Fine structure in nuclear resonances.
Angular distribution of outcoming gamma radiation associated with giant dipole resonance in Si super 28 nucleus
SILICON↗
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Comparison of Measured Leakage Current Distributions with Calculated Damage Energy Distributions in HgCdTe
This paper presents a combined Monte Carlo and analytic approach to the calculation of the pixel-to-pixel distribution of proton-induced damage in a HgCdTe sensor array and compares the results to measured dark current distributions after damage by 63 MeV protons. The moments of the Coulombic, nuclear elastic and nuclear inelastic damage distribution were extracted from Monte Carlo simulations and combined to form a damage distribution using the analytic techniques first described in [I]. The calculations show that the high energy recoils from the nuclear inelastic reactions (calculated using the Monte Car10 code MCNPX [2]) produce a pronounced skewing of the damage energy distribution. The nuclear elastic component (also calculated using the MCNPX) has a negligible effect on the shape of the damage distribution. The Coulombic contribution was calculated using MRED [3,4], a Geant4 [4,5] application. The comparison with the dark current distribution strongly suggests that mechanisms which are not linearly correlated with nonionizing damage produced according to collision kinematics are responsible for the observed dark current increases. This has important implications for the process of predicting the on-orbit dark current response of the HgCdTe sensor array.
Marshall, C. J.↗
Remarks on the theory of mu-mesic atoms
Nuclear charge distribution model for muon-meson atomic calculations
CHARGE DISTRIBUTION↗
A digital computer program for calculating steady temperature and density distributions in a gas containing heat sources
Computer program for calculating axial temperature and density distribution in nuclear reactor gas exposed to neutron flux
GAS HEATING↗
Black holes in elliptical galaxies
Black hole objects in elliptical galaxies, using stellar light and velocity dispersion distribution in nuclear regions for mass limits
Burbidge, G. R.↗
Explosive nucleosynthesis and the composition of metal-poor stars
Stellar initial metal content effect on nuclear abundance distributions synthesized under explosive carbon, oxygen and silicon burning conditions
Arnett, W. D.↗
Cosmic electrons
The published literature on cosmic electrons is summarized. The primary and secondary sources of cosmic electrons are discussed, and the propagation of the electrons in the interstellar medium is studied with respect to energy loss mechanisms, age distributions, and spectral modifications during flight. Various portions of the electron and positron spectra are then considered in relation to problems of astrophysics. New information is presented on such topics as the origin of low-energy positrons, the decay kinematics of the pi-mu-e process, the application of age distributions for nuclear cosmic rays to cosmic electrons, and the possibility of nonidentical sources for cosmic electrons and protons.
Ramaty, R.↗
A Monte Carlo model for the gardening of the lunar regolith
The processes of movement and turnover of the lunar regolith are described by a Monte Carlo model. The movement of material by the direct cratering process is the dominant mode, but slumping is also included for angles exceeding the static angle of repose. Using a group of interrelated computer programs, a large number of properties are calculated, including topography, formation of layers, depth of the disturbed layer, nuclear-track distributions, and cosmogenic nuclides. In the most complex program, the history of a 36-point square array is followed for times up to 400 million years. The histories generated are complex and exhibit great variety. Because a crater covers much less area than its ejecta blanket, there is a tendency for the height change at a test point to exhibit periods of slow accumulation followed by sudden excavation. In general, the agreement with experiment and observation seems good, but two areas of disagreement stand out. First, the calculated surface is rougher than that observed. Second, the observed bombardment ages, of the order 400 million are shorter than expected (by perhaps a factor of 5).
Arnold, J. R.↗
Experiment K305: Quantitative analysis of selected bone parameters. Supplement 1: Effects of weightlessness on osteoblast differentiation in rat molar periodontium
The morphometric analysis of periodontal ligament (PDL), the osteogenic interface between tooth and bone, is described. Immediately post-flight, PDL width and total cell number were decreased. Frequency distributions of nuclear volume revealed that presumptive preosteoblasts were particularly depressed. Depleted numbers of preosteoblasts may be an important factor in the mechanism of inhibited bone formation during weightlessness.
