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NASA NTRS1975

Gamma ray astronomy

A review is given of recent papers in the field of gamma-ray astronomy. It is mentioned that the strongest feature of the gamma ray sky is the general radiation from the galactic plane which is superimposed on what appears to be a general diffuse radiation, whose properties are not yet defined. The observation of high-energy galactic gamma radiation by the SAS-2 satellite is considered as is the observation of point and localized sources (the Crab Nebula, Vela, etc.). The observation of diffusive gamma radiation is discussed.

Fichtel, C. E.↗

NASA NTRS1976

The origin of OB associations and extended regions of high-energy activity in the Galaxy through supernova cascade processes

A number of observed characteristics of OB associations, including their positive total energies and subgroup structure, are explained by the hypothesis that an association is composed of stars formed by compression of interstellar clouds in blast waves of several generations of supernovae. The later-generation supernovae are simply the most massive stars formed by this mechanism in the earlier generations. This supernova cascade process may also be responsible for giant loops such as the North Polar Spur. Enhanced high-energy gamma-ray emission is expected from the loops.

Ogelman, H. B.↗

NASA NTRS1976

Search for gamma ray bursts with coincident balloon flights

A search was conducted for cosmic gamma ray bursts of small size and of sufficient frequency of occurrence to be detected during a one day observation program. Two similar detectors, successfully balloon-borne from launch sites in South Dakota and Texas, achieved about 20 hours of simultaneous operation at several millibars atmospheric depth, with continuous separation of over 1,500 km. Fluctuations of the counting rates of less than 150 keV photons with temporal structures from microseconds to several minutes were compared in order to detect coincident or associated responses from the two instruments. No coincident gamma-ray burst events were detected. The resulting integral size spectrum of small bursts, from this and from all other searches, remains a spectrum of upper limits, consistent with an extrapolation of the size spectrum of the largest known bursts, fitting a power low of index -1.5.

Cline, T. L.↗

NASA NTRS1977

Diffuse galactic gamma ray lines

The production rate of 4.44-MeV line for a variety of assumed cosmic ray spectra is evaluated. These results are compared with reported galactic gamma-ray line intensities and are consistent with a low energy cosmic ray density which increases toward the galactic center in proportion to the molecular gas density.

Lingenfelter, R. E.↗

NASA NTRS1977

Gamma rays and large scale galactic structure

Gamma ray astronomy is now beginning to provide a new look at the galactic structure and the distribution of cosmic rays, both electrons and nucleons, within the galaxy. The observations are consistent with a galactic spiral-arm model in which the cosmic rays are linearly coupled to the interstellar gas on the scale of the spiral arms. The agreement between the predictions of the model and the observations for regions of the plane where both 21-cm and 2.6-mm CO surveys exist emphasizes the need to extend these observations to include the entire plane. Future gamma-ray observations with more sensitivity and better angular resolutions, combined with these radio surveys, should shed new light on the distribution of cosmic rays, the nature of the galaxy, and the location and intensity of the spiral arms.

Kniffen, D. A.↗

NASA NTRS1977

Geology orbiter comparison study

Instrument requirements of planetary geology orbiters were examined with the objective of determining the feasibility of applying standard instrument designs to a host of terrestrial targets. Within the basic discipline area of geochemistry, gamma-ray, X-ray fluorescence, and atomic spectroscopy remote sensing techniques were considered. Within the discipline area of geophysics, the complementary techniques of gravimetry and radar were studied. Experiments using these techniques were analyzed for comparison at the Moon, Mercury, Mars and the Galilean satellites. On the basis of these comparative assessments, the adaptability of each sensing technique was judged as a basic technique for many targets, as a single instrument applied to many targets, as a single instrument used in different mission modes, and as an instrument capability for nongeoscience objectives.

