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At least 145 records · Page 8

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NASA NTRS1979

The reaction Mg/n, alpha/Ne at 14.1 and 14.7 MeV - Cross sections and implications for meteorites

Mass spectroscopic analysis of neutron-irradiated targets of natural magnesium yields cross sections of 59 + or - 14, 160 + or - 8, and 11.0 + or - 3.3 mb for Ne-20, Ne-21 and Ne-22, respectively, at 14.1 MeV and of 94 + or - 8, 152 or - 12, and 13.0 + or - 2.0 mb at 14.7 Mev. Using these cross sections, calculations modeling cosmic ray interactions in stony meteoroids of radii 20 and 26 cm predict that between the surface and the center the Ne-22/Ne-21 ratio falls more than 10% while the Ne-21 production rate rises about 30%. Calculations are compared with published neon measurements for the Keyes and St. Severin meteorites.

Reedy, R. C.↗

NASA NTRS1979

Detection of 6.13 MeV gamma-rays within and at the top of the atmosphere

A measurement of the 6.13 MeV gamma-radiation from excited O-16 in the atmosphere has been made with a high-resolution Ge(Li) spectrometer flown from Palestine, Texas, 1974 June 10, on a high-altitude balloon. This measurement, made both within and near the top of the atmosphere, is found to have the same general profile as predicted by a semiempirical model, but gives a flux about a factor of 2 higher than predicted. Certain variations in the intensity at float altitude have led to the hypothesis of an extraterrestrial source of 6.13 MeV gamma-radiation from the galactic anticenter. The 3 sigma upper limits for a number of other astrophysically significant line fluxes are also given. The data presented here are consistent with either a source or a no-source hypothesis.

Willett, J. B.↗

NASA NTRS1981

Measurement of the 0.511 MeV gamma ray line from the Galactic Center

The detection of the 0.511 MeV electron positron annihilation line coming from the Galactic Center to provide the means to estimate the rate of positron production and to test some theoretical sources of positrons is addressed. The results of the measurements of the 0.511 MeV line flux made with a gamma ray experiment on board a stratospheric balloon are presented. The detector field of view looked at the galactic longitude range -31 deg l(II) +41 deg. The observed flux is 0.0067 (+ or - 0.0005) photons 1/cm(2)5 which is in very good agreement with the expected flux when assuming that the Galactic Center is a line source emitting uniformly.

Jardim, J. O. D.↗

NASA NTRS1980

The energy spectrum of cosmic ray electrons, 5-150 MeV in late 1978 and early 1979

The University of Chicago cosmic ray electron detector was launched aboard the ISEE-3 spacecraft on August 12, 1978. The energy spectrum of electrons measured in 1978 is nearly identical to that measured in 1968 by OGO-5. During the period of our investigation we observe quiet time increases in the electron flux which are of Jovian origin. These increases are restricted to energies below about 25 MeV but appear to have an energy dependence that peaks around 15 MeV.

Evenson, P.↗

NASA NTRS1981

Centaurus A /NGC 5128/ at 2 keV-2.3 MeV - HEAO 1 observations and implications

The active-nucleus galaxy Centaurus A has been studied at 2 keV-2.3 MeV using data from the UCSD/MIT hard X-ray and low-energy gamma-ray instrument and the GSFC/CIT cosmic X-ray experiment on HEAO-1. It is found that an E exp -1.60 + or - 0.03 power law spectrum breaking to E exp -2.0 + or - 0.2 at 140 keV best describes the January and July 1978 data. The average intensity was 50% higher during the January observations. Upper limits to unresolved lines at 511 keV and 1.6 MeV were found to be 6.5 x 10 to the -4th photons/sq cm-s and 2.2 x 10 to the -4th photons/sq cm-s, respectively, at the 90% confidence level. The present data are consistent with the detailed calculations of the synchrotron self-Compton mechanism; they may also agree, marginally, with the predictions of emission from spherical accretion onto black holes.

Baity, W. A.↗

NASA NTRS1981

Limit on galactic 6.13 MeV gamma-ray line

The University of New Hampshire large gamma-ray detector observed the galactic center region during a balloon flight from Alice Springs, Australia on 1977 November 21-22. The absence of any observable line at 6.13 MeV or its escape-peak energy makes it possible to place an upper limit of 8.1 x 10 to the -4th photons/(sq cm s) at the 99% confidence level on the 0-16 de-excitation line at this energy from the galactic disk in the direction of the center. This limit restricts the interpretation given by Willett et al. (1979) of the line at 6.13 MeV which they observed while viewing the galactic anticenter. The present results indicate that it is highly unlikely that the line which these authors report is due to dark nebulae or the quiet sun. Possible explanations for their observation are atmospheric background, local production in the detector, a localized cosmic source in the direction of the galactic anticenter, or a statistical fluctuation.

