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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1974

In-situ measurement of the rate of U-235 fission induced by lunar neutrons

The depth profile of the neutron-induced fission rate of U-235 was directly measured to a depth of 350 g/sq cm by the Apollo 17 Lunar Neutron Probe Experiment. The fission rate rises sharply from the surface to a broad maximum from 110 to 160 g/sq cm and drops off at greater depths. The shape of the theoretical depth profile of Lingenfelter et al (1972) fits the measured capture rates well at all depths. The absolute magnitude of the experimental fission rates are (11 plus or minus 17)% lower than those calculated theoretically. The excellent agreement between theory and experiment implies that conclusions drawn previously by interpreting lunar sample data with the theoretical capture rates will not require revision. In particular, lunar surface processes, rather than uncertainties in the capture rates, are required to explain the relatively low neutron fluences observed for surface soil samples compared to the fluences expected for a uniformly mixed regolith.

Woolum, D. S.↗
NASA NTRS1972

Neutron spectral measurements in the upper atmosphere

An experiment to measure neutrons in the upper atmosphere was performed on a balloon flight from Palestine, Texas, at an altitude of about 32 km. The experimental arrangement is discussed briefly, and results of a preliminary analysis of the data for neutrons in the energy range 3 to 30 MeV are given.

Zobel, W.↗
NASA NTRS1994

Asymptotic Giant Branch stars as a source of short-lived radioactive nuclei in the solar nebula

We carried out a theoretical evaluation of the contribution of Asymptotic Giant Branch (AGB) stars to some short-lived (10(exp 6) less than or equal to Tau-bar less than or equal to 2 x 10(exp 7) yr) isotopes in the Interstellar Medium (ISM) and in the early solar system using stellar model calculations for thermally pulsing evolutionary phases of low-mass stars. The yields of s-process nuclei in the convective He-shell for different neutron exposures tau(sub 0) were obtained, and AGB stars were shown to produce several radioactive nuclei (especially Pd-107, Pb-205, Fe-60, Zr-93, Tc-99, Cs-135, and Hf-182) in diferent amounts. Assuming either contamination of the solar nebula from a single AGB star or models for continuous injection and mixing from many stars into the ISM, we calculate the ratios of radioactive to stable nuclei at the epoch of the Sun's formation. The dilution factor between the AGB ejecta and the early solar system matter is obtained by matching the observed Pd-107/Pd-108 and depends on the value of tau(sub 0). It is found that small masses M(sub He) of He-shell material (10(exp -4)-10(exp -7) solar mass) enriched in s-process nuclei are sufficient to contaminate 1 solar mass of the ISM to produce the Pd-107 found in the early solar system. Predictions are made for all of the other radioactive isotopes. The optimal model to explain several observed radioactive species at different states of the proto-solar nebula involves a single AGB star with a low neutron exposure (tau(sub 0) = 0.03 mbarn(sup -1)) which contaminated the cloud with a dilution factor of M(sub He)/solar mass approximately 1.5 x 10(exp -4). This will also contribute newly synthesized stable s-process nuclei in the amount of approximately 10(exp -4) of their abundances already present in the proto-solar cloud. Variations in the degree of homogenization (approximately 30%) of the injected material may account for some of the small general isotopic anomalies found in meteorites. It is also found that Fe-60 is produced in small but significant quantities that may be sufficient to explain the observations if the time elapsed delta from the contamination of the ISM to the formation of protoplanetary bodies is not higher than delta = 5 x 10(exp 6) yr. If delta is longer, up to 10 x 10(exp 6) yr, this would require the single AGB star to experience enhanced neutron densities (n(sub n) approximately 3 x 10(exp 9)n/cu cm) in the s-processing zone in order to compensate for the branching at Fe-59. The alternative model of long-term continuous ejection of matter from many AGB stars does not appear to match the observations. We also estimate the Al-26 production from the H-shell and find that the Al-26 abundance in the early solar system may be readily explained in a self-consistent manner. Moreover, Al-26 from AGB stars may contribute substantially to the galactic Al-26 gamma-source, while no significant gamma-flux from Co-60 (deriving from Fe-60 decay) is to be expected.

Wasserburg, G. J.↗
NASA NTRS1968

Detection sensitivities in 3-8 MeV neutron activation

Study of detection sensitivities of 73 radioactive elements using fast unmoderated neutrons includes experiments for irradiation, cooling and counting conditions. The gamma ray emission spectra is used to identify the unknown material.

