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At least 487 records · Page 27

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NASA NTRS1975

Propagation of Sound at Moderate and High Intensities in Absorbent and Hard-Walled Cylindrical Ducts

The propagation of plane wave and higher order acoustic modes in both hard-walled and absorbent cylindrical ducts was studied at moderate sound intensities where the linear wave equation is valid, and at high intensities where nonlinear effects can be observed. The experiments were conducted with an anechoically terminated twelve-inch inside-diameter transite pipe. Various types of sound sources were mounted at one end of the duct to generate the desired acoustic fields within the duct. Arrays of conventional loudspeakers were used to generate plane waves and higher order acoustic modes at moderate intensities, and an array of four high intensity electro-pneumatic sound sources was used for the experiments in the nonlinear region. The attenuation of absorbent liners made of several different materials was obtained at moderate intensities for both plane waves and high order modes. It was found that the characteristics of the liners studied did not change appreciably at high intensities.

Mcdaniel, Oliver Herbert↗

NASA NTRS1974

The UCSD/MIT hard X-ray and low energy gamma ray experiment for HEAO-A

An HEAO-A spacecraft is described which is scheduled for launch in April 1977 to survey the sky in the 0.15 keV to 10 MeV range of X-rays with improved sensitivity and angular resolution. The spectral and time variations of discrete sources, and the spectrum and isotropy of the diffuse component will be measured. A hard X-ray and low energy gamma ray experiment is described which will use as an array of seven (NaI)(Tl)/CsI(Na) phoswich detectors and eight large CsI(Na) shields. Detector events are pulse-height analyzed with a selectable time resolution of 0.1, 0.005, or 0.00002 sec. The instrument background is expected to be comprised of approximately equal parts of diffuse gamma rays entering the aperture, radioactive decay of spallation products, and gamma ray leakage through the shields. Detailed background predictions are presented, and sensitivity estimates indicate that the Crab nebula should be detected up to approximately 5 MeV. Approximate 1% isotropy limits for the diffuse component of cosmic gamma rays should be achieved, and its spectrum measured with improved precision.

Matteson, J. L.↗

NASA NTRS1975

Nonlinear propagation of a wave packet in a hard-walled circular duct

The method of multiple scales is used to derive a nonlinear Schroedinger equation for the temporal and spatial modulation of the amplitudes and the phases of waves propagating in a hard-walled circular duct. This equation is used to show that monochromatic waves are stable and to determine the amplitude dependance of the cutoff frequencies.

Nayfeh, A. H.↗

NASA NTRS1974

Nonlinear propagation of a wave packet in a hard-walled circular duct

Nayfeh, A. H.↗

NASA NTRS1976

A study of the cosmic sources of hard X-rays

An actively-shielded, high-energy X-ray telescope was launched onboard OSO-8 on 21 June 1975. The primary objectives of this experiment are the measurements of the energy spectrum of discrete cosmic X-ray sources in the range 20 keV to 3 MeV and of the temporal variations in the intensity of each source detected with a time resolution of 0.3 msec. This detector provides the highest duty factor and the finest time resolution of any of its kind for observations over a period of up to 10 days. The background spectrum of this detector in orbit was monitored continuously since shortly after launch. The minimum detectable source strength is estimated to be between 10,000 and 100,000 photons/sq-cm-sec keV, limited primary by the effects of induced radioactivity. From 16 July through 18 July 1975, the X-ray binary, Cen X-3, was observed with the hard X-ray telescope.

Dennis, B. R.↗

NASA NTRS1975

The Hard X-ray experiment on the Astronomical Netherlands Satellite

The Hard X-ray Experiment flown on the Astronomical Netherlands Satellite is described. The instrument consists of two parts. One is a large-area detector of about 60 sq cm in total area, sensitive in the energy range between 1.5 and 30 keV. Two counters comprise this detector, each collimated 10 min by 3 deg and offset in the narrow direction by 4 min. The other part is a Bragg-crystal assembly consisting of two PET crystals and counters aligned to search for the silicon emission lines near 2 keV. Instrument characteristics and orbital operations are described.

