206538240
A Shorter 146Sm Half-Life Measured and Implications for 146Sm-142Nd Chronology in the Solar System
A New Lease on Half-Life Radiometric dating relies on measuring the abundance of long-lived radionuclides relative to the abundance of their radiogenic decay products—a process determined by the original radionuclide's half-life. For primordial radionuclides that decay slowly, such as 146Sm decaying to 142Nd, this method provides the timing of some of the earliest processes in solar system history. Using accelerator mass spectrometry, Kinoshita et al. (p. 1614) provide a revised estimate for the 146Sm half-life of ∼68.7 million years, which is 30% shorter than the previously accepted value. This shorter half-life suggests that reductions need to be made in the estimated ages for differentiation of Earth's mantle and the solidification of the Moon's magma ocean and for other more recent processes. Mantle differentiation of Earth, the Moon, and Mars occurred earlier and over a shorter time scale than previously estimated. The extinct p-process nuclide 146Sm serves as an astrophysical and geochemical chronometer through measurements of isotopic anomalies of its α-decay daughter 142Nd. Based on analyses of 146Sm/147Sm α-activity and atom ratios, we determined the half-life of 146Sm to be 68 ± 7 (1σ) million years, which is shorter than the currently used value of 103 ± 5 million years. This half-life value implies a higher initial 146Sm abundance in the early solar system, (146Sm/144Sm)0 = 0.0094 ± 0.0005 (2σ), than previously estimated. Terrestrial, lunar, and martian planetary silicate mantle differentiation events dated with 146Sm-142Nd converge to a shorter time span and in general to earlier times, due to the combined effect of the new 146Sm half-life and (146Sm/144Sm)0 values.
109652400
Commissioning of the ATLAS reconstruction software with first data
The ATLAS detector is being commissioned using all types of physics data available: cosmic rays, beam-halo and beam-gas events. In addition to putting in place the trigger and data acquisition chains, commissioning of the full software chain is a main goal. This is interesting not only to ensure that the reconstruction, monitoring and simulation chains are ready to deal with LHC physics data, but also to understand the detector performance in view of achieving the physics requirements.
118399911
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.
19225231
Simple relations between mean passage times and Kramers' stationary rate.
The classical problem of the escape time of a metastable potential well in a thermal environment is generally studied by various quantities like Kramers' stationary escape rate, mean first passage time, nonlinear relaxation time, or mean last passage time. In addition, numerical simulations lead to the definition of other quantities as the long-time limit escape rate and the transient time. In this paper, we propose some simple analytical relations between all these quantities. In particular, we point out the hypothesis used to evaluate these various times in order to clarify their comparison and applicability, and show how average times include the transient time and the long-time limit of the escape rate.
120833641
Examining fine potential energy effects in high-energy fission dynamics
The potential energy surface plays a decisive role in nuclear fission. Together with inertia and viscosity, it influences the trajectory of the system, and the properties of the fission fragments result from the puzzling interplay between static and dynamical effects. A careful study on the influence of the parametrization of the potential energy landscape in heavy-ion-induced fission is performed. Dynamical calculations are done within the stochastic Langevin approach in a three-dimensional deformation space. Various prescriptions of the potential energy surface are considered, probing two different Liquid Drop models and the deformation dependence of the Wigner/congruence energy. A wide set of observables, including cross sections, particle multiplicities, and integral, as well as isotopic and isobaric, distributions of fission and evaporation products, is analyzed. Nuclei close to the Businaro-Gallone point are confirmed to be well suited for investigating the Liquid Drop parametrization, while the influence of the deformation-dependent Wigner/congruence energy is difficult to demonstrate unambiguously in fission at high excitation energies.
121241106
Isotopic production cross sections of the residual nuclei in spallation reactions induced by 136Xe projectiles on proton at 500 A MeV
Around 270 medium-mass residual nuclei, formed in spallation reactions induced by 136Xe projectiles impinging on a liquid hydrogen target at 500 A MeV, have been unambiguously identified at GSI using the magnetic spectrometer FRS. The individual production cross sections and the longitudinal momentum distributions have been determined with high accuracy. These data represent an important constraint for theoretical models describing spallation reactions. © 2013 Elsevier B.V.