196 research outputs found
Sealed operation, and circulation and purification of gas in the HARPO TPC
HARPO is a time projection chamber (TPC) demonstrator of a gamma-ray
telescope and polarimeter in the MeV-GeV range, for a future space mission. We
present the evolution of the TPC performance over a five month sealed-mode
operation, by the analysis of cosmic-ray data, followed by the fast and
complete recovery of the initial gas properties using a lightweight gas
circulation and purification system.Comment: Proceedings_MPGD2015, EPJ Web of Conference
Micromegas TPC studies at high magnetic fields using the charge dispersion signal
The International Linear Collider (ILC) Time Projection Chamber (TPC)
transverse space-point resolution goal is 100 microns for all tracks including
stiff 90 degree tracks with the full 2 meter drift. A Micro Pattern Gas
Detector (MPGD) readout TPC can achieve the target resolution with existing
techniques using 1 mm or narrower pads at the expense of increased detector
cost and complexity. The new MPGD readout technique of charge dispersion can
achieve good resolution without resorting to narrow pads. This has been
demonstrated previously for 2 mm x 6 mm pads with GEMs and Micromegas in cosmic
ray tests and in a KEK beam test in a 1 Tesla magnet. We have recently tested a
Micromegas-TPC using the charge dispersion readout concept in a high field
super-conducting magnet at DESY. The measured Micromegas gain was found to be
constant within 0.5% for magnetic fields up to 5 Tesla. With the strong
suppression of transverse diffusion at high magnetic fields, we measure a flat
50 micron resolution at 5 Tesla over the full 15 cm drift length of our
prototype TPC.Comment: 7 pages, 3 figure
Measurement of 1.7 to 74 MeV polarised gamma rays with the HARPO TPC
Current {\gamma}-ray telescopes based on photon conversions to
electron-positron pairs, such as Fermi, use tungsten converters. They suffer of
limited angular resolution at low energies, and their sensitivity drops below 1
GeV. The low multiple scattering in a gaseous detector gives access to higher
angular resolution in the MeV-GeV range, and to the linear polarisation of the
photons through the azimuthal angle of the electron-positron pair.
HARPO is an R&D program to characterise the operation of a TPC (Time
Projection Chamber) as a high angular-resolution and sensitivity telescope and
polarimeter for {\gamma} rays from cosmic sources. It represents a first step
towards a future space instrument. A 30 cm cubic TPC demonstrator was built,
and filled with 2 bar argon-based gas. It was put in a polarised {\gamma}-ray
beam at the NewSUBARU accelerator in Japan in November 2014. Data were taken at
different photon energies from 1.7 MeV to 74 MeV, and with different
polarisation configurations. The electronics setup is described, with an
emphasis on the trigger system. The event reconstruction algorithm is quickly
described, and preliminary measurements of the polarisation of 11 MeVphotons
are shown.Comment: Proceedings VCI201
New determinations of gamma-ray line intensities of the Ep = 550 keV and Ep = 1747 keV resonances of the 13-C(p,gamma)14-N reaction
Gamma-ray angular distributions for the resonances at Ep = 550 keV and 1747
keV of the radiative capture reaction 13-C(p,g)14-N have been measured, using
intense proton beams on isotopically pure 13-C targets. Relative intensities
for the strongest transitions were extracted with an accuracy of typically five
per cent, making these resonances new useful gamma-ray standards for efficiency
calibration in the energy range Egamma = 1.6 to 9 MeV.Comment: 17 pages, 6 figures, Nuclear Instruments and Methods, Sec. A,
accepte
Characterization of microbulk detectors in argon- and neon-based mixtures
A recent Micromegas manufacturing technique, so called Microbulk, has been
developed, improving the uniformity and stability of this kind of detectors.