Gursky, H.↗

NASA NTRS1975

Pulse pile-up in hard X-ray detector systems

When pulse-height spectra are measured by a nuclear detection system at high counting rates, the probability that two or more pulses will arrive within the resolving time of the system is significant. This phenomenon, pulse pile-up, distorts the pulse-height spectrum and must be considered in the interpretation of spectra taken at high counting rates. A computational technique for the simulation of pile-up is developed. The model is examined in the three regimes where (1) the time between pulses is long compared to the detector-system resolving time, (2) the time between pulses is comparable to the resolving time, and (3) many pulses occur within the resolving time. The technique is used to model the solar hard X-ray experiment on the OSO-7 satellite; comparison of the model with data taken during three large flares shows excellent agreement. The paper also describes rule-of-thumb tests for pile-up and identifies the important detector design factors for minimizing pile-up, i.e., thick entrance windows and short resolving times in the system electronics.

Datlowe, D. W.↗

NASA NTRS1976

UCSD OSO-7 observations of the hard X-ray spectrum and variability of Centaurus A

Results are reported for four X-ray scans of the region containing Cen A. It is found that the X-ray source had a hard number spectrum (spectral index of -1.2) during these observations and that the intensity in the range from 10 to 100 keV apparently increased by 230% with no detectable change in spectral shape between two observations 210 days apart. Either a Compton-synchrotron mechanism or thermal bremsstrahlung at any temperature greater than 200 keV is suggested as the source of the X-rays. It is noted that the present observations, together with a similar detection of another galaxy, may establish a distinct class of extragalactic X-ray objects with flat and highly absorbed spectra.

Mushotzky, R. F.↗

NASA NTRS1976

A fast transient source of hard X-rays at high galactic latitude

An extremely short-lived transient X-ray source has been detected with the SAS-3 satellite at a high galactic latitude (about -51 deg). The source, designated MX 2346-65, had a duration of between 45 s and 2200 s as well as a very hard spectrum. This source had not been detected in previous sky surveys and was not seen again by SAS-3 in 7 days of further periodic observations. Possible explanations of this type of event are discussed.

Rappaport, S.↗

NASA NTRS1977

Backscatter of hard X-rays in the solar atmosphere

The solar photosphere backscatters a substantial fraction of the hard X rays from solar flares incident upon it. This reflection was studied using a Monte Carlo simulation which takes into account Compton scattering and photo-electric absorption. Both isotropic and anisotropic X ray sources are considered. The bremsstrahlung from an anisotropic distribution of electrons are evaluated. By taking the reflection into account, the inconsistency is removed between recent observational data regarding the center-to-limb variation of solar X ray emission and the predictions of models in which accelerated electrons are moving down toward the photosphere.

Bai, T.↗

NASA NTRS1977

Graphical method for predicting life of a rocket thrust chamber with half-hard zirconium-copper liner and electroformed nickel closeout

A method for estimating the life of a regeneratively cooled rocket thrust chamber was developed and is based on the hot-gas wall temperature and the temperature difference between the hot-gas wall and the outside surface of the closeout. This method permits a quick estimate of the life of a thrust chamber when design changes or test-cycle variations are considered. Strain range and life are presented graphically as functions of these temperature parameters for a typical high-performance rocket thrust chamber with a half-hard zirconium-copper liner and an electroformed nickel closeout.

