69 research outputs found
Broad levels in O and their relevance for the astrophysical s-process
Levels in O affect the astrophysical s-process in two opposite ways.
The neutron production is enhanced by resonances in the
C(,)O reaction at excitation energies around 7 MeV in
O, and the number of available neutrons is reduced by low-lying
resonances in the O(,)O reaction corresponding to
levels in O with excitation energies of MeV. The present work uses
the F(,)O reaction to determine absolute widths of the
relevant levels in O. The results improve the uncertainties of the
previously adopted values and resolve a discrepancy between recent studies for
the level close to the threshold of the C(,)O
reaction. In addition, improved excitation energies and widths are provided for
several states in O up to excitation energies close to 8 MeV.Comment: 5 pages, 2 figures, Phys. Rev. C (in press
ATLAS monitored drift tube chambers for super-LHC
After the high-luminosity upgrade of the Large Hadron Collider (LHC) at CERN,
the ATLAS muon spectrometer is expected to work at 10 times increased
background rates of gammas and neutrons. This is challenging as the momentum
resolution of the spectrometer is expected to be 10 %. This requires a single
tube resolution of the muon drift tubes of 80 mum. At background rates around
1000 Hz/cm2 space charge effects will lead in the slow and non-linear AR:CO2 =
93:7 gas mixture to a degradation of the drift-tube spatial resolution. This
was studied before experimentally for gammas and low energetic neutrons. Almost
no information exists for fast neutrons. Therefore, we organized our studies
under the following aspects: - We investigated the influence of 11 MeV neutrons
on the position resolution of ATLAS MDT chambers. At flux densities between 4
and 16 kHz/cm2, almost no influence on the position resolution was found, it
degrades by only 10 mum at a detection efficiency of only 4*10-4. - We
investigated inert gas mixtures on fastness and linearity of their
position-drifttime (r-t) relation. At a reduction of the maximum drift time by
a factor of 2, the use of the present hardware and electronics might be
possible. For our experimental studies we used our Munich cosmic ray facility.
Two gas mixtures show almost identical position resolution as the standard gas.
- For spectrometer regions of highest background rates we contributed to the
investigation of newly developed 15 mm drift tubes. Position resolutions have
been measured as a function of gamma background rates between 0 and 1400
Hz/cm2. - Garfield simulations have been performed to simulate space charge
effects due to gamma irradiation. Results will be presented for the standard
geometry as well as for the new 15 mm drift tubes.Comment: 3 pages, 7 figures, conferenc
High Rate Proton Irradiation of 15mm Muon Drifttubes
Future LHC luminosity upgrades will significantly increase the amount of
background hits from photons, neutrons and protons in the detectors of the
ATLAS muon spectrometer. At the proposed LHC peak luminosity of 5*10^34
1/cm^2s, background hit rates of more than 10 kHz/cm^2 are expected in the
innermost forward region, leading to a loss of performance of the current
tracking chambers. Based on the ATLAS Monitored Drift Tube chambers, a new high
rate capable drift tube detecor using tubes with a reduced diameter of 15mm was
developed. To test the response to highly ionizing particles, a prototype
chamber of 46 15mm drift tubes was irradiated with a 20 MeV proton beam at the
tandem accelerator at the Maier-Leibnitz Laboratory, Munich. Three tubes in a
planar layer were irradiated while all other tubes were used for reconstruction
of cosmic muon tracks through irradiated and non-irradiated parts of the
chamber. To determine the rate capability of the 15mm drift-tubes we
investigated the effect of the proton hit rate on pulse height, efficiency and
spatial resolution of the cosmic muon signals
High-Rate Capable Floating Strip Micromegas
We report on the optimization of discharge insensitive floating strip
Micromegas (MICRO-MEsh GASeous) detectors, fit for use in high-energy muon
spectrometers. The suitability of these detectors for particle tracking is
shown in high-background environments and at very high particle fluxes up to
60MHz/cm. Measurement and simulation of the microscopic discharge behavior
have demonstrated the excellent discharge tolerance. A floating strip
Micromegas with an active area of 48cm50cm with 1920 copper anode
strips exhibits in 120GeV pion beams a spatial resolution of 50m at
detection efficiencies above 95%. Pulse height, spatial resolution and
detection efficiency are homogeneous over the detector. Reconstruction of
particle track inclination in a single detector plane is discussed, optimum
angular resolutions below are observed. Systematic deviations of this
TPC-method are fully understood. The reconstruction capabilities for
minimum ionizing muons are investigated in a 6.4cm6.4cm floating strip
Micromegas under intense background irradiation of the whole active area with
20MeV protons at a rate of 550kHz. The spatial resolution for muons is not
distorted by space charge effects. A 6.4cm6.4cm floating strip
Micromegas doublet with low material budget is investigated in highly ionizing
proton and carbon ion beams at particle rates between 2MHz and 2GHz. Stable
operation up to the highest rates is observed, spatial resolution, detection
efficiencies, the multi-hit and high-rate capability are discussed.Comment: Presented at ICHEP 2014, accepted for publication in Nuclear Physics
B Proceedings Supplement
Performance of Drift-Tube Detectors at High Counting Rates for High-Luminosity LHC Upgrades
The performance of pressurized drift-tube detectors at very high background
rates has been studied at the Gamma Irradiation Facility (GIF) at CERN and in
an intense 20 MeV proton beam at the Munich Van-der-Graaf tandem accelerator
for applications in large-area precision muon tracking at high-luminosity
upgrades of the Large Hadron Collider (LHC). The ATLAS muon drifttube (MDT)
chambers with 30 mm tube diameter have been designed to cope with and neutron
background hit rates of up to 500 Hz/square cm. Background rates of up to 14
kHz/square cm are expected at LHC upgrades. The test results with standard MDT
readout electronics show that the reduction of the drift-tube diameter to 15
mm, while leaving the operating parameters unchanged, vastly increases the rate
capability well beyond the requirements. The development of new small-diameter
muon drift-tube (sMDT) chambers for LHC upgrades is completed. Further
improvements of tracking efficiency and spatial resolution at high counting
rates will be achieved with upgraded readout electronics employing improved
signal shaping for high counting rates
Could the GSI Oscillations be Observed in a Standard Electron Capture Decay Experiment?
The electron-capture decay of 180Re has been investigated to search for
oscillations in the decay probability as reported from a recent measurement at
GSI, Darmstadt. The production period was kept short compared to the reported
oscillation period. No such oscillation was observed, indicating that the
reported oscillations would not have been observable in a conventional
experiment with radioactive atoms in a solid environment but must have to do
with the unique conditions in the GSI experiment where hydrogen-like ions are
moving independently in a storage ring and decaying directly by a true two-body
decay to a long-lived (ground-) state. Our finding could restrict possible
theoretical interpretations of the oscillations.Comment: 6 pages, 4 figure
A gas analyzer for the internal polarized target of the HERMES experiment
A gas analyzer has been developed for the internal polarized target of the HERMES experiment at DESY in order to determine the relative amount of atomic and molecular hydrogen or deuterium in a gas sample. The precise quantitative knowledge of this ratio is crucial because the nucleons in atoms and molecules contribute differently to the average nuclear polarization of the target gas. A new calibration technique used to derive the relative sensitivity to atoms and molecules is presented. As an example, it is shown how the gas analyzer is used within the HERMES environment to divide the molecules in the gas sample into an unpolarized and a potentially polarized fraction
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