Roberts, W. E.↗
An abrasion-ablation model description of galactic heavy-ion fragmentation
The fragmentation of high-energy galactic heavy ions by nuclear interactions with arbitrary target nuclei is described within the context of a simple abrasion-ablation fragmentation model. The abrasion part of the theory utilizes a quantum-mechanical formalism based upon an optical model potential approximation to the exact nucleus-nucleus multiple-scattering series. Nuclear charge distributions of the excited prefragments are calculated using either a hypergeometric distribution or a method based upon the zero-point oscillations of the giant dipole resonance. The excitation energy of the prefragment is estimated from the geometric clean-cut abrasion-ablation model. The decay probabilities for the various particle emission channels, in the ablation stage of the fragmentation, are obtained from the EVAP-4 Monte Carlo computer program. Elemental production cross sections for 1.88-GeV/nucleon iron colliding with carbon, silver, and lead targets are calculated and compared with experimental data and with the predictions from the semiempirical relations of Silberberg and Tsao.
Townsend, L. W.↗
Optical model calculations of heavy-ion target fragmentation
The fragmentation of target nuclei by relativistic protons and heavy ions is described within the context of a simple abrasion-ablation-final-state interaction model. Abrasion is described by a quantum mechanical formalism utilizing an optical model potential approximation. Nuclear charge distributions of the excited prefragments are calculated by both a hypergeometric distribution and a method based upon the zero-point oscillations of the giant dipole resonance. Excitation energies are estimated from the excess surface energy resulting from the abrasion process and the additional energy deposited by frictional spectator interactions of the abraded nucleons. The ablation probabilities are obtained from the EVA-3 computer program. Isotope production cross sections for the spallation of copper targets by relativistic protons and for the fragmenting of carbon targets by relativistic carbon, neon, and iron projectiles are calculated and compared with available experimental data.
Townsend, L. W.↗
20 kHz Space Station power system
The Space Station represents the next major U.S. commitment in space. The efficient delivery of power to multiple user loads is key to that success. In 1969, NASA Lewis Research Center began a series of studies with component and circuit developments that led to the high frequency bi-directional, four quadrant resonant driven converter. Additional studies and subsequent developments into the early 1980's have shown how the high frequency ac power system could provide overall advantages to many aerospace power systems. Because of its wide versatility, it also has outstanding advantages for the Space Station Program and its wide range of users. High frequency ac power provides higher efficiency, lower cost, and improved safety. The 20 kHz power system has exceptional flexibility, is inherently user friendly, and is compatible with all types of energy sources - photovoltaic, solar dynamic, rotating machines or nuclear Lewis distribution system testbed. The testbed demonstrates flexibility, versatility, and transparency to user technology as well as high efficiency, low mass, and reduced volume.
Hansen, Irving G.↗
Theoretical investigation of gas-surface interactions
A Dirac-Hartree-Fock code was developed for polyatomic molecules. The program uses integrals over symmetry-adapted real spherical harmonic Gaussian basis functions generated by a modification of the MOLECULE integrals program. A single Gaussian function is used for the nuclear charge distribution, to ensure proper boundary conditions at the nuclei. The Gaussian primitive functions are chosen to satisfy the kinetic balance condition. However, contracted functions which do not necessarily satisfy this condition may be used. The Fock matrix is constructed in the scalar basis and transformed to a jj-coupled 2-spinor basis before diagonalization. The program was tested against numerical results for atoms with a Gaussian nucleus and diatomic molecules with point nuclei. The energies converge on the numerical values as the basis set size is increased. Full use of molecular symmetry (restricted to D sub 2h and subgroups) is yet to be implemented.