Cutts, J. A. J.↗

NASA NTRS1976

Terrestrial bodies orbiter, LUNAR - A system for detailed global measurements of a planet's properties

The NASA Lunar Polar Orbiter, to be launched by a Delta vehicle in 1980, is designed to perform a global exploration of such moon properties as the gravity field, figure, and surface composition, as well as a lunar resource survey. The paper reviews the mission, spacecraft, and data system concepts in terms of baseline mission sequence, choice of orbits, spacecraft configuration requirements, mission sequence requirements and data system and operations. Scientific experiments planned for the Orbiter mission include geophysical altimetry and gravity experiments, heat flow experiment, magnetic field experiments, measurements of the moon's natural gamma-ray spectrum, X-ray fluorescence of lunar surface materials, sunlight reflectance spectroscopy, and spectro-stereo imaging. Characteristics of instruments designed for use in these investigations are examined. Through the use of advanced sensors and data-system techniques, coupled with well-developed spacecraft systems, the mission promises to return a very large quantity and variety of lunar data at a cost comparable with that of simpler missions in the past.

Burke, J. D.↗

NASA NTRS1976

Electrons in a closed galaxy model of cosmic rays

The paper considers the consistency of positrons and electrons with a propagation model in which the cosmic rays are stopped by nuclear collisions or energy losses before they can escape from the Galaxy (the closed-galaxy model). The fact that no inconsistency is found between the predictions and the data implies that the protons which produce the positrons by nuclear reactions could have their origin in a large number of distant sources, as opposed to the heavier nuclei which in this model come from a more limited set of sources. The closed-galaxy model predicts steep electron and positron spectra at high energies. None of these are inconsistent with present measurements; but future measurements of the spectrum of high-energy positrons could provide a definite test for the model. The closed-galaxy model also predicts that the interstellar electron intensity below a few GeV is larger than that implied by other models. The consequence of this result is that electron bremsstrahlung is responsible for about 50% of the galactic gamma-ray emission at photon energies greater than 100 MeV

Ramaty, R.↗

NASA NTRS1977

The high energy astronomy observatories

The forthcoming NASA project of orbiting High Energy Astronomy Observatories (HEAO's) designed to probe the universe by tracing celestial radiations and particles is outlined. Solutions to engineering problems concerning HEAO's which are integrated, yet built to function independently are discussed, including the onboard digital processor, mirror assembly and the thermal shield. The principle of maximal efficiency with minimal cost and the potential capability of the project to provide explanations to black holes, pulsars and gamma-ray bursts are also stressed. The first satellite is scheduled for launch in April 1977.

Neighbors, A. K.↗

NASA NTRS1976

The relevance of solar flares to astrophysics

The physical mechanisms associated with solar flares are reviewed. The relevance of flare mechanisms to other astrophysical phenomena is discussed. In this context, specific models of quasars and radio galaxies, Sco X-1 and gamma-ray bursts are examined.

Sturrock, P. A.↗

NASA NTRS1976

Electron transport and the magnetic field in the Crab Nebula

Observational data obtained at radio, optical, and X-ray frequencies on the size of the emission region in the Crab Nebula are used to examine various energy-transport models. It is shown that the data cannot be fitted by diffusion models, even when the energy-injection region is extended. A more favorable comparison is made with an extended source model in which bulk motion is the transport mechanism. Assuming a point source of electrons and a uniform nebular magnetic field, a perpendicular magnetic-field component of less than 0.0008 gauss is determined. Combining this with the lower limit obtained from gamma-ray observations, a perpendicular field component of between 0.0005 and 0.0008 gauss is obtained. A break in the size dependence is predicted at about 12 microns and 45 keV. Above 45 keV, the size should decrease as the inverse square root of proton energy.

Wolff, R. S.↗

NASA NTRS1976

Non-thermal processes in large solar flares

The paper examines the significance of particle acceleration processes in large solar flares, and the importance of accelerated particles in the generation of other flare phenomena. Comprehensive observations of the August 1972 series of large flares were used to obtain quantitative estimates of the energetic particle populations, the electromagnetic emissions, and the mass ejections. The fluxes of energetic electrons and protons were derived from observed X-ray and gamma-ray emissions, respectively. The energy input into the solar atmosphere from the collisional losses of these energetic particles is computed as a function of overlying column density, under the assumption that particles are accelerated at a point high in the solar atmosphere and then precipitate down to denser layers. The flash phase radiation, soft X-ray flare plasma, and the interplanetary shock wave can be consistently and quantitatively explained as a result of the interaction of nonrelativistic electrons with the solar atmosphere.