Dunphy, P. P.↗

NASA NTRS1981

Interplanetary propagation of less than 1-MeV protons in nonimpulsive energetic particle events

An investigation is conducted regarding the existence of measurable transverse diffusive transport for 0.3- to 0.5 MeV protons. The study focuses on 1-hour averaged proton anisotropy data measured by detectors on board the earth-orbiting IMP 7 and 8 spacecraft. The data is restricted to nonimpulsive energetic particle events which are not explicitly associated with solar flares. It is established to within the limits of the measurements using 1-hour averages, that in nonimpulsive 0.3- to 0.5-MeV proton events as measured in the Newtonian inertial frame of the solar system, the anisotropy is predominantly transverse to the magnetic field and most hourly averages of the transverse component are essentially due to the particles' E x B drift.

Zwickl, R. D.↗

NASA NTRS1981

On the origin of the MeV energy nucleon flux associated with CIRs

Measurements of recurrent enhancements of interplanetary nucleon flux in the MeV energy range are presented and interpreted. The features recur at the solar rotation period in association with stream-stream plasma interaction regions corotating with the sun. At distances from the sun less than 1 AU, the maximum intensities of the hydrogen and helium components increase with increasing distance between 300%/AU and 600%/AU. A model is proposed which predicts the acceleration of nuclei from keV plasma energies to MeV energies by means of transit time damping of magnetosonic waves as solar wind plasma flows from the sun. Numerical solutions of the transport equation are derived to demonstrate that the model does not reproduce radial variations of hydrogen and helium fluxes, and estimates are made of the radial diffusive mean free path. Finally, the observations are found to be consistent with nucleon acceleration at the CIR shocks beyond 1 AU with subsequent diffusion toward the sun.

Christon, S. P.↗

NASA NTRS1981

Origin of 0.511 MeV emission from the Galactic center

The observations of 0.511 MeV positron annihilation radiation from the direction of the Galactic center are reviewed, and the implications of the line intensity, the shape of the line, and the position and angular size of the line emission region are discussed. These observations appear to require a compact source at the Galactic center, and within the constraints of both the 0.511 MeV observations and the hard X-ray, infrared and radio observations, a model is proposed of positron production and annihilation around a massive black hole in the Galactic nucleus.

Lingenfelter, R. E.↗

NASA NTRS1982

The energy spectrum of 20 keV-20 MeV electrons accelerated in large solar flares

IMP 6, 7, and 8 measurements of the energy spectrum of 20 keV to 20 MeV electrons observed from large solar flares are presented. To minimize propagation effects, only events from flares at W30 deg to W90 deg solar longitude are considered. The energy spectra are constructed using the maximum flux observed at each energy. It is shown that these spectra are representative of the spectra of the electrons escaping from the sun over this range of energies. It is found that every event shows the same spectral shape: a double power law with a smooth transition around 100-200 keV and power law exponents of 0.6-2.0 below and 2.4-4.3 above. The more intense the event, the harder the observed spectrum; in certain cases, the spectra are observed to steepen above 3 MeV.

Lin, R. P.↗

NASA NTRS1982

A high-resolution measurement of the 2.223 MeV neutron capture line in a solar flare

An intense solar flare lasting 40 s was observed by the HEAO 3 gamma-ray spectrometer on November 9, 1979, at 3:05 UT. The flare was observed in four high-resolution germanium detectors as well as in five CsI shield detectors over an energy range of 100 keV to above 5 MeV. Of particular interest is a line feature at 2.2248 + or - 0.0010 MeV. The precise energy measurement provides unambiguous evidence that this is the (H-1)(n, gamma)(H-2) line resulting from neutron capture on hydrogen. An upper limit of 5 keV is found for the natural line width. The time dependence of the neutron capture line is discussed as well as the overall characteristics of the November 9 flare.

Prince, T. A.↗

NASA NTRS1982

Pitch angle distributions of geomagnetically trapped MeV helium ions during quiet times

It is noted that during geomagnetically quiet conditions, energetic radiation belt helium ion fluxes at MeV energies have been found to exhibit characteristic radial profiles and large pitch angle anisotropies. Compiling data from many experiments, a systematic dependence of this anisotropy with helium ion energy is deduced. Provided a certain approximation holds for the observed pitch angle distributions, an empirical relation is deduced involving the helium ion energy. The range of the total ion energy here is 0.59-9 MeV (148-2250 keV per nucleon). These values are obtained for L shells in the range where L is approximately 2 to 5. The results are compared with theoretical expectations, and a qualitative explanation for the observed trend is suggested.