Dyall, Kenneth G.↗
Abrasion-ablation model for neutron production in heavy ion collisions
In intermediate energy nucleus-nucleus collisions, neutron production at forward angles is observed to occur with a Gaussian shape that is centered near the beam energy and extends to energies well above that of the beam. This paper presents an abrasion-ablation model for making quantitative predictions of the neutron spectrum. To describe neutrons produced from the abrasion step of the reaction where the projectile and target overlap, we use the Glauber model and include effects of final-state interactions. We then use the prefragment mass distribution from abrasion with a statistical evaporation model to estimate the neutron spectrum resulting from ablation. Measurements of neutron production from Ne and Nb beams are compared with calculations, and good agreement is found.
NASA Discipline Radiation Health↗
Comparison of Measured Dark Current Distributions with Calculated Damage Energy Distributions in HgCdTe
This paper presents a combined Monte Carlo and analytic approach to the calculation of the pixel-to-pixel distribution of proton-induced damage in a HgCdTe sensor array and compares the results to measured dark current distributions after damage by 63 MeV protons. The moments of the Coulombic, nuclear elastic and nuclear inelastic damage distributions were extracted from Monte Carlo simulations and combined to form a damage distribution using the analytic techniques first described in [1]. The calculations show that the high energy recoils from the nuclear inelastic reactions (calculated using the Monte Carlo code MCNPX [2]) produce a pronounced skewing of the damage energy distribution. While the nuclear elastic component (also calculated using the MCNPX) contributes only a small fraction of the total nonionizing damage energy, its inclusion in the shape of the damage across the array is significant. The Coulombic contribution was calculated using MRED [3-5], a Geant4 [4,6] application. The comparison with the dark current distribution strongly suggests that mechanisms which are not linearly correlated with nonionizing damage produced according to collision kinematics are responsible for the observed dark current increases. This has important implications for the process of predicting the on-orbit dark current response of the HgCdTe sensor array.
Marshall, C. J.↗
Electron energy distribution in a helium plasma created by nuclear radiations
An integral balance technique for calculation of the electron energy distribution in a radiation-induced plasma is described. Results predict W-values reasonably well and compare favorably with more complicated Monte-Carlo calculations. The distribution found differs from that in a normal electrical discharge and is of interest in radiation-pumped laser research.
Lo, R. H.↗
Electron capture in carbon dwarf supernovae
The rates of electron capture on heavier elements under the extreme conditions predicted for dwarf star supernovae have been computed, incorporating modifications that seem to be indicated by present experimental results. An estimate of the maximum possible value of such rates is also given. The distribution of nuclei in nuclear statistical equilibrium has been calculated for the range of expected supernovae conditions, including the effects of the temperature dependence of nuclear partition functions. These nuclide abundance distributions are then used to compute nuclear equilibrium thermodynamic properties. The effects of the electron capture on such equilibrium matter are discussed. In the context of the 'carbon detonation' supernova model, the dwarf central density required to ensure core collapse to a neutron star configuration is found to be slightly higher than that obtained by Bruenn (1972) with the electron capture rates of Hansen (1966).-
Mazurek, T. J.↗
Numerical models for cometary dust environments
The development of a numerical model for the dust environment near a comet is described. The major components of the model are a simple treatment of the acceleration of the dust by the gas outflow from an isothermal icy nucleus, a detailed treatment of the dust particle trajectories controlled by solar gravity and radiation pressure, including the formation of the dust envelope and tail, and a simple treatment of the radiation from the dust in the visible and thermal infrared. The results of recent comet observations are used to estimate parameter values for the model, including dust-to-gas ratios by light and by mass, the distribution of dust and gas emission on the nuclear surface, the distributions of dust particle size, density, mass, and radiative properties, and other quantities. Gas and dust production rates as functions of time are derived from observed light curves using the simple photometric theory. Sample results are presented for the dust concentration, flux and fluence along the trajectory of a spacecraft which flies by Comet P/Halley in 1986.
Divine, N.↗