Kasper, H. J.↗

NASA NTRS1977

Studies on solar hard X-Rays and gamma-rays: Compton backscatter, anisotropy, polarization and evidence for two phases of acceleration

Observations of solar X-rays and gamma-rays from large flares show that the hard X-ray spectrum extends into the gamma ray region, where a flattening in the spectrum of the continuum emission is observed above about 1 MeV. This emission is believed to be due to bremsstrahlung. In addition to electron-proton collisions, at energies greater than approximately 500 keV, bremsstrahlung due to electron-electron collisions becomes significant. Bremsstrahlung production was calculated for a variety of electron spectra extending from the nonrelativistic region to relativistic energies and electron-electron bremsstrahlung is taken into account. By comparing these calculations with data, it is shown that the flattening in the spectrum of the continuum emission can be best explained by an electron spectrum consisting of two distinctive components. This evidence, together with information on the X-ray and gamma ray time profiles, implied the existence of two phases of acceleration. The first phase accelerates electrons mainly up to about several hundred keV; the second phase accelerates a small fraction of the electrons accelerated in the first phase to relativistic energies and accelerates protons to tens and hundreds of MeV.

Bai, T.↗

NASA NTRS1977

Friction and wear properties of three hard refractory coatings applied by radiofrequency sputtering

The adherence, friction, and wear properties of thin hard refractory compound coatings applied to 440C bearing steel by radiofrequency sputtering were investigated. Friction and wear tests were done with nonconforming pin on disk specimens. The compounds examined were chromium carbide, molybdenum silicide, and titanium carbide. The adherence, friction, and wear were markedly improved by the application of a bias voltage to the bearing steel substrate during coating deposition. Analysis by X-ray photoelectron spectroscopy indicated that the improvement may be due to a reduction in impurities in bias deposited coatings. A fivefold reduction in oxygen concentration in MoSi2 coating by biasing was noted. Chromium carbide was not effective as an antiwear coating. Molybdenum silicide provided some reduction in both friction and wear. Titanium carbide exhibited excellent friction and antiwear properties at light loads. Plastic flow and transfer of the coating material onto the pin specimen appears to be important in achieving low friction and wear.

Brainard, W. A.↗

NASA NTRS1976

Hard X-ray imaging facility for space shuttle: A scientific and conceptual engineering study

A shuttle-accommodated instrument for imaging hard X-rays in the study of nonthermal particles and high temperature particles in various solar and cosmic phenomena was defined and its feasibility demonstrated. The imaging system configuration is described as well as the electronics, aspect systems, mechanical and thermal properties and the ground support equipment.

Peterson, L. E.↗

NASA NTRS1978

Experiments on asteroids using hard landers

Hard lander missions to asteroids are examined using the Westphal penetrator study as a basis. Imagery and chemical information are considered to be the most significant science to be obtained. The latter, particularly a detailed chemical analysis performed on an uncontaminated sample, may answer questions about the relationships of asteroids to meteorites and the place of asteroids in theories of the formation of the solar system.

Turkevich, A.↗

NASA NTRS1978

Hard X-ray imaging from the solar probe

The solar probe offers a platform with particular advantages for studying solar nonthermal plasma processes via the observations of hard X-radiation from energetic electrons in the chromosphere and corona, these include (1) high sensitivity, (2) a second line of sign (in addition to the earth's) that can aid in three dimensional reconstruction of the source distribution, and, (3) the possibility of correlation with direct measurements of the nonthermal particles from the probe itself.

Hudson, H. S.↗

NASA NTRS1978

A mean spherical model for soft potentials: The hard core revealed as a perturbation

The mean spherical approximation for fluids is extended to treat the case of dense systems interacting via soft-potentials. The extension takes the form of a generalized statement concerning the behavior of the direct correlation function c(r) and radial distribution g(r). From a detailed analysis that views the hard core portion of a potential as a perturbation on the whole, a specific model is proposed which possesses analytic solutions for both Coulomb and Yukawa potentials, in addition to certain other remarkable properties. A variational principle for the model leads to a relatively simple method for obtaining numerical solutions.

Rosenfeld, Y.↗

NASA NTRS1980

Torque-wrench extender for hard-to-reach fasteners

Extension kit for torque wrench tightens and loosens captive fasteners in hard-to-reach places. Kit consists of four universal socket joints and extender rod enclosed in greased-packed tube. Extension kit replaces snap-on adapter and flexible drive shaft.

Selik, S.↗