Excellent energy resolutions have been obtained, reaching values as low as 11%
FWHM at 5.9 keV in Ar+5%iC4H10. This detector has other advantages like its
flexible structure, low material budget and high radio-purity. Two microbulk
detectors with gaps of 50 and 25 um have been characterized in argon- and
neon-based mixtures with ethane, isobutane and cyclohexane. The results will be
presented and discussed. The gain curves have been fitted to the Rose-Korff
gain model and dependences of the electron mean free path and the threshold
energy for ionization have been obtained. The possible relation between these
two parameters and the energy resolution will be also discussed.Comment: Submitted to the Journal of Instrumentatio
Supercooled liquid water cloud observed, analysed, and modelled at the top of the planetary boundary layer above Dome C, Antarctica
Abstract. A comprehensive analysis of the water budget over the Dome C (Concordia,
Antarctica) station has been performed during the austral summer 2018–2019
as part of the Year of Polar Prediction (YOPP) international campaign. Thin
(∼100 m deep) supercooled liquid water (SLW) clouds have been
detected and analysed using remotely sensed observations at the station
(tropospheric depolarization lidar, the H2O Antarctica Microwave Stratospheric and Tropospheric
Radiometer (HAMSTRAD), net
surface radiation from the Baseline Surface Radiation Network (BSRN)), radiosondes, and satellite observations (CALIOP, Cloud-Aerosol LIdar with Orthogonal Polarization/CALIPSO, Cloud Aerosol Lidar and Infrared
Pathfinder Satellite Observations) combined with a specific
configuration of the numerical weather prediction model: ARPEGE-SH (Action
de Recherche Petite Echelle Grande Echelle – Southern Hemisphere). The
analysis shows that SLW clouds were present from November to March, with the
greatest frequency occurring in December and January when ∼50 % of the days in summer time exhibited SLW clouds for at least 1 h. Two case studies are used to illustrate this phenomenon. On 24 December 2018, the atmospheric planetary boundary layer (PBL) evolved
following a typical diurnal variation, which is to say with a warm and dry
mixing layer at local noon thicker than the cold and dry stable layer at
local midnight. Our study showed that the SLW clouds were observed at Dome C
within the entrainment and the capping inversion zones at the top of the
PBL. ARPEGE-SH was not able to correctly estimate the ratio between liquid
and solid water inside the clouds with the liquid water path (LWP) strongly
underestimated by a factor of 1000 compared to observations. The lack of
simulated SLW in the model impacted the net surface radiation that was 20–30 W m−2 higher in the BSRN observations than in the ARPEGE-SH
calculations, mainly attributable to the BSRN longwave downward surface
radiation being 50 W m−2 greater than that of ARPEGE-SH. The second
case study took place on 20 December 2018, when a warm and wet episode
impacted the PBL with no clear diurnal cycle of the PBL top. SLW cloud
appearance within the entrainment and capping inversion zones coincided with
the warm and wet event. The amount of liquid water measured by HAMSTRAD was
∼20 times greater in this perturbed PBL than in the typical
PBL. Since ARPEGE-SH was not able to accurately reproduce these SLW clouds,
the discrepancy between the observed and calculated net surface radiation
was even greater than in the typical PBL case, reaching +50 W m−2,
mainly attributable to the downwelling longwave surface radiation from BSRN
being 100 W m−2 greater than that of ARPEGE-SH. The model was then run
with a new partition function favouring liquid water for temperatures below
−20 down to −40 ∘C. In this test mode, ARPEGE-SH has
been able to generate SLW clouds with modelled LWP and net surface radiation
consistent with observations during the typical case, whereas, during the
perturbed case, the modelled LWP was 10 times less than the observations and
the modelled net surface radiation remained lower than the observations by
∼50 W m−2. Accurately modelling the presence of SLW
clouds appears crucial to correctly simulate the surface energy budget over
the Antarctic Plateau
Micromegas detector developments for MIMAC
The aim of the MIMAC project is to detect non-baryonic Dark Matter with a
directional TPC. The recent Micromegas efforts towards building a large size
detector will be described, in particular the characterization measurements of
a prototype detector of 10 10 cm with a 2 dimensional readout
plane. Track reconstruction with alpha particles will be shown.Comment: 8 pages, 7 figures Proceedings of the 3rd International conference on
Directional Detection of Dark Matter (CYGNUS 2011), Aussois, France, 8-10
June 2011; corrections on author affiliation
MICROMEGAS chambers for hadronic calorimetry at a future linear collider
Prototypes of MICROMEGAS chambers, using bulk technology and analog readout,
with 1x1cm2 readout segmentation have been built and tested. Measurements in
