Broad levels in 17^{17}O and their relevance for the astrophysical s-process

Levels in $^{17}$O affect the astrophysical s-process in two opposite ways. The neutron production is enhanced by resonances in the $^{13}$C($\alpha$,$n$)$^{16}$O reaction at excitation energies around 7 MeV in $^{17}$O, and the number of available neutrons is reduced by low-lying resonances in the $^{16}$O($n$,$\gamma$)$^{17}$O reaction corresponding to levels in $^{17}$O with excitation energies of $4-5$ MeV. The present work uses the $^{19}$F($d$,$\alpha$)$^{17}$O reaction to determine absolute widths of the relevant levels in $^{17}$O. The results improve the uncertainties of the previously adopted values and resolve a discrepancy between recent studies for the $1/2^+$ level close to the threshold of the $^{13}$C($\alpha$,$n$)$^{16}$O reaction. In addition, improved excitation energies and widths are provided for several states in $^{17}$O up to excitation energies close to 8 MeV.Comment: 5 pages, 2 figures, Phys. Rev. C (in press

Long-term irradiation of an ATLAS NSW SM2 Micromegas quadruplet using an AmBe neutron source

The NSW Micromegas chambers in the ATLAS forward muon spectrometer are subject to background rates of 15-20 kHz/cm under HL-LHC conditions. The innermost detector area closest to the LHC beam pipe will accumulate a charge of C cm−2 year−1 under these rates. Due to the late change of the detector gas from non-aging Ar:CO 93:7 vol% to the more HV stable ternary mixture Ar:CO :iC H10 93:5:2 vol% and the known vulnerability of wire chambers to Hydrocarbon-containing gas mixtures a three-year-long aging study has been performed. An SM2 series module of the NSW Micromegas quadruplets was irradiated at LMU in Garching/Munich using a 10 GBq AmBe neutron source emitting MeV n/s as well as 4.4 MeV gammas/s and 60 keV gammas/s. The SM2 chamber was irradiated in a region of several 10 cm in size with a dose rate well exceeding the HL-LHC equivalent local charge densities for three years. In between the irradiation periods the performance of the SM2 chamber regarding spatial resolution and efficiency on cosmic muon tracking was tested several times. We report on the irradiation and the performance studies of the SM2 Micromegas quadruplet and conclude that no sign of loss in performance has been observed in contradiction to an earlier experience using drift tube wire chambers

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

Long-term irradiation study of sMDT drift tubes with an integrated accumulated charge of 60 C per wire using beta-electrons from a 90Sr source

Two ATLAS sMDT drift tubes have been irradiated for almost 1 year using a 90Sr beta-decay source. An integrated charge of 62 C has been accumulated on each of both anode wires over an anode-wire region of about 7.5 cm. Taking into account the intensity distribution of the irradiation corresponds to a maximum accumulated line charge density of about 14 C/cm. At the innermost position of the ATLAS forward muon spectrometer 10 C/cm are expected for 10 years of high-luminosity LHC operation and for this detector type at gas gain 20000. To investigate potential outgassing, the endplug region of the drift-tubes, where no gas amplification occurs, was irradiated additionally using about half the beta-electrons emitted from the source. The other beta-electrons were irradiating an active part of the gas volume for monitoring purpose. During four months the endplugs were irradiated by 5 C/cm equivalent. All observed anode currents were very stable over the whole period of irradiation and thus no sign of deterioration in the performance of both drift tubes was observed. This indicates that no ageing effects occurred and that no performance loss due to outgassing of any plastic surfaces has been observed. All components that have potential contact to the detector gas Ar:CO with a mixture of 93:7 (percent volume) have been carefully and properly chosen. The required cleanliness of all tube- and gas components has been achieved during construction and operation of these drift tubes

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$^2$. Measurement and simulation of the microscopic discharge behavior have demonstrated the excellent discharge tolerance. A floating strip Micromegas with an active area of 48cm$\times$50cm with 1920 copper anode strips exhibits in 120GeV pion beams a spatial resolution of 50$\mu$m 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 $5^\circ$ are observed. Systematic deviations of this $\mu$TPC-method are fully understood. The reconstruction capabilities for minimum ionizing muons are investigated in a 6.4cm$\times$6.4cm floating strip Micromegas under intense background irradiation of the whole active area with 20MeV protons at a rate of 550kHz. The spatial resolution for muons is not distorted by space charge effects. A 6.4cm$\times$6.4cm 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