within the atoms-in-molecules (AIM) framework gave detailed
information on the charge density distributio
A Micromegas-based low-background x-ray detector coupled to a slumped-glass telescope for axion research
We report on the design, construction and operation of a low background x-ray
detection line composed of a shielded Micromegas (micromesh gaseous structure)
detector of the microbulk technique. The detector is made from radiopure
materials and is placed at the focal point of a ~5 cm diameter, 1.3 m
focal-length, cone-approximation Wolter I x-ray telescope (XRT) comprised of
thermally-formed (or "slumped") glass substrates deposited with multilayer
coatings. The system has been conceived as a technological pathfinder for the
future International Axion Observatory (IAXO), as it combines two of the
techniques (optic and detector) proposed in the conceptual design of the
project. It is innovative for two reasons: it is the first time an x-ray optic
has been designed and fabricated specifically for axion research, and the first
time a Micromegas detector has been operated with an x-ray optic. The line has
been installed at one end of the CERN Axion Solar Telescope (CAST) magnet and
is currently looking for solar axions. The combination of the XRT and
Micromegas detector provides the best signal-to-noise ratio obtained so far by
any detection system of the CAST experiment with a background rate of
5.410counts per hour in the energy region-of-interest and
signal spot area.Comment: 21 pages, 16 figure
Status of IGEX dark matter search at Canfranc Underground Laboratory
One IGEX 76Ge double-beta decay detector is currently operating in the
Canfranc Underground Laboratory in a search for dark matter WIMPs, through the
Ge nuclear recoil produced by the WIMP elastic scattering. In this talk we
report on the on-going efforts to understand and eventually reject the
background at low energy. These efforts have led to the improvement of the
neutron shielding and to partial reduction of the background, but still the
remaining events are not totally identified. A tritium contamination or
muon-induced neutrons are considered as possible sources, simulations and
experimental test being still under progress. According to the success of this
study we comment the prospects of the experiment as well as those of its future
extension, the GEDEON dark matter experiment.Comment: 6 pages, 3 figures, talk given at 4th International Workshop on the
Identification of Dark Matter, York, September 200
Background assessment for the TREX Dark Matter experiment
TREX-DM is conceived to look for low-mass Weakly Interacting Massive
Particles (WIMPs) using a gas Time Projection Chamber equipped with micromegas
readout planes at the Canfranc Underground Laboratory. The detector can hold in
the active volume 20 l of pressurized gas up to 10 bar, corresponding to 0.30
kg of Ar or 0.16 kg of Ne. The micromegas are read with a self-triggered
acquisition, allowing for thresholds below 0.4 keV (electron equivalent). A low
background level in the lowest energy region is another essential requirement.
To assess the expected background, all the relevant sources have been
considered, including the measured fluxes of gamma radiation, muons and
neutrons at the Canfranc Laboratory, together with the activity of most of the
components used in the detector and ancillary systems, obtained in a complete
assay program. The background contributions have been simulated by means of a
dedicated application based on Geant4 and a custom-made code for the detector
response. The background model developed for the detector presently installed
in Canfranc points to levels from 1 to 10 counts keV-1 kg-1 d-1 in the region
of interest, making TREX-DM competitive in the search for low-mass WIMPs. A
roadmap to further decrease it down to 0.1 counts keV-1 kg-1 d-1 is underway.Comment: Final version, as accepted in Eur. Phys. J.
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