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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Radiation damage of the HEAO C-1 germanium detectors

The effects of radiation damage from proton bombardment of the four HEAO C-1 high purity germanium detectors have been measured and compared to predictions. Because of the presence of numerous gamma-ray lines in the detector background spectra and because of the relatively long exposure time of the HEAO 3 satellite to cosmic-ray and trapped protons, it has been possible to measure both the energy and time dependence of radiation damage. After 100 d in orbit, each of the four detectors has been exposed to approximately 3 x 10 to the 7th protons/sq cm, and the average energy resolution at 1460 keV had degraded from 3.2 keV fwhm to 8.6 keV fwhm. The lines were all broadened to the low energy side although the line profile was different for each of the four detectors. The damage-related contribution to the degradation in energy resolution was found to be linear in energy and proton influence.

Mahoney, W. A.↗
NASA NTRS1981

Hard X-ray spectrum of Cygnus X-1

Long-term measurements of the hard X-ray spectrum from 3 keV to 8 MeV of the black-hole candidate Cygnus X-1 in its low state are reported. Observations were made from October 26 to November 18, 1977 with the A2 (Cosmic X-ray) and A4 (Hard X-ray and Low-Energy Gamma-Ray) experiments on board HEAO 1 in the spacecraft's scanning mode. The measured spectrum below 200 keV is found to agree well with previous spectra which have been fit by a model of the Compton scattering of optical or UV photons in a very hot plasma of electron temperature 32.4 keV and optical depth 3.9 or 1.6 for spherical or disk geometry, respectively. At energies above 300 keV, however, flux excess is observed which may be accounted for by a distribution of electron temperatures from 15 to about 100 keV.

Nolan, P. L.↗
NASA NTRS1981

Confinement of cosmic rays in molecular clouds

The consequences of cosmic ray production by a supernova in a molecular cloud are discussed. Self-trapping problems for a higher flux of cosmic rays in a molecular cloud are focused on. The column density of molecular clouds is probably too high to explain the majority of galactic cosmic ray sources, even allowing for fortuitous asymmetry in the placement of the supernova in the cloud, however, measurements of antiproton flux suggest that some cosmic ray sources do have a high column density. The large predicted gamma-ray luminosity of such a cloud invites comparison with COS-B sources.

Zweibel, E. G.↗
NASA NTRS1981

A European X-ray astrophysics mission

Five instruments (Bragg Spectrometer, Large Area Proportional and Scintillation Counter Detectors, Wide Field X-ray Cameras and a Gamma-Ray Burst Monitor) are discussed and estimates of their performance are given. Their scientific aims are summarized and sample observing programmes are discussed.

Culhane, J. L.↗
NASA NTRS1981

Spectroscopy in the 10 keV to 10 MeV range

Spectral lines in the 10 keV to 1 MeV range carry information of fundamental importance on many astronomical objects. Since the lines are directly related to specific physical processes this information is model independent and gives the physical conditions in the objects. At the sensitivities achieved to date, approximately 0.0001 to 0.001 phsq cm. sec for steady sources and approximately 0.01 to 1 ph/sq cm sec for transient sources, lines were detected from the galactic center, gamma-ray bursts and transients, X-ray pulsators, the Crab pulsar and solar flares. Future instruments with a factor of approximately 100 sensitivity improvement will allow detailed spectroscopic study of these classes of objects as well as supernova remnants, active galaxies and the interstellar medium. This sensitivity improvement can be obtained through the use of detector technology already proven in balloon and satellite instruments.

Matteson, J. L.↗
NASA NTRS1981

Absence of J/pi/, T = 0/+/, 1 level excitations in residual nuclei from /pi /+ or -/,2N/ reactions

Gamma-ray spectra taken in prompt coincidence with incident 180-MeV pi/+/, 220-MeV pi/-/ and 0-MeV pi/-/ on the self-conjugate targets C-12 and S-32 reveal that the cross sections/yields are equal to or near zero for production of the J/pi/,T = 0/+/, 1 levels of the odd Z, odd N nuclei resulting from np removal. It is thought that the 0-MeV pi/-/ results can be explained by selection rules governing one-step quasi-free interactions but that the delta(1232) region results cannot. This indicates that there may be additional restrictions upon the pion absorption mechanism.

