AXTAR: Mission Design Concept

The Advanced X-ray Timing Array (AXTAR) is a mission concept for X-ray timing of compact objects that combines very large collecting area, broadband spectral coverage, high time resolution, highly flexible scheduling, and an ability to respond promptly to time-critical targets of opportunity. It is optimized for submillisecond timing of bright Galactic X-ray sources in order to study phenomena at the natural time scales of neutron star surfaces and black hole event horizons, thus probing the physics of ultradense matter, strongly curved spacetimes, and intense magnetic fields. AXTAR's main instrument, the Large Area Timing Array (LATA) is a collimated instrument with 2-50 keV coverage and over 3 square meters effective area. The LATA is made up of an array of supermodules that house 2-mm thick silicon pixel detectors. AXTAR will provide a significant improvement in effective area (a factor of 7 at 4 keV and a factor of 36 at 30 keV) over the RXTE PCA. AXTAR will also carry a sensitive Sky Monitor (SM) that acts as a trigger for pointed observations of X-ray transients in addition to providing high duty cycle monitoring of the X-ray sky. We review the science goals and technical concept for AXTAR and present results from a preliminary mission design study.Comment: 19 pages, 10 figures, to be published in Space Telescopes and Instrumentation 2010: Ultraviolet to Gamma Ray, Proceedings of SPIE Volume 773

Meter-scale spark X-ray spectrumstatistics

X-ray emission by sparks implies bremsstrahlung from a population of energetic electrons, but the details of this process remain a mystery. We present detailed statistical analysis of X-ray spectra detected by multiple detectors during sparks produced by 1 MV negative high-voltage pulses with 1 $\mu$s risetime. With over 900 shots, we statistically analyze the signals, assuming that the distribution of spark X-ray fluence behaves as a power law and that the energy spectrum of X-rays detectable after traversing $\sim$2 m of air and a thin aluminum shield is exponential. We then determine the parameters of those distributions by fitting cumulative distribution functions to the observations. The fit results match the observations very well if the mean of the exponential X-ray energy distribution is 86 $\pm$ 7 keV and the spark X-ray fluence power law distribution has index -1.29 $\pm$ 0.04 and spans at least 3 orders of magnitude in fluence

A quantum-mechanical perspective on linear response theory within polarizable embedding

The derivation of linear response theory within polarizable embedding is carried out from a rigorous quantum-mechanical treatment of a composite system. Two different subsystem decompositions (symmetric and nonsymmetric) of the linear response function are presented, and the pole structures as well as residues of the individual terms are analyzed and discussed. This theoretical analysis clarifies which form of the response function to use in polarizable embedding, and we highlight complications in separating out subsystem contributions to molecular properties. For example, based on the nonsymmetric decomposition of the complex linear response function, we derive conservation laws for integrated absorption cross sections, providing a solid basis for proper calculations of the intersubsystem intensity borrowing inherent to coupled subsystems and how that can lead to negative subsystem intensities. We finally identify steps and approximations required to achieve the transition from a quantum-mechanical description of the composite system to polarizable embedding with a classical treatment of the environment, thus providing a thorough justification for the descriptions used in polarizable embedding models

Aerospace Medicine and Biology: A continuing bibliography with indexes, supplement 145

This bibliography lists 301 reports, articles, and other documents introduced into the NASA scientific and technical information system in August 1975

Developing Time-Resolved Synchrotron Infrared Spectroscopy for Spectroelectrochemical Measurements

This thesis details my work in developing spectroelectrochemical platforms utilizing synchrotron infrared radiation (SIR) and time-resolved FTIR to study the kinetics of electrochemical reactions on second to millisecond scale with high chemical sensitivity. It will cover development of spectroelectrochemical cells and procedures compatible with the mid-IR beamline at the Canadian Light Source to study irreversible electrocatalytic processes with rapid scan FTIR in reflection mode. The thesis demonstrates application of SIR-based rapid scan FTIR to spatially map catalytic activity on heterogenous PtNi electrode and provides proof-of-principle for its capability for combinatorial screening for binary electrocatalysts. The thesis discusses the development of time-resolved step scan FTIR in attenuated total reflection - surface enhancing infrared absorption spectroscopy configuration (ATR-SEIRAS) to improve signal-to-noise of the measurement for increased detection limits and time-resolution, before demonstrating the utility of the developed step scan ATR-SEIRAS platform by investigation of the kinetics of conformational changes within self-assembled monolayers (SAM) of ferrocene alkanethiols. Overall, the work described in this thesis outlines the advancement of SIR-based spectroelectrochemical platforms within the group to a point, where they can be directly applied to investigation of dynamics processes within electrochemical reaction

Developing Time-Resolved Synchrotron Infrared Spectroscopy for Spectroelectrochemical Measurements

This thesis details my work in developing spectroelectrochemical platforms utilizing synchrotron infrared radiation (SIR) and time-resolved FTIR to study the kinetics of electrochemical reactions on second to millisecond scale with high chemical sensitivity. It will cover development of spectroelectrochemical cells and procedures compatible with the mid-IR beamline at the Canadian Light Source to study irreversible electrocatalytic processes with rapid scan FTIR in reflection mode. The thesis demonstrates application of SIR-based rapid scan FTIR to spatially map catalytic activity on heterogenous PtNi electrode and provides proof-of-principle for its capability for combinatorial screening for binary electrocatalysts. The thesis discusses the development of time-resolved step scan FTIR in attenuated total reflection - surface enhancing infrared absorption spectroscopy configuration (ATR-SEIRAS) to improve signal-to-noise of the measurement for increased detection limits and time-resolution, before demonstrating the utility of the developed step scan ATR-SEIRAS platform by investigation of the kinetics of conformational changes within self-assembled monolayers (SAM) of ferrocene alkanethiols. Overall, the work described in this thesis outlines the advancement of SIR-based spectroelectrochemical platforms within the group to a point, where they can be directly applied to investigation of dynamics processes within electrochemical reaction