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At least 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Photon and radiowave emission from peeling pressure sensitive adhesives in air

During separation of an adhesive from a polymer substrate in air, intense bursts of photons ('phE', for photon emission) and long wavelength electromagnetic radiation ('RE', for radiowave emission), similar to those reported earlier by Deryagin, et al. (1978) have been observed. In this paper, careful measurements of the phE time distributions, as well as time correlations between bursts of phE and RE, are reported. These results support the view that patches of electrical charge produced by charge separation between dissimilar materials lead to microdischarges in and near the crack tip. The role of these discharges in producing sustained phE after the discharge has been extinguished is also discussed.

Donaldson, E. E.↗
NASA NTRS1985

A preliminary look at astrometric accuracy as a function of photon counts

The importance of photon counts as a factor in the accuracy of astrometric positions is illustrated by a preliminary look at data collected with the Allegheny Observatory's photon-counting detector, the Multichannel Astrometric Photometer. Twenty estimates are plotted of the standard errors, per hour, of astrometric positions obtained with the photometer and the blue-light lens of the observatory's 30-inch refractor. The agreement between the estimated standard errors in the X and Y coordinates supports the conclusion that, as predicted by Drake, the accuracy is dependent upon the count rate.

Gatewood, G.↗
NASA NTRS1987

Photon counts from stellar occultation sources

The feasibility of using stars as radiation sources for Earth atmospheric occultation experiments is investigated. Exoatmospheric photon counts of the order of 10 to the 6th power photons/sq cm/sec are realized for the 15 visually brightest stars. Most photon counts appear to be marginally detectable unless photomultiplier or cascade detection devices can be used.

Buglia, James J.↗
NASA NTRS1985

CCD charge collection efficiency and the photon transfer technique

The charge-coupled device (CCD) has shown unprecendented performance as a photon detector in the areas of spectral response, charge transfer, and readout noise. Recent experience indicates, however, that the full potential for the CCD's charge collection efficiency (CCE) lies well beyond that which is realized in currently available devices. A definition of CCE performance is presented and a standard test tool (the photon transfer technique) for measuring and optimizing this important CCD parameter is introduced. CCE characteristics for different types of CCDs are compared; the primary limitations in achieving high CCE performance are discussed, and the prospects for future improvement are outlined.

Janesick, J.↗
NASA NTRS1988

Solid-State Single-Photon Counter

Avalanche photodiode used to detect single photons if cooled to optimum temperature and overbiased beyond breakdown voltage. Offers two to three times sensitvity of photomultiplier tubes commonly used for photon detection. When cooled to reduce number of thermal carriers, avalanche photodiode reverse-biased beyond normal breakdown voltage to give it extremely-high internal gain of 10 to seventh power to 10 to eigth power. Advantages of solid-state devices to applications requiring low-level light detection, optical communication, astronomy, remote sensing, optical metrology, and optical signal processing.

Robinson, D. L.↗
NASA NTRS1987

Particle acceleration and production of energetic photons in SN1987A

A pulsar wind model for the acceleration of particles in SN1987A is discussed. The expected photon flux is investigated in terms of the spectrum of parent protons and electrons, the nature of the region in which they propagate after acceleration, and the magnetic field and radiation environment which determines the subsequent fate of produced photons. The model is found to produce observable signals if the spin period of the pulsar is 10 ms or less.

Gaisser, T. K.↗
NASA NTRS1987

Photon detection with cooled avalanche photodiodes

Commercial avalanche photodiodes have been operated as single-photon detectors at an optimum operating temperature and bias voltage. These detectors were found to be 1.5-3 times more sensitive than presently available photomultiplier tubes (PMTs). Both single-photon detection probability and detector noise increase with bias voltage; detection probabilities greater than twice that of a PMT were obtained with detector noise levels below 100 counts per second. Higher probabilities were measured at higher noise levels. The sources of noise and their dependence on temperature and bias voltage are discussed.

