Supernovae and Positron Annihilation

Radioactive nuclei, especially those created in SN explosion, have long been suggested to be important contributors of galactic positrons. In this paper we describe the findings of three independent OSSE/SMM/TGRS studies of positron annihilation radiation, demonstrating that the three studies are largely in agreement as to the distribution of galactic annihilation radiation. We then assess the predicted yields and distributions of SN-synthesized radionuclei, determining that they are marginally compatible with the findings of the annihilation radiation studies.Comment: 7 pages, accepted for publication in New Astronomy Reviews (Astronomy with Radioactivites III

An Emergentist Account of Collective Cognition in Collaborative Problem Solving

As a first step toward an emergentist theory of collective cognition in collaborative problem solving, we present a proto-theoretical account of how one might conceive and model the intersubjective processes that organize collective cognition into one or another--convergent, divergent, or tensive--cognitive regime. To explore the sufficiency of our emergentist proposal we instantiate a minimalist model of intersubjective convergence and simulate the tuning of collective cognition using data from an empirical study of small-group, collaborative problem solving. Using the results of this empirical simulation, we test a number of preliminary hypotheses with regard to patterns of interaction, how those patterns affect a cognitive regime, and how that cognitive regime affects the efficacy of a problem-solving group

Both Ligand- and Cell-Specific Parameters Control Ligand Agonism in a Kinetic Model of G Protein–Coupled Receptor Signaling

G protein–coupled receptors (GPCRs) exist in multiple dynamic states (e.g., ligand-bound, inactive, G protein–coupled) that influence G protein activation and ultimately response generation. In quantitative models of GPCR signaling that incorporate these varied states, parameter values are often uncharacterized or varied over large ranges, making identification of important parameters and signaling outcomes difficult to intuit. Here we identify the ligand- and cell-specific parameters that are important determinants of cell-response behavior in a dynamic model of GPCR signaling using parameter variation and sensitivity analysis. The character of response (i.e., positive/neutral/inverse agonism) is, not surprisingly, significantly influenced by a ligand's ability to bias the receptor into an active conformation. We also find that several cell-specific parameters, including the ratio of active to inactive receptor species, the rate constant for G protein activation, and expression levels of receptors and G proteins also dramatically influence agonism. Expressing either receptor or G protein in numbers several fold above or below endogenous levels may result in system behavior inconsistent with that measured in endogenous systems. Finally, small variations in cell-specific parameters identified by sensitivity analysis as significant determinants of response behavior are found to change ligand-induced responses from positive to negative, a phenomenon termed protean agonism. Our findings offer an explanation for protean agonism reported in β2-adrenergic and α2A-adrenergic receptor systems

A Cryogenic Infrared Calibration Target

A compact cryogenic calibration target is presented that has a peak diffuse reflectance, $R \le 0.003$, from $800-4,800\,{\rm cm}^{-1}$ $(12-2\,\mu$m). Upon expanding the spectral range under consideration to $400-10,000\,{\rm cm}^{-1}$ $(25-1\,\mu$m) the observed performance gracefully degrades to $R \le 0.02$ at the band edges. In the implementation described, a high-thermal-conductivity metallic substrate is textured with a pyramidal tiling and subsequently coated with a thin lossy dielectric coating that enables high absorption and thermal uniformity across the target. The resulting target assembly is lightweight, has a low-geometric profile, and has survived repeated thermal cycling from room temperature to $\sim4\,$K. Basic design considerations, governing equations, and test data for realizing the structure described are provided. The optical properties of selected absorptive materials -- Acktar Fractal Black, Aeroglaze Z306, and Stycast 2850 FT epoxy loaded with stainless steel powder -- are characterized and presented

On the morphology of the electron-positron annihilation emission as seen by SPI/INTEGRAL

The 511 keV positron annihilation emission remains a mysterious component of the high energy emission of our Galaxy. Its study was one of the key scientific objective of the SPI spectrometer on-board the INTEGRAL satellite. In fact, a lot of observing time has been dedicated to the Galactic disk with a particular emphasis on the central region. A crucial issue in such an analysis concerns the reduction technique used to treat this huge quantity of data, and more particularly the background modeling. Our method, after validation through a variety of tests, is based on detector pattern determination per ~6 month periods, together with a normalisation variable on a few hour timescale. The Galactic bulge is detected at a level of ~70 sigma allowing more detailed investigations. The main result is that the bulge morphology can be modelled with two axisymmetric Gaussians of 3.2 deg. and 11.8 deg. FWHM and respective fluxes of 2.5 and 5.4 x 10^-4 photons/(cm^2.s^1). We found a possible shift of the bulge centre towards negative longitude at l=-0.6 +/- 0.2 degrees. In addition to the bulge, a more extended structure is detected significantly with flux ranging from 1.7 to 2.9 x10^-3 photons/(cm^2.s^1) depending on its assumed geometry (pure disk or disk plus halo). The disk emission is also found to be symmetric within the limits of the statistical errors.Comment: This paper has 12 pages and 14 figures. Accepted for publication by the Astrophysical Journa

Low Energy Smart Phone for the Homeless

For our senior design project, we conducted research on the Android mobile operating system, looking for ways to improve it for the homeless. The problem with the current system is that it is designed for wealthier consumers, while less fortunate consumers such as the homeless have different requirements for them. In our researched, we attemepted to reconfigure the android operating system so that it would be more tailored to the homeless. We first researched what features the homeless value in a mobile device. We then determined how we could improve the battery life at a user configuration level based on the requirements of the homeless. Following these initial improvements, we determined how we could improve the battery life of the android system by modifying the source code. We found that more conservative use of the processor has a noticable affect on the systems battery life. We recommend developing a version of Android that uses the processor more conservatively by default to save battery life

Monte Carlo calibration of the SMM gamma ray spectrometer for high energy gamma rays and neutrons

The Gamma Ray Spectrometer (GRS) on the Solar Maximum Mission spacecraft was primarily designed and calibrated for nuclear gamma ray line measurements, but also has a high energy mode which allows the detection of gamma rays at energies above 10 MeV and solar neutrons above 20 MeV. The GRS response has been extrapolated until now for high energy gamma rays from an early design study employing Monte Carlo calculations. The response to 50 to 600 MeV solar neutrons was estimated from a simple model which did not consider secondary charged particles escaping into the veto shields. In view of numerous detections by the GRS of solar flares emitting high energy gamma rays, including at least two emitting directly detectable neutrons, the calibration of the high energy mode in the flight model has been recalculated by the use of more sophisticated Monte Carlo computer codes. New results presented show that the GRS response to gamma rays above 20 MeV and to neutrons above 100 MeV is significantly lower than the earlier estimates

Spectrum of the gamma-ray diffuse component observed from HEAO-1

The spectrum of the diffuse X and gamma ray background was measured between 15 keV and 4 MeV with the scintillation detectors aboard the HEAO 1 satellite. The apertures of the detectors were modulated on time scales of hours and the difference in counting rates measured the diffuse component flux. The observed spectrum is presented and compared with other measurements. At least two components are indicated, one below -100 keV and the other above. Possible origins are discussed

SMM detection of interstellar Al-26 gamma radiation

The gamma ray spectrometer on the Solar Maximum Mission Satellite has detected the interstellar Al-26 line when the Galactic center traversed its aperture. The center of the emission is consistent with the location of the Galactic center, but the spatial distribution is presently not well defined. The total flux in the direction of the Galactic center is 4.3 + or - 0.4) x .0001 gamma/sq cm-s-rad for an assumed population I distribution