Ar/iC4H10 (95/5) and Ar/CO2 (80/20) are reported. The dependency of the
prototypes gas gain versus pressure, gas temperature and amplification gap
thickness variations has been measured with an 55Fe source and a method for
temperature and pressure correction of data is presented. A stack of four
chambers has been tested in 200GeV/c and 7GeV/c muon and pion beams
respectively. Measurements of response uniformity, detection efficiency and hit
multiplicity are reported. A bulk MICROMEGAS prototype with embedded digital
readout electronics has been assembled and tested. The chamber layout and first
results are presented
A new chemistry-climate tropospheric and stratospheric model MOCAGE-Climat: evaluation of the present-day climatology and sensitivity to surface processes
International audienceWe present the chemistry-climate configuration of the Météo-France Chemistry and Transport Model, MOCAGE-Climat. MOCAGE-Climat is a state-of-the-art model that simulates the global distribution of ozone and its precursors (82 chemical species) both in the troposphere and the stratosphere, up to the mid-mesosphere (~70 km). Surface processes (emissions, dry deposition), convection, and scavenging are explicitly described in the model that has been driven by the ECMWF operational analyses of the period 2000–2005, on T21 and T42 horizontal grids and 60 hybrid vertical levels, with and without a procedure that reduces calculations in the boundary layer, and with on-line or climatological deposition velocities. Model outputs have been compared to available observations, both from satellites (TOMS, HALOE, SMR, SCIAMACHY, MOPITT) and in-situ instrument measurements (ozone sondes, MOZAIC and aircraft campaigns) at climatological timescales. The distribution of long-lived species is in fair agreement with observations in the stratosphere putting apart shortcomings linked to the large-scale circulation. The variability of the ozone column, both spatially and temporarily, is satisfactory. However, the too fast Brewer-Dobson circulation accumulates too much ozone in the lower to mid-stratosphere at the end of winter. Ozone in the UTLS region does not show any systematic bias. In the troposphere better agreement with ozone sonde measurements is obtained at mid and high latitudes than in the tropics and differences with observations are the lowest in summer. Simulations using a simplified boundary layer lead to ozone differences between the model and the observations up to the mid-troposphere. NOx in the lowest troposphere is in general overestimated, especially in the winter months over the northern hemisphere, which might result from a positive bias in OH. Dry deposition fluxes of O3 and nitrogen species are within the range of values reported by recent inter-comparison model exercises. The use of climatological deposition velocities versus deposition velocities calculated on-line had greatest impact on HNO3 and NO2 in the troposphere
The INTEGRAL spectrometer SPI: performance of point-source data analysis
The performance of the SPI point-source data analysis system is assessed using a combination of simulations and of observations gathered during the first year of INTEGRAL operations. External error estimates are derived by comparing source positions and fluxes obtained from independent analyses. When the source detection significance provided by the spiros imaging reconstruction program increases from ∼10 to ∼100, the errors decrease as the inverse of the detection significance, with values from ∼10 to ∼1 arcmin in positions, and from ∼10 to ∼1 per cent in relative flux. These errors are dominated by Poisson counting noise. Our error estimates are consistent with those provided by the spiros program. With higher detection significance, the accuracy is ultimately limited to ∼0.5 arcmin in position and ∼1 per cent in relative flux by other types of errors. In these cases, spiros underestimates the true errors as it only takes into account the Poisson counting noise. At the low signal-to-noise ratio end, spiros is also too optimistic, and the number of spurious detections is significantly higher than would be expected from statistical noise fluctuations only. The analysis results do not depend significantly on the target off-axis angle, or on the number of pointings considered, provided that this number is larger than ∼15. Realistic source confusion tests are carried out by adding simulated data to the observation of the Crab nebula and pulsar. Reliable flux values can be obtained for sources separated by angles as small as 0.5 deg if their positions are known and kept fixed in the data deconvolution process. However, when spiros is searching for best source positions, if two sources are separated by less than ∼2 deg, it may only find a single spurious source located between the real ones (with a flux equal to the sum of the fluxes). Finally, users of the spiros program can find a number of important recommendations in the conclusio
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