408 research outputs found
Neutron spectroscopy with the Spherical Proportional Counter
A novel large volume spherical proportional counter, recently developed, is
used for neutron measurements. Gas mixtures of with and
pure are studied for thermal and fast neutron detection, providing a
new way for the neutron spectroscopy. The neutrons are detected via the
and reactions. Here we
provide studies of the optimum gas mixture, the gas pressure and the most
appropriate high voltage supply on the sensor of the detector in order to
achieve the maximum amplification and better resolution. The detector is tested
for thermal and fast neutrons detection with a and a
neutron source. The atmospheric neutrons are successfully
measured from thermal up to several MeV, well separated from the cosmic ray
background. A comparison of the spherical proportional counter with the current
available neutron counters is also given.Comment: 7 pages, 10 figure
The discrimination capabilities of Micromegas detectors at low energy
The latest generation of Micromegas detectors show a good energy resolution,
spatial resolution and low threshold, which make them idoneous in low energy
applications. Two micromegas detectors have been built for dark matter
experiments: CAST, which uses a dipole magnet to convert axion into detectable
x-ray photons, and MIMAC, which aims to reconstruct the tracks of low energy
nuclear recoils in a mixture of CF4 and CHF3. These readouts have been
respectively built with the microbulk and bulk techniques, which show different
gain, electron transmission and energy resolutions. The detectors and the
operation conditions will be described in detail as well as their
discrimination capabilities for low energy photons will be discussed.Comment: To be published in the proceedings of the TIPP2011 conference
(Physics Procedia
Subclinical VZV reactivation in immunocompetent children hospitalized in the ICU associated with prolonged fever duration*
AbstractA prospective observational study was conducted to examine whether asymptomatic VZV reactivation occurs in immunocompetent children hospitalized in an ICU and its impact on clinical outcome. A secondary aim was to test the hypothesis that vaccinated children have a lower risk of reactivation than naturally infected children. Forty immunocompetent paediatric ICU patients and healthy controls were enrolled. Patients were prospectively followed for 28 days. Clinical data were collected and varicella exposure was recorded. Admission serum levels of TNF-a, cortisol and VZV-IgG were measured. Blood and saliva samples were collected for VZV-DNA detection via real-time PCR. As a comparison, the detection of HSV-DNA was also examined. Healthy children matched for age and varicella exposure type (infection or vaccination) were also included. VZV reactivation was observed in 17% (7/39) of children. Children with VZV reactivation had extended duration of fever (OR = 1.17; 95% CI, 1.02–1.34). None of the varicella-vaccinated children or healthy controls had detectable VZV-DNA in any blood or saliva samples examined. HSV-DNA was detected in saliva from 33% of ICU children and 2.6% of healthy controls. Among children with viral reactivation, typing revealed wild-type VZV and HSV-1. In conclusion, VZV reactivation occurs in immunocompetent children under severe stress and is associated with prolonged duration of fever
Lowering the background level and the energy threshold of Micromegas x-ray detectors for axion searches
Axion helioscopes search for solar axions by their conversion in x-rays in
the presence of high magnetic fields. The use of low background x-ray detectors
is an essential component contributing to the sensitivity of these searches. In
this work, we review the recent advances on Micromegas detectors used in the
CERN Axion Solar Telescope (CAST) and proposed for the future International
Axion Observatory (IAXO). The actual setup in CAST has achieved background
levels below 10 keV cm s, a factor 100 lower than
the first generation of Micromegas detectors. This reduction is based on active
and passive shielding techniques, the selection of radiopure materials, offline
discrimination techniques and the high granularity of the readout. We describe
in detail the background model of the detector, based on its operation at CAST
site and at the Canfranc Underground Laboratory (LSC), as well as on Geant4
simulations. The best levels currently achieved at LSC are low than 10
keV cm s and show good prospects for the application of
this technology in IAXO. Finally, we present some ideas and results for
reducing the energy threshold of these detectors below 1 keV, using
high-transparent windows, autotrigger electronics and studying the cluster
shape at different energies. As a high flux of axion-like-particles is expected
in this energy range, a sub-keV threshold detector could enlarge the physics
case of axion helioscopes.Comment: Proceedings of 3rd International Conference on Technology and
Instrumentation in Particle Physics (TIPP 2014
Micromegas operation in high pressure xenon: charge and scintillation readout
The operational characteristics of a Micromegas operating in pure xenon at
the pressure range of 1 to 10 bar are investigated. The maximum charge gain
achieved in each pressure is approximately constant, around 4x10^2, for xenon
pressures up to 5 bar and decreasing slowly above this pressure down to values
somewhat above 10^2 at 10 bar. The MM presents the highest gains for xenon
pressures above 4 bar, when compared to other micropattern gaseous multipliers.
The lowest energy resolution obtained for X-rays of 22.1 keV exhibits a steady
increase with pressure, from 12% at 1bar to about 32% at 10 bar. The effective
scintillation yield, defined as the number of photons exiting through the MM
mesh holes per primary electron produced in the conversion region was
calculated. This yield is about 2x10^2 photons per primary electron at 1 bar,
increasing to about 6x10^2 at 5 bar and, then, decreasing again to 2x10^2 at 10
bar. The readout of this scintillation by a suitable photosensor will result in
higher gains but with increased statistical fluctuations.Comment: 22 pages, 11 figure
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