Robinson, D. L.↗
NASA NTRS1987

Photon noise and constant-volume operators

In a previous paper, a class of constant volume (CV) operators designed to maximize spatial resolution in the presence of photon noise was introduced. In this paper, the statistical properties are derived of the output images created by CV operators applied to photon-noisy versions of input images consisting of edges, spots, and gratings. The results are used to calculate the input image parameters that would permit any test image to be discriminated from a uniform field with a given level of reliability and consequently to compare the predictions of a CV operator model with psychophysical data. These results also allow the potential usefulness of CV operators in artificial image processing applications to be assessed.

Yellott, John I., Jr.↗
NASA NTRS1987

Cooled avalanche photodiode used for photon detection

Commercial avalanche photodiodes have been operated as single-photon detectors at an optimum operating temperature and bias voltage. These detectors were found to be 1.5 to 3 times more sensitive than presently-available photomultiplier tubes (PPMTs). Both single-photon detection probability and detector noise increase with bias voltage; detection probabilities greater than 25 percent were obtained with detector noise levels comparable to the noise of a PMT; higher probabilities were measured at higher noise levels. The sources of noise and their dependence on temperature and bias voltage are discussed.

Robinson, Deborah L.↗
NASA NTRS1989

The grand unified photon spectrum: A coherent view of the diffuse extragalactic background radiation

The spectrum of diffuse extragalactic background radiation (DEBRA) at wavelengths from 10(exp 5) to 10(exp -24) cm is presented in a coherent fashion. Each wavelength region, from the radio to ultra-high energy photons and cosmic rays, is treated both separately and as part of the grand unified photon spectrum (GUPS). A discussion of, and references to, the relevant literature for each wavelength region is included. This review should provide a useful tool for those interested in diffuse backgrounds, the epoch of galaxy formation, astrophysical/cosmological constraints to particle properties, exotic early Universe processes, and many other astrophysical and cosmological enterprises. As a worked example, researchers derive the cosmological constraints to an unstable-neutrino spies (with arbitrary branching ratio to a radiative decay mode) that follow from the GUPS.

Ressell, M. Ted↗
NASA NTRS1989

The solid state photomultiplier: Status of photon counting beyond the near-infrared

Rockwell International's Solid State Photomultiplier (SSPM) is an impurity-band avalanche device which can count individual photons with wavelengths between 0.4 and 28 micrometers. Its response to a photon is a pulse of between 10(exp 4) and 10(exp 5) conduction electrons, making it an important device for use in phenomenology. The characteristics of the SSPM make it a potentially important device for use in astronomical applications. Contract NAS2-12400 was initiated in June 1986 to conduct modeling and characterization studies of the SSPM to provide a basis for assessing its use in astronomical systems. Some SSPM models and results of measurements which characterize the group of SSPMs recently fabricated on this contract are discussed.

Hays, K. M.↗
NASA NTRS1990

Photonic processing at NASA Ames Research Center

The Photonic Processing group is engaged in applied research on optical processors in support of the Ames vision to lead the development of autonomous intelligent systems. Optical processors, in conjunction with numeric and symbolic processors, are needed to provide the powerful processing capability that is required for many future agency missions. The research program emphasizes application of analog optical processing, where free-space propagation between components allows natural implementations of algorithms requiring a large degree of parallel computation. Special consideration is given in the Ames program to the integration of optical processors into larger, heterogeneous computational systems. Demonstration of the effective integration of optical processors within a broader knowledge-based system is essential to evaluate their potential for dependable operation in an autonomous environment such as space. The Ames Photonics program is currently addressing several areas of interest. One of the efforts is to develop an optical correlator system with two programmable spatial light modulators (SLMs) to perform distortion invariant pattern recognition. Another area of research is optical neural networks, also for use in distortion-invariant pattern recognition.

