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NASA NTRS1973

Diffuse X and gamma radiation.

The most reliable measurements in the energy range from 0.1 MeV to 100 MeV are presented in graphs. Studies in the region from 1 keV to 100 keV are discussed, giving attention to nonthermal mechanisms, thermal emission from a uniform intergalactic medium, the contribution of discrete extragalactic sources to the isotropic X-ray background, discrete diffuse X-ray sources in galaxy clusters, the contribution of galaxy clusters to the isotropic X-ray background, and fluctuations in the X-ray background. Diffuse X rays below 1 keV are also considered together with diffuse galactic X and gamma rays above 1 keV and the significance of diffuse X and gamma radiation for astronomy.

Silk, J.↗

NASA NTRS1973

Introduction to gasdynamics of explosions

Questions concerning the genesis and sustenance of an explosion are investigated, giving attention to the mechanics of explosions, the gasdynamics of explosions, aspects of technological significance, and future prospects. The dynamics of exothermic processes is discussed together with the most prominent effects of explosions. Blast waves are considered, taking into account conservation principles, blast wave transformation, conservative equations in nondimensional form, the equation of state, Eulerian space profiles, Eulerian time profiles, Lagrangian time profiles, boundary conditions and integral relations, and self-similar flow fields.

Oppenheim, A. K.↗

NASA NTRS1981

Interpretations and implications of gamma-ray lines from solar flares, the galactic centre and gamma-ray transients

Gamma-ray line emission from the Sun results from the nuclear interactions of energetic protons and nuclei with the solar atmosphere. These interactions produce gamma-ray lines from neutron capture, positron annihilation, and nuclear deexcitation. Observation of such gamma-rays can provide unique information on high energy processes at the Sun. Details of solar gamma-ray spectroscopy are discussed along with the galactic center 0.511 MeV line. The richness of astronomy at 0.511 MeV is indicated by the great variety of astrophysical positron production mechanisms and by the many astrophysical sites where such mechanisms could operate. Attention is also given to lines from gamma-ray transients, and the prospects for gamma-ray line detections, taking into account gamma-ray lines from processes of nucleosynthesis and lines from low-energy cosmic ray interactions.

Ramaty, R.↗

NASA NTRS1985

Plasma waves and instabilities

An account is given of the waves and instabilities occurring at collisionless shocks, with attention to the mechanisms responsible for the generation of these waves. The transition region of the shock usually involves an abrupt broadband burst of electrostatic noise that extends from below the lower hybrid resonance to near the electron plasma frequency, and by a broadband burst of whistler mode EM noise below the electron cyclotron frequency. Electrostatic lower hybrid waves are also noted. Upstream of the shock, electron plasma oscillations, ion acoustic waves, and intense ULF MHD waves are often observed. The region downstream of the shock is usually very chaotic; electrostatic waves often extend long distances into the downstream region, together with whistler mode emissions.

Gurnett, Donald A.↗

NASA NTRS1993

A detailed study of sheared Alfven waves and their possible application to gamma-ray bursts

Strong particle acceleration in neutron-star environments may result in the upscattering of soft photons to gamma-ray energies. Sheared Alfven waves, which may be generated by several plausible disturbances in the stellar crust, can energize the charges to Lorentz factors in excess of 10 exp 5. However, a full understanding of the physics of these MHD waves is lacking due to the complications arising from the charge separation within the sheared region. Here, we present a detailed study of this mechanism, with particular attention to a comparison of the nonrelativistic and relativistic domains. We find that the charge separation is consistent with our previous classical MHD solutions and that it provides a natural description of the sheared current via a relativistic flow. These waves can apparently produce the particle distributions that are necessary to produce the gamma-ray fluxes observed in gamma-ray bursts, regardless of whether the sources are Galactic or extragalactic.

Fatuzzo, Marco↗

NASA NTRS1983

Mechanisms of degradation of graphite composites in a simulated space environment

Attention is given to degradation mechanisms for graphite/polysulfone and graphite/epoxy laminates exposed to ultraviolet and high-energy electron radiation in vacuum up to 960 equivalent sun hours and to 10 to the 9th rads, respectively. The materials showed good electron radiation stability as indicated by the low G values for gas formation and no evidence of mechanical property changes. Quantum yields for gas formation indicate poor stability to ultraviolet radiation. Mechanical property measurements did not show significant changes up to 960 ESH, with the possible exception of P1700/C6000. The main products of irradiation were identified as hydrogen and methane, along with high levels of CO and CO2.