Fritz, T. A.↗

NASA NTRS1982

Solar energetic photon transients - 50 keV-100 MeV

The observed properties of solar transient photon impulses between a few keV and 100 MeV recorded by the SMM are compared and reasons for continued monitoring of solar photon transients are discussed. Various solar flare events observed are reviewed, along with gamma ray measurements. It is calculated that the emission of energetic photons is due to ion and electron interactions with matter, with both species undergoing acceleration at close time intervals. The gamma ray events and microwave emissions showed time structures similar to previously recorded X ray emissions. Bursts of all three radiation types have displayed a quasi-periodicity of less than 10 sec. Data on the 2.223 MeV neutron capture line have shown a decay in line intensity to take about 50 sec, consistent with Monte Carlo calculation for a He-3/H abundance of 0.00005.

Chupp, E. L.↗

NASA NTRS1983

Magnetic drifts at Io - Depletion of 10-MeV electrons at Voyager 1 encounter due to a forbidden zone

A model for magnetic field lines is used to track the Voyager spacecraft trajectory past Io back to the equatorial plane in order to determine the cause of an observed depletion of 10 MeV electrons in Io's orbit. The field lines are in the vicinity of an Alfven wing, a noncompressive field perturbation. It is suggested that the Jovian magnetospheric drift features influence the behavior of particles around Io, which also has a zone of reduced convection velocity. An analytical model is developed that demonstrates that a forbidden zone may exist around Io due to the gradient drifts and reduced convection velocity, and projections of particles convecting to the Io surface agree well with the Voyager data for particle concentrations. Discrepancies in the measured 9 MeV fluxes with respect to Pioneer data indicate that a possible change in the Io conductance in the interval between the passages of the two spacecraft.

Goldstein, B. E.↗

NASA NTRS1985

Abundance of low energy (50-150 MeV) antiprotons in cosmic rays

The progress is presented of the nuclear emulsion experiment to determine abundance of low energy antiprotons in cosmic rays. No antiprotons have been detected so far at upper limit of p/p less than or similar to 4 x .0001 in the energy range 50 MeV to 15 MeV.

Apparao, K. M. V.↗

NASA NTRS1985

On the detectability of key-MeV solar protons through their nonthermal Lyman-alpha emission

The intensity and timescale of nonthermal Doppler-shifted hydrogen L alpha photon emission as diagnostics of 10 keV to 10 MeV protons bombarding the solar chromosphere during flares are investigated. The steady-state excitation and ionization balance of the proton beam are determined, taking into account all important atomic interactions with the ambient chromosphere. For a proton energy flux comparable to the electron energy flux commonly inferred for large flares, L alpha wing intensities orders of magnitude larger than observed nonflaring values were found. Investigation of timescales for ionization and charge exchange leads researchers to conclude that over a wide range of values of mean proton energy and beam parameters, Doppler-shifted nonthermal L alpha emission is a useful observational diagnostic of the presence of 10 keV to 10 MeV superthermal proton beams in the solar flare chromosphere.

Canfield, R. C.↗

NASA NTRS1987

Performance of AlGaAs, GaAs and InGaAs cells after 1 MeV electron irradiation

Electron irradiations were made on three different types of III-V cells. AlGaAs, GaAs, and InGaAs cells with bandgaps of approximately 1.72, 1.43, and 1.1 eV, respectively, were tested. All of the cells were concentrator cells and performance data from one sun to beyond 100x AMO were taken. The total 1 MeV electron fluence was 3 times 10 to the 15th power e/square cm with data taken at several intermediate fluences. Cell performance is presented as a function of electron fluence for various concentration ratios and two different temperatures (25 and 80 C). Since these three cell types are potential candidates for the individual cells in a cascade structure, it is possible to calculate the loss in performance of cascade cells under 1 MeV irradiation. Data are presented which show the calculated performance of both series-connected and separately connected cascade cells.

Curtis, Henry B.↗

NASA NTRS1988

Directivity of 100 keV-1 MeV photon sources in solar flares

Stereoscopic observations of 0.1-1.0 MeV photon sources in solar flares made with spectrometers aboard the ISEE 3 and PVO (Pioneer Venus Orbiter) have been analyzed to determine the directivity of the photon sources and its possible dependence on photon energy. During the period October 1, 1978-October 31, 1980, a total of 44 solar flares were observed simultaneously by the two instruments. Of these, 39 flares were in full view of both the instruments, the remaining five being partially occulted by the photosphere from the line of sight of at least one instrument. The view angles theta(P) and theta(I) of the PVO and ISEE 3 instruments with respect to the outward solar radius at the flare site varied from one flare to another and were in the range 9-88 deg. The difference between the two view angles varied from 1 deg to 66 deg. The observations of differential photon energy spectra averaged over more than about 16 s do not indicate any systematic directivity. In most flares the directivity of 0.1-1.0 MeV photon sources is found to be less than about 2.5.

Kane, S. R.↗