Stronach, C. E.↗
NASA NTRS1980

Nuclear gamma rays from 720-MeV alpha-induced reactions on Al-27 and Si-28

Prompt gamma rays from the interaction of 720-MeV alpha particles with Al-27 and Si-28 were detected and analyzed to identify residual nuclei and to determine cross sections for production of specific levels. No gamma-ray transitions were detected from nuclei heavier than the target. From Doppler broadening, the momentum of the residual nuclei was estimated. The results are compared with previous results for 140- and 1600-MeV alphas on Al-27 and approximately 200-MeV positive or negative pions on Al-27 and Si-28 and fitted to a spallation-yield formula.

Lieb, B. J.↗
NASA NTRS1981

Annihilation radiation from a hot e/+/-e/-/ plasma

The radiation from electron-positron annihilations in a plasma of temperature above 10 to the 8th K is investigated as a possible source of the emission line at energies between 400 and 460 keV frequently seen in gamma-ray bursts. The annihilation rate and luminosity of an optically thin electron-positron plasma and the energy distribution of the resulting annihilation radiation are calculated by the use of a Monte Carlo technique as functions of temperature. Results indicate the annihilation spectrum to be peaked at an energy of 0.511 MeV plus a temperature-dependent blueshift, and the annihilation line to be significantly temperature-broadened. The widths of the observed burst emission lines set an upper limit of 3 x 10 to the 8th K on the temperature of any pair annihilation region in burst sources, which is considerably lower than the typical kinetic temperatures of the radiating particles. It is thus inferred that either the annihilation region is nonthermal, or spatially distinct from the burst site.

Ramaty, R.↗
NASA NTRS1981

HEAO 3 measurements of the atmospheric positron annihilation line

The atmospheric positron annihilation line at 511 keV was the first gamma-ray line observed which originates from processes of astrophysical importance. Measurements of the properties of a positron annihilation line make it possible to determine many of the physical conditions present at the source of emission. An investigation was conducted of the 511-keV line measured with the HEAO C-1 experiment. The HEAO C-1 experiment consisted of a cluster of four, large, high-purity germanium detectors surrounded by an active anticoincidence CsI shield. A strong atmospheric line was measured at 511.07 + or - 0.10 keV. The net atmospheric emission has been accurately determined as a function of geomagnetic latitude and varies from 0.01 photons/sq cm-sec-sr near the magnetic equator to 0.035 photons/sq cm-sec-sr at a latitude of 55 deg.

Mahoney, W. A.↗
NASA NTRS1981

The radiolysis and radioracemization of amino acids on silica surfaces

Results are presented of experiments on the radioracemization of amino acids in the presence of silica surfaces such as may have been found on the prebiotic earth. L-leucine and a DL-leucine mixture deposited on samples of 1-quartz and an amorphous silica preparation (Syloid 63) was subjected to Co-60 gamma-ray irradiation, then analyzed by gas chromatography to determine the radiolysis and racemization rates. The quartz surface is found to have a marginal efficacy in enhancing radiolysis when compared with a crystalline L-leucine control, although enhancing radioracemization symmetrically by a factor of two. Both the radiolysis and radioracemization of L-leucine and DL-leucine on a Syloid-63 silica surface are observed to increase with increasing radiation dose, and to be substantially greater than in the crystalline controls. Additional experiments with the nonprotein amino acid isovaline deposited on Syloid 63 confirm the greater radiolysis susceptibility of amino acids deposited on silica with respect to the crystalline state, although racemization is not observed. The observations suggest that the presence of a silica surface would have a deleterious effect on any mechanism for the origin of molecular chirality relying on stereoselective beta-radiolysis.

Bonner, W. A.↗
NASA NTRS1980

Enhanced orbital geochemical images by the Laplacian subtraction method

Orbital geochemical studies of the moon based on X-ray fluorescence and gamma-ray spectroscopy have been successful. There are, however, difficulties if geologic units or other features to be investigated are too small. Investigations have been conducted with the objective to improve with the aid of image enhancement methods both spatial resolution and contrast for the geochemical experiments. A description is provided of the Laplacian subtraction method of enhancement. It was found that the application of a least-squares version of this method to the Mg/Al ratios obtained by orbital X-ray fluorescence improved geochemical images considerably. The spatial resolution increased by about a factor of 2 and the contrast also increased. The Laplacian of an image can also be used to detect very small 'hot' or 'cold' spots in an image.

Schonfeld, E.↗
NASA NTRS1981

Confinement and acceleration of cosmic rays in galactic superbubbles

The role of the newly discovered galactic superbubbles on the confinement and acceleration of cosmic rays is examined. It is shown that these structures naturally account for the lifetime and the local origin of the cosmic rays, as well as for the location of galactic gamma-ray sources not associated with pulsars.