Ochoa, Ellen↗
NASA NTRS1988

Photon degradation of AlGaAs/GaAs solar cells

The behavior of GaAs solar cells after photon illumination for a prolonged exposure time is presented. More than 55 OMCVD AlGaAs/GaAs solar cells were exposed for over 400 h to AM0 photons at 28 C in three separate, well-controlled runs. Significant degradation of solar cell efficiency was observed in two out of three runs. Although noticeable losses in the open-circuit voltage, fill factor, and maximum power were observed, no change in the short-circuit current was found. In one of the runs, no change was seen in either the test cells or the control cells. Each cell in this run was protected with a coverglass. The cells in this run had thicker buffer layers and thinner window layers than the cells in the other two runs.

Anspaugh, Bruce↗
NASA NTRS1989

Production and transfer of UV photons in non-homogeneous spherical clouds

Due to screening by dust particles, the UV radiation field of interstellar origin is practically inexistent within very dense interstellar clouds. However, it appears possible that the cosmic-ray excitation of the Lyman and Werner systems of the hydrogen molecule could originate a chemically-significant flux of UV photons even within such dense clouds. Computations of photon fluxes were carried out for two different models of radial density distribution inside the cloud (gas and dust densities were assumed to have the same density distribution). The result strongly support the suggestion that the low energy cosmic rays may generate inside dense interstellar clouds UV radiation fields that may significantly contribute to the gas and dust evolution.

Aiello, Santi↗
NASA NTRS1991

Cyclotron harmonics in accreting pulsars and gamma-ray bursters - Effect of two-photon processes

The radiative transfer at the cyclotron first, second, and third harmonics is discussed for simplified accreting pulsar and gamma-ray burster emission regions. Two-photon scattering as well as two-photon emission, which play a major role in determining the line-strength ratio is included. Stimulated effects are also important for accurately modeling the optically thick line and continuum regions. These calculations are compared with recent observations of accreting pulsars and gamma-ray burst sources, showing a qualitative agreement. The cyclotron harmonic lines should be strongly polarized.

Alexander, S. G.↗
NASA NTRS1991

Plastic scintillator block as photon beam monitor for EGRET calibration

The EGRET (Energetic Gamma Ray Experiment Telescope) detector has been calibrated at SLAC (Stanford Linear Accelerator) and, to a lesser degree, at the MIT Bates Linear Accelerator Center. To monitor the photon beams for the calibration, a plastic scintillator block, 5 cm x 5 cm in cross section, 15 cm in length, and viewed by a single photomultiplier tube, was used for the entire beam energy range of 15 MeV to 10 GeV. The design operation, and method of analysis of the beam intensity are presented. A mathematical framework has been developed to treat the general case of a beam with multiphoton beam pulses and with a background component. A procedure to deal with the fluctuations of the beam intensity over a data-taking period was also developed. The photon beam monitor is physically sturdy, electronically steady, simple to construct, and easy to operate. Its major merits lie in its sheer simplicity of construction and operation and in the wide energy range it can cover.

Lin, Y. C.↗
NASA NTRS1991

A simplified formulation of photon transport in leaf canopies with scatterers of finite dimensions

A formalism for photon transport in leaf canopies with finite-dimensional scattering centers that mutually cross-shade was recently proposed resulting in an equation set which is rather complicated. In this paper, we propose a simplification of the correction factors that describe the correlation in photon fates. These simplified correction factors permit the derivation of a modified integro-differential transfer equation with two additional terms that arise from considerations of finite size scatterers. A numerical procedure for the solution of this transfer equation in the framework of a successive collisions approach is also presented.

Myneni, Ranga B.↗
NASA NTRS1991

Gamma ray pulsar analysis from photon pobability maps

We present a new method of analyzing skymap-type gamma ray data. Each photon event is replaced by a probability distribution on the sky corresponding to the observing instrument's point spread function. The skymap produced by this process may be used for source detection or identification. Most important, the use of these photon weights for pulsar analysis promises significant improvement over traditional techniques.

Brown, Lawrence E.↗