Giori, C.↗

NASA NTRS1981

Mechanical heating in the transition region

Attention is focused on the energy balance in the transition region and the role that mechanical heating plays in determining the temperature density structure of this region in a stellar atmosphere. Because of its role as the interface layer through which mass and energy flow between the chromospheres and corona, direct deposition of mechanical energy is a relatively unimportant factor in the overall energy balance in the transition region, except in the uppermost layers where the temperature approaches coronal values.

Withbroe, G.↗

NASA NTRS1993

Physical mechanism of comet outbursts - An experimental result

Attention is given to an experimental investigation of the physical mechanism of comet outbursts which is consistent with the general picture of mantle presence on comets and clarifies the relation of mantles to eruptive activity. The experiment and closeup observation of Comet P/Halley suggest a result different from most mathematical models in that the release of gas pressure does not occur only from uniform gas flow out of the entire surface. In some active comets near perihelion within a few AU of the sun, gas production rates and disturbance of the surface may be so high that the outflow is nearly continuous, with the regolith being entirely stripped away, as in many of the models. The present model provides a cyclic eruption and recharge mechanism which is lacking in most other models.

Hartmann, William K.↗

NASA NTRS1992

Small-crack growth behavior in high-strength aluminum alloys - A NASA/CAE cooperative program

NASA and the Chinese Aeronautics Establishment have participated in a Fatigue and Fracture Mechanics Cooperative Program giving attention to small-crack effects in high-strength Al alloys. Experimental and analytical investigations studied crack initiation and growth behavior of small cracks in both bare 7075-T6 and clad LC9cs Al alloys, with a view to the improvement of their fracture-mechanics analyses in the cases of surface crack and corner crack configurations. It is also expected that superior life-prediction methods will be developed in order to correlate and predict small crack and large crack growth with various load histories.

Newman, J. C., Jr.↗

NASA NTRS1977

Annual Conference on Nuclear and Space Radiation Effects, 14th, College of William and Mary, Williamsburg, Va., July 12-15, 1977, Proceedings

Topics related to processing and hardness assurance are considered, taking into account the radiation hardening of CMOS technologies, technological advances in the manufacture of radiation-hardened CMOS integrated circuits, CMOS hardness assurance through process controls and optimized design procedures, the application of operational amplifiers to hardened systems, a hard off-the-shelf SG1524 pulse width modulator, and the gamma-induced voltage breakdown anomaly in a Schottky diode. Basic mechanisms are examined, giving attention to chemical and structural aspects of the irradiation behavior of SiO2 films on silicon, experimental observations of the chemistry of the SiO2/Si interface, leakage current phenomena in irradiated SOS devices, the avalanche injection of holes into SiO2, the low-temperature radiation response of Al2O3 gate insulators, and neutron damage mechanisms in silicon at 10 K. Other subjects discussed are related to radiation effects in devices and circuits, space radiation effects, and aspects of simulation, energy deposition, and dosimetry.

Stahl, R. H.↗

NASA NTRS1988

Focus drive mechanism for the International Ultraviolet Explorer

The mechanical design of the compact and robust focus drive apparatus developed for the IUE's scientific instrumentation is based on a novel planetary drive that converts the angular increments of a permanent magnet stepping motor into simultaneous linear displacements of three ball screws supporting the secondary mirror. Significant torque margins are thus produced while yielding exceptionally smooth and reliable operation. Attention is given to mechanical drive materials-selection criteria.

Schuster, Ludwig E.↗

NASA NTRS1982

Dynamic surface measurements on a model helicopter rotor during blade slap at high angles of attack

The modern helicopter offers a unique operational capability to both the public and private sectors. However, the use of the helicopter may become severely limited due to the radiated noise generated by the rotor system. A description is presented of some of the experimental results obtained with a model helicopter rotor in an anechoic wind tunnel with regard to blade stall as a source mechanism of blade slap. Attention is given to dynamic rotor blade surface phenomena and the resulting far field impulsive noise from the model helicopter rotor at high angles of attack and low tip speed. The results of the investigation strongly implicates the boundary layer as playing an important role in blade slap due to blade/vortex interaction (BVI) in a highly loaded rotor. Intermittent stall cannot be ruled out as a possible source mechanism for blade slap. This implies that blade surface characteristics, airfoil shape and local Reynolds number may now be used as tools to reduce the resultant far-field sound pressure levels in helicopters.

