A Study of the LXeGRIT Detection Efficiency for MeV Gamma-Rays during the 2000 Balloon Flight Campaign

LXeGRIT - Liquid Xenon Gamma-Ray Imaging Telescope - is the first prototype of a Compton telescope for \MeV \g-ray astrophysics based on a LXe time projection chamber. One of the most relevant figures of merit for a Compton telescope is the detection efficiency for \g-rays, which depends on diverse contributions such as detector geometry and passive materials, trigger efficiency, dead time, etc. A detailed study of the efficiency of the LXeGRIT instrument, based both on laboratory measurements and Monte Carlo simulations, is presented in this paper.Comment: 20 pages, 15 figures; submitted to NIM

The Mechanism of Hydrogen Evolution During Anodic Polarization of Aluminium

AbstractIn this work, a model to account for the superfluous hydrogen evolution mechanism during anodic polarization of Al is proposed. The model is based on the assumption that the simultaneous presence of an anodic current, produced at some distance from the corrosion front, and of conditions that promote local depassivation such as, for example, the presence of chlorides, induces localized rupture of the pre-existing oxide/hydroxide film. This local depassivation leads to the formation of regions where the electrolyte is either in contact with the metal or separated only by a poorly protective salt film. Here, due to the large potential difference available, hydrogen evolves. The model is validated via electrochemical polarization assisted with in-situ image visualization of pure Al and Al/Cu system in experimental conditions that promote stable oxide film formation or induce local film rupture. Hydrogen streams from the active corrosion sites, increasing upon anodic polarization, are observed only in the presence of a depassivating media (chloride) and a remote cathodic current, provided either via galvanic coupling to copper or via external polarization

Compton Imaging of MeV Gamma-Rays with the Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT)

The Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) is the first realization of a liquid xenon time projection chamber for Compton imaging of MeV gamma-ray sources in astrophysics. By measuring the energy deposit and the three spatial coordinates of individual gamma-ray scattering points, the location of the source in the sky is inferred with Compton kinematics reconstruction. The angular resolution is determined by the detector's energy and spatial resolutions, as well as by the separation in space between the first and second scattering. The imaging response of LXeGRIT was established with gamma-rays from radioactive sources, during calibration and integration at the Columbia Astrophysics Laboratory, prior to the 2000 balloon flight mission. In this paper we describe in detail the various steps involved in imaging sources with LXeGRIT and present experimental results on angular resolution and other parameters which characterize its performance as a Compton telescope.Comment: 22 pages, 20 figures, submitted to NIM

Correlation between electrochemical impedance measurements and corrosion rate of magnesium investigated by real-time hydrogen measurement and optical imaging

The corrosion behaviour of magnesium in chloride-containing aqueous environment was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) performed simultaneously with real-time hydrogen evolution measurements and optical imaging of the corroding surface. The potentiodynamic investigation revealed substantial deviations from linearity in close proximity of the corrosion potential. In particular, differences in the slope of the current/potential curves were observed for small polarizations above or below the corrosion potential. These observations, suggest that the usual method based on the use of the Stern–Geary equation to convert a value of resistance into a value of corrosion current is inadequate. Nonetheless, a very good correlation between values of resistances estimated by EIS and corrosion currents obtained from real-time hydrogen measurement was found. Real-time hydrogen measurement also enabled, for the first time, direct measurement of an ‘apparent’ Stern–Geary coefficient for magnesium. In order to rationalize the complex behaviours experimentally observed, an electrical model for the corroding magnesium surface is presented

Correlation between electrochemical impedance measurements and corrosion rate of magnesium investigated by real-time hydrogen measurement and optical imaging

The corrosion behaviour of magnesium in chloride-containing aqueous environment was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) performed simultaneously with real-time hydrogen evolution measurements and optical imaging of the corroding surface. The potentiodynamic investigation revealed substantial deviations from linearity in close proximity of the corrosion potential. In particular, differences in the slope of the current/potential curves were observed for small polarizations above or below the corrosion potential. These observations, suggest that the usual method based on the use of the Stern–Geary equation to convert a value of resistance into a value of corrosion current is inadequate. Nonetheless, a very good correlation between values of resistances estimated by EIS and corrosion currents obtained from real-time hydrogen measurement was found. Real-time hydrogen measurement also enabled, for the first time, direct measurement of an 'apparent' Stern–Geary coefficient for magnesium. In order to rationalize the complex behaviours experimentally observed, an electrical model for the corroding magnesium surface is presented

a mathematical description accounting for the superfluous hydrogen evolution and the inductive behaviour observed during electrochemical measurements on magnesium

Abstract When electrochemical techniques are used to probe the surface of corroding magnesium with the aim of obtaining quantitative information on the corrosion process, two peculiarities are generally observed: i) with anodic polarization, the rate of hydrogen evolution increases instead of decreasing and ii) during electrochemical impedance spectroscopy measurements, an inductive contribution is often observed at the low-frequency end of the spectra. The presence of these two phenomena clearly has an impact on the methodology that should be applied to correctly estimate corrosion rates from electrochemical data. The aim of this work is to provide a general mathematical description of the corroding magnesium surface that, under minimal a priori assumptions regarding the reaction kinetics, can account simultaneously for both superfluous hydrogen evolution and inductive response. The mathematical results are consistent with the suggestion that the superfluous hydrogen evolution is mainly related to the increase of the surface of the active corrosion front during anodic polarization. Further, the obtained results show that the inductive response is expected when, at the corrosion front, oxidation of magnesium proceeds faster than hydrogen evolution

A novel germline mutation of PDGFR-β might be associated with clinical response of colorectal cancer to regorafenib

This is the first description of a germline mutation of the PDGFR-β gene, that correlates with response to regorafenib in a patient with metastatic colorectal cance