Lin, R. P.↗

NASA NTRS1976

Surface chemistry of selected lunar regions

A completely new analysis has been carried out on the data from the Apollo 15 and 16 gamma-ray spectrometer experiments. The components of the continuum background have been estimated. The elements Th, K, Fe, and Mg give useful results; for Ti are significant only for a few high-Ti regions. Errors are given, and the results are checked by other methods. Concentrations are reported for about 60 lunar regions; the ground track has been subdivided in various ways. The borders of the maria seem well-defined chemically, while the distribution of KREEP is broad. This wide distribution requires emplacement of KREEP before the era of mare formation. Its high concentration in western mare soils seems to require major vertical mixing.

Bielefeld, M. J.↗

NASA NTRS1977

Cosmic rays in a dynamical halo. I - Age and matter traversal distributions and anisotropy for nuclei. II - Electrons

Recent measurements of the mean lifetime of galactic cosmic rays (at least 10 million yr) have suggested a dynamical halo model of cosmic-ray confinement to the Galaxy. A one-dimensional version of the model is analyzed to determine the ranges of parameters required to give self-consistent values of the mean lifetime, matter traversed, anisotropy, and interstellar gamma-ray flux. It is found that the ranges of the parameters are: disk scale height of 4000-10,000 pc, average convection velocity of no more than 60 km/s, and diffusion coefficient of (3-10) by 10 to the 28th power sq cm/sec for particles with charges of the order of 10 GV. Some consequences for this model of a much shorter lifetime are also discussed. The energy spectrum and spatial distribution of electrons in the dynamical halo model are considered. A gradual steepening of the electron energy spectrum beginning at several GeV energy is found, as indicated by observations. The decrease of electron density with height in the halo depends on the particle energy and may be much steeper than that for nuclei. The models give a halo with a scale height of the order of 3 kpc for several-GeV electrons, which is shown to be consistent with radio observations.

Owens, A. J.↗

NASA NTRS1977

Neutral particle background in cosmic ray telescopes composed of silicon solid state detectors

The energy loss-spectrum of secondary charged particles produced by the interaction of gamma-rays and energetic neutrons in silicon solid state detectors has been measured with a satellite-borne cosmic ray telescope. In the satellite measurements presented here two distinct neutral background effects are identified: secondary protons and alpha particles with energies of about 2 to 100 MeV produced by neutron interactions, and secondary electrons with energies of about 0.2 to 10 MeV produced by X-ray interactions. The implications of this neutral background for satellite measurements of low energy cosmic rays are discussed, and suggestions are given for applying these results to other detector systems in order to estimate background contamination and optimize detector system design.

Mewaldt, R. A.↗

NASA NTRS1977

Design and performance of an actively collimated phoswich system for X-ray astronomy

The design and performance of a phoswich type scintillation detector system having 34 sq cm effective collecting area and a sensitivity of 3 x 10 to the -5th power photons/sq cm/s/keV for balloon-borne cosmic X-ray source observations are discussed. The various shield anticoincidence systems are evaluated for effectiveness in reduction of the system background. The total background of nominally 4 x 10 to the -4th power counts/sq cm/s/keV is analyzed and found to consist of aperture flux, K-escape X-rays from the shielding, leakage, and degradation of gamma-rays through the shielding, and several particle induced effects. The background analysis suggests an ultimate practical upper limit of about 8 x 10 to the -5th power counts/sq cm/s/keV might be attainable beyond which further sensitivity improvements will require increased collecting area or a basically differing design approach.

Matteson, J. L.↗

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.↗