Hubbard, J. E., Jr.↗

NASA NTRS1992

Toxicological implications of extended space flights

This paper draws attention to the needs and mechanisms for shielding crewmembers on long-duration space flights from hazards related to chemical toxicants. Specific attention is given to existing data on sources of impaired performance, namely, neurotoxicants, respiratory infections, pulmonary function. The behavioral effects associated with long-term exposure to volatile organic solvents can impair crucial functional parameters of space flight and mission objectives. Respiratory infections contribute to performance decrements of up to 20 percent, and pulmonary function can be impaired by contaminants such as ozone leading to reduced performance. It is concluded that these and other sources of toxicologically induced performance reductions be studied since they impinge on vehicle design and mission objectives.

Weiss, Bernard↗

NASA NTRS1977

Fundamental electronic mechanisms limiting the performance of solar cells

Attention is focused on distortion in the energy band, and carrier recombination and generation rates (lifetimes), as the two dominant mechanisms. Spatial dependences associated with these two mechanisms, in the direction normal to the surface illuminated by the sun and in the direction tangential to that surface, are also emphasized as crucial factors in governing the efficiency of solar cells. Electronic parameters for the set of differential equations characterizing transport, recombination, and generation of carriers, and interband and band-bound transition rates, are studied.

Lindholm, F. A.↗

NASA NTRS1971

Supernova explosions.

The recent history of theoretical investigations of the supernova mechanism is considered, giving attention also to a number of nuclear physical problems which have yet to be solved in connection with the thermonuclear detonation. A variety of different processes of nucleo-synthesis are expected to occur in association with the supernova explosions. Aspects of the chemical evolution of the galaxy are discussed including the cosmic ray production of lithium, beryllium, and boron in the interstellar medium. Various hypotheses to account for the very large amount of light that comes from a supernova explosion are also examined.

Cameron, A. G. W.↗

NASA NTRS1972

Earth's magnetospheric processes; Proceedings of the Symposium, Cortina, Italy, August 30-September 10, 1971.

New data obtained by satellites, rockets, aircraft, and ground-based observations are interpreted in papers dealing with the magnetospheric structure and processes, particle distributions, magnetic and electric fields, plasma convection, particle acceleration and diffusion mechanisms, and substorm phenomena. Attention is given to particle populations in different magnetospheric regions, auroral particle precipitation patterns, effects of electric fields on plasma convection, VLF phenomena, a high-energy proton model for the inner radiation belt, substorm behavior of plasma sheet particles, and interpretations of magnetic field variations during substorms. Individual items are announced in this issue.

Mccormac, B. M.↗

NASA NTRS1973

CdS-metal contact at higher current densities.

An investigation is conducted concerning the mechanisms by which a steady flow of current proceeds through the contact when an external voltage is applied. The main characteristics of current mechanisms are examined, giving attention to photoemission from the cathode, thermionic emission, minority-carrier extraction, and the tunneling of electrons. A high-field domain analysis is conducted together with experimental studies. Particular attention is given to the range in which tunneling predominates.

Stirn, R. J.↗

NASA NTRS1974

Model calculations for diffuse molecular clouds

A steady state isobaric cloud model is developed. The pressure, thermal, electrical, and chemical balance equations are solved simultaneously with a simple one dimensional approximation to the equation of radiative transfer appropriate to diffuse clouds. Cooling is mainly by CII fine structure transitions, and a variety of heating mechanisms are considered. Particular attention is given to the abundance variation of H2. Inhomogeneous density distributions are obtained because of the attenuation of the interstellar UV field and the conversion from atomic to molecular hyrodgen. The effects of changing the model parameters are described and the applicability of the model to OAO-3 observations is discussed. Good qualitative agreement with the fractional H2 abundance determinations has been obtained. The observed kinetic temperatures near 80 K can also be achieved by grain photoelectron heating. The problem of the electron density is solved taking special account of the various hydrogen ions as well as heavier ones.

Glassgold, A. E.↗