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At least 919 records · Page 51

Stabilities of nitrogen containing heterocyclic radicals and geometrical influences on non-radiative processes in organic molecules

Stabilities of nitrogen containing heterocyclic radicals were studied to detect radicals of the type R-N-R, and to theoretically rationalize their electronic structure. The computation of simple potential energy surfaces for ground and excited states is discussed along with the photophysical properties of indolizine. Methods of calculation and problems associated with the calculations are presented. Results, tables, diagrams, discussions, and references are included.

Evleth, E. M.↗

White dwarfs as infrared sources in the galactic centre.

It is shown that quantized magnetic bremsstrahlung radiation in intense magnetic fields from the surface layers of white dwarfs could serve as one of the possible explanations for the infrared emission from the center of the galaxy that has been detected by Hoffmann et al. (1971). It is assumed, in particular, that the infrared radiation is from clusters of white dwarfs.

Chow, T. L.↗

Trabecular Bone Mechanical Properties and Fractal Dimension

Countermeasures for reducing bone loss and muscle atrophy due to extended exposure to the microgravity environment of space are continuing to be developed and improved. An important component of this effort is finite element modeling of the lower extremity and spinal column. These models will permit analysis and evaluation specific to each individual and thereby provide more efficient and effective exercise protocols. Inflight countermeasures and post-flight rehabilitation can then be customized and targeted on a case-by-case basis. Recent Summer Faculty Fellowship participants have focused upon finite element mesh generation, muscle force estimation, and fractal calculations of trabecular bone microstructure. Methods have been developed for generating the three-dimensional geometry of the femur from serial section magnetic resonance images (MRI). The use of MRI as an imaging modality avoids excessive exposure to radiation associated with X-ray based methods. These images can also detect trabecular bone microstructure and architecture. The goal of the current research is to determine the degree to which the fractal dimension of trabecular architecture can be used to predict the mechanical properties of trabecular bone tissue. The elastic modulus and the ultimate strength (or strain) can then be estimated from non-invasive, non-radiating imaging and incorporated into the finite element models to more accurately represent the bone tissue of each individual of interest. Trabecular bone specimens from the proximal tibia are being studied in this first phase of the work. Detailed protocols and procedures have been developed for carrying test specimens through all of the steps of a multi-faceted test program. The test program begins with MRI and X-ray imaging of the whole bones before excising a smaller workpiece from the proximal tibia region. High resolution MRI scans are then made and the piece further cut into slabs (roughly 1 cm thick). The slabs are X-rayed again and also scanned using dual-energy X-ray absorptiometry (DEXA). Cube specimens are then cut from the slabs and tested mechanically in compression. Correlations between mechanical properties and fractal dimension will then be examined to assess and quantify the predictive capability of the fractal calculations.

Hogan, Harry A.↗

Verification and Quantification of Single Event Effects on High Speed SRAM in Terrestrial Environments

As integrated circuits become more sensitive to charged particles and neutrons, anomalous performance due to single event effects (SEE) is a concern and requires experimental verification and quantification. The Center for Applied Radiation Research (CARR) at Prairie View A&M University has developed experiments as a participant in the NASA ER-2 Flight Program, the APEX balloon flight program and the Student Launch Program. Other high altitude and ground level experiments of interest to DoD and commercial applications are being developed. The experiment characterizes the SEE behavior of high speed and high density SRAM's. The system includes a PC-104 computer unit, an optical drive for storage, a test board with the components under test, and a latchup detection and reset unit. The test program will continuously monitor the stored checkerboard data pattern in the SW and record errors. Since both the computer and the optical drive contain integrated circuits, they are also vulnerable to radiation effects. A latchup detection unit with discrete components will monitor the test program and reset the system when necessary. The first results will be obtained from the NASA ER-2 flights, which are now planned to take place in early 1998 from Dryden Research Center in California. The series of flights, at altitudes up to 70,000 feet, and a variety of flight profiles should yield a distribution of conditions for correlating SEES. SEE measurements will be performed from the time of aircraft power-up on the ground throughout the flight regime until systems power-off after landing.

Huff, H.↗

Astrophysical data on 5 eV to 1 keV radiation from the radiative decay of fundamental particles - Current limits and prospects for improvement

Line emission from the decay of fundamental particles, integrated over cosmological distances, can give rise to detectable spectral features in the diffuse astronomical background between 5 eV and 1 keV. Spectroscopic observations may allow these features to be separated from line emission from the numerous local sources of radiation. The current observational status and existing evidence for such features are reviewed. No definitive detections of nongalactic line features have been made. Several local sources of background mask the features at many wavelengths and confuse the interpretation of the data. No systematic spectral observations have been carried out to date. Upcoming experiments which can be expected to provide significantly better constraints on the presence of spectral features in the diffuse background from 5 eV to 1 keV are reviewed.

Bowyer, Stuart↗

Sensor detects hydrocarbon oil contaminants in fluid lines

Sensor with ultraviolet light monitors and detects hydrocarbon oil contaminants present in fluid lines. The light causes the oil particles to fluoresce. This light emitted by the oil particle is detected by a photocell which is relatively insensitive to ultraviolet radiation.

Roth, B.↗

Surface temperature mapping with infrared photographic pyrometry

Infrared photographic pyrometry method measures and maps the temperature distribution on a heated surface with accuracy and precision. This method involves the collection, detection and measurement of a narrow bandwidth of emitted infrared radiation. Standard commercially-available equipment is used, together with systematic procedures.

Pollack, F. G.↗

Vibrating ribbon bolometer: A concept

Incident electromagnetic radiation pulses maintain bolometer's sensitive element in state of resonant oscillation and correlate amplitude of oscillation with incident radiation power. Bolometer can be employed with other devices for detecting motion of ribbon.