Kafatos, M.↗
NASA NTRS1981

Origins of the low energy relativistic interplanetary electrons

Electron measurements in the energy range 2-25 MeV on the Pioneer 10 spacecraft are studied from 1 to 21.5 AU. It is found that in this radial range, interplanetary low energy electron fluxes are of Jovian origin, based on the decreasing electron intensity from about 6 to 21.5 AU, a negative gradient from about 11 to 21.5 AU, and the constant spectral index observed from 1 to 21.5 AU. The upper limit of the galactic flux is estimated at 12 MeV and standard assumptions are applied to solar modulation. It is found that at 1 AU, the expected flux of galactic origin is a factor 300 or more below the observed quiet time flux, and the extrapolated interstellar flux level is consistent with estimates based on galactic diffuse radio and gamma-ray emissions.

Eraker, J. H.↗
NASA NTRS1982

Compositional variation in the Hadley Apennine region

Orbital geochemical data in the Hadley Apennine region are related to typical rock compositions and used in determining the distribution of soils derived from the rock types found in this region. Orbital XRF Mg/Si and Al/Si intensities are the orbital data that are used primarily. These data are corrected for spurious interorbit variation using a modification of a previously developed method. The corrected values are than converted to % MgO and % Al2O3, respectively, from theoretical considerations, and as such are compared with similar concentrations for typical lunar rocks and soils of the Apollo 15 landing site. The relationship of the XRF values to Fe, Ti, and Th concentrations, derived from gamma-ray observations, is also considered. It is established that the orbital geochemistry data for this region are consistent with the presence of a mixture of ANT suite and Fra Mauro basalt components frequently dominated by a KREEP basalt component toward the west and by a mafic pyroclastic component toward the east.

Clark, P. E.↗
NASA NTRS1982

Thorium concentrations in the lunar surface. V - Deconvolution of the central highlands region

The distribution of thorium in the lunar central highlands measured from orbit by the Apollo 16 gamma-ray spectrometer is subjected to a deconvolution analysis to yield improved spatial resolution and contrast. Use of two overlapping data fields for complete coverage also provides a demonstration of the technique's ability to model concentrations several degrees beyond the data track. Deconvolution reveals an association between Th concentration and the Kant Plateau, Descartes Mountain and Cayley plains surface formations. The Kant Plateau and Descartes Mountains model with Th less than 1 part per million, which is typical of farside highlands but is infrequently seen over any other nearside highland portions of the Apollo 15 and 16 ground tracks. It is noted that, if the Cayley plains are the result of basin-forming impact ejecta, the distribution of Th concentration with longitude supports an origin from the Imbrium basin rather than the Nectaris or Orientale basins. Nectaris basin materials are found to have a Th concentration similar to that of the Descartes Mountains, evidence that the latter may have been emplaced as Nectaris basin impact deposits.

Metzger, A. E.↗
NASA NTRS1981

Thermonuclear processes on accreting neutron stars

Theoretical models for X-ray burst sources that invoke thermonuclear flashes on the surface layers of an accreting neutron star are discussed. The historical development of X-ray burst observation is summarized, and a physical picture of a neutron star undergoing accretion is drawn. Detailed numerical computations of the evolution of the surface layers of such a star are reviewed. The need for general relativistic corrections to the model is pointed out. Finally, comparisons are made with observations of X-ray bursts, the rapid burster, fast X-ray transients, X-ray pulsars, and gamma-ray burst sources.

Joss, P. C.↗
NASA NTRS1988

Supernova 1987A

Supernova 1987A (February 23, 1987) in the Large Magellanic Cloud is the brightest supernova to be observed since SN 1604 AD (Kepler). Detection of a burst of neutrinos indicates that a neutron star was formed. Radioactive decay of about 0.07 solar mass of Co-56 is responsible for the observed optical light as well as hard X-rays and gamma-ray lines. Ultraviolet, optical, and infrared 'light echoes' and soft X-rays provide information on the distribution of circumstellar matter and the evolution of the progenitor star.

Mccray, Richard↗
NASA NTRS1971

Neutron Activation Analysis of Water - A Review

Recent developments in this field are emphasized. After a brief review of basic principles, topics discussed include sources of neutrons, pre-irradiation physical and chemical treatment of samples, neutron capture and gamma-ray analysis, and selected applications. Applications of neutron activation analysis of water have increased rapidly within the last few years and may be expected to increase in the future.

Buchanan, John D.↗