Dimeff, J.↗

Turbulent transport measurements with a laser Doppler velocimeter

The power spectrum of phototube current from a laser Doppler velocimeter operating in the heterodyne mode has been computed. The spectrum is obtained in terms of the space time correlation function of the fluid. The spectral width and shape predicted by the theory are in agreement with experiment. For normal operating parameters the time average spectrum contains information only for times shorter than the Lagrangian integral time scale of the turbulence. To examine the long time behavior, one must use either extremely small scattering angles, much longer wavelength radiation or a different mode of signal analysis, e.g., FM detection.

Edwards, R. V.↗

Solar flare produced pulsations in meter and X-ray wavelengths

The mechanism responsible for the modulation of synchrotron radiation in the meter wave range by a solar flare is examined. The proposed explanation is based on the interaction of a flare generated shock front with a magnetic flux tube extending out into the corona over the flare. As the shock wave propagates through the flux tube, Alfven waves are developing and travelling up the tube toward each other, thereby accelerating electrons and protons by the Fermi mechanism and thus filling the top of the flux tube with energetic electrons. Radial oscillations develop as the shock front becomes parallel to the magnetic field direction and the intensity of the synchrotron radiation is modulated by these oscillations. An OSO-5 experiment detected a hard X-ray burst associated with the flare that produced modulated radio emission and X-ray modulations that are attributed to density fluctuations in the X-ray bursts.

Frost, K. J.↗

Turbulent transport measurements with a laser Doppler velocimeter.

The power spectrum of phototube current from a laser Doppler velocimeter operating in the heterodyne mode has been computed. The spectral width and shape predicted by the theory are in agreement with experiment. For normal operating parameters the time-average spectrum contains information only for times shorter than the Lagrangian-integral time scale of the turbulence. To examine the long-time behavior, one must use either extremely small scattering angles, much-longer-wavelength radiation, or a different mode of signal analysis, e.g., FM detection.

Edwards, R. V.↗

Coupling characteristics of thin-film metal-oxide-metal diodes at 10.6 microns

Direct detection experiments have demonstrated the coherent coupling of 10.6 micrometer radiation into photolithographically fabricated metal-oxide-metal tunnel junctions. A CO2 laser beam mechanically chopped at 1 KHz was focused at a variable angle of incidence with a power density of about 10 W/sq cm at the diodes. Diodes in which the junction resistance was much greater than the lead resistance displayed angular characteristics dominated by coherent antenna coupling.

Wang, S. Y.↗

Mass spectrometer has wide angular acceptance

Higher mass resolution is achieved by replacing conventional mass/velocity spectrometers with system of planar ac-modulated grids. System gives focusing for all angles of incidence. Ac-modulation and detection scheme minimizes sensitivity to light and other types of radiation.

Neugebauer, M. M.↗

Imaging of X-ray aurorae from Spacelab

The Atmospheric X-ray Emission Telescope (AXET) is designed to image and measure the spatial, temporal, and spectral distributions of X-ray aurorae produced in the upper atmosphere by precipitating energetic electrons. The bremsstrahlung-produced X radiation penetrates to stratospheric depths where it can modify ozone, conductivity, and other critical atmospheric parameters. Remote sensing of this radiation from topside will provide vital data on solar-terrestrial relationships and mechanisms that could help trigger tropospheric response to solar activity. The X rays also provide direct information concerning the dynamics of radiation-belt processes within the magnetosphere. AXET is designed to detect sources up to 50 keV and to image them below 25 keV by means of directionally sensitive proportional counters with passive collimators. It will be mounted in the Space Shuttle on a single fixed platform designed to optimize viewing conditions for the anticipated orbits and aspects of early Spacelab missions.

Goldberg, R. A.↗

Active cavity radiometer type IV

A new cavity pyrheliometer, the active cavity radiometer type IV (ACR IV), has been developed for the measurement of total solar optical irradiance. Analysis predicts its ability to measure at the solar constant level with 0.1% uncertainty in SI units. In comparison tests ACR IVs have consistently demonstrated 0.3% higher results than the World Radiometric Reference scale. A prototype has been tested, and a flight instrument has been developed and flown in a sounding rocket experiment to determine the solar constant. ACR IV instrumentation is being developed for flight experiments on the Spacelab I and Solar Maximum missions to monitor the total solar output of optical radiation as part of a long-term program to detect variations of climatological significance.

Willson, R. C.↗

Role of the doubly stochastic Neyman type-A and Thomas counting distributions in photon detection

The sums of Meyman type-A and Thomas random variables are shown to retain their form under the constant multiplication parameter constraint. The conditions under which the two random variables converge in distribution to the fixed multiplicative Poisson, and to the Gaussian, are presented. It is shown that the latter result is important, in that it provides a solution to likelihood-ratio detection, estimation and discrimination problems in the presence of many kinds of signal and noise. Among the explicit applications presented are: (1) the photon-counter scintillation detection of nuclear particles when particle flux is low, (2) the photon-counting detection of weak optical signals in the presence of ionizing radiation, and (3) the design of a star-scanner spacecraft guidance system adequate for hostile space environments.

Teich, M. C.↗

The muon content of gamma-ray showers

The result of a calculation of the expected number of muons in gamma ray initiated and cosmic ray initiated air showers using a realistic model of hadronic collisions in an effort to understand the available experimental results and to assess the feasibility of using the muon content of showers as a veto to reject cosmic ray initiated showers in ultra-high energy gamma ray astronomy are reported. The possibility of observing very-high energy gamma-ray sources by detecting narrow angle anisotropies in the high energy muon background radiation are considered.

Edwards, P. G.↗