Wahlgren, M. A.↗
NASA NTRS1972

The ORNL-SNAP shielding program

The effort in the ORNL-SNAP shielding program is directed toward the development and verification of computer codes using numerical solutions to the transport equation for the design of optimized radiation shields for SNAP power systems. A brief discussion is given for the major areas of the SNAP shielding program, which are cross-section development, transport code development, and integral experiments. Detailed results are presented for the integral experiments utilizing the TSF-SNAP reactor. Calculated results are compared with experiments for neutron and gamma-ray spectra from the bare reactor and as transmitted through slab shields.

Mynatt, F. R.↗
NASA NTRS1972

Effect of multiple spin species on spherical shell neutron transmission analysis

A series of Monte Carlo calculations were performed in order to evaluate the effect of separated against merged spin statistics on the analysis of spherical shell neutron transmission experiments for gold. It is shown that the use of separated spin statistics results in larger average capture cross sections of gold at 24 KeV. This effect is explained by stronger windows in the total cross section caused by the interference between potential and J(+) resonances and by J(+) and J(-) resonance overlap allowed by the use of separated spin statistics.

Semler, T. T.↗
NASA NTRS1974

Surface history of some Apollo 17 lunar soils

Cosmic ray track densities in Apollo 17 soil samples are used to infer surface exposure times of soils from a trench at Van Serg Crater, from on and near a boulder at Camelot Crater, and from the position of the heat flow and neutron flux experiments (the ALSEP site). The topmost 2 cm of soil at Van Serg was exposed for 11 m.y., the top cm at Camelot for 36 m.y. A layering chronology and average deposition rate are proposed for the trench. For all soils the median track densities imply predispositional irradiation in the top 15 cm of the lunar surface for times that were long compared with the actual residence in the stratigraphic positions from which the soils were collected. Van Serg crater is inferred to have been formed approximately 24 m.y. ago.

Fleischer, R. L.↗
NASA NTRS1986

Hydrostatic compression of Fe(1-x)O wuestite

Hydrostatic compression measurements on Fe(0.95)O wuestite up to 12 GPa yield a room temperature value for the isothermal bulk modulus of K(ot) = 157 (+ or - 10) GPa at zero pressure. This result is in accord with previous hydrostatic and nonhydrostatic measurements of K(ot) for wuestites of composition: 0.89 = Fe/O 0.95. Dynamic measurements of the bulk modulus by ultrasonic, shock-wave and neutron-scattering experiments tend to yield a larger value: K(ot) approximately 180 GPa. The discrepancy between static and dynamic values cannot be explained by the variation of K(ot) with composition, as has been proposed. This conclusion is based on high-precision compression data and on theoretical models of the effects of defects on elastic constants. Barring serious errors in the published measurements, the available data suggest that wuestite exhibits a volume relaxation under pressure.

Jeanloz, R.↗
NASA NTRS1990

Electron energy spectrum and magnetic interactions in high-T(sub c) superconductors

The character of magnetic interactions in La-Sr-Cu-O and Y-Ba-Cu-O systems is of primary importance for analysis of high-T(sub c) superconductivity in these compounds. Neutron diffraction experiments showed the antiferromagnetic ground state for nonsuperconducting La2CuO4 and YBa2Cu3O6 with the strongest antiferromagnetic superexchange being in the ab plane. The nonsuperconducting '1-2-3' system has two Neel temperatures T sub N1 and T sub N2. The first one corresponds to the ordering of Cu atoms in the CuO2 planes; T sub N2 reflects the antiferromagnetic ordering of magnetic moments in CuO chains relatively to the moments in the planes T sub N1 and T sub N2 depend strongly on the oxygen content. Researchers describe magnetic interactions in high-T superconductors based on the Linear Muffin-Tin Orbitals (LMTO) band structure calculations. Exchange interaction parameters can be defined from the effective Heisenberg hamiltonian. When the magnetic moments are not too large, as copper magnetic moments in superconducting oxides, J sub ij parameters can be defined through the non-local magnetic susceptibility of spin restricted solution for the crystal. The results of nonlocal magnetic susceptibility calculations and the values of exchange interaction parameters for La CuO and YBa2Cu3O7 systems are given in tabular form. Strong anisotropy of exchange interactions in the ab plane and along the c axis in La2CuO4 is obviously seen. The value of Neel temperature found agrees well with the experimental data available. In the planes of '1-2-3' system there are quite strong antiferromagnetic Cu-O and O-O interaction which appear due to holes in oxygen subbands. These results are in line with the magnetic model of oxygen holes pairing in high-T(sub c) superconductors.

Turshevski, S. A.↗
NASA NTRS1991

Electron energy spectrum and magnetic interactions in high-Tc superconductors

The character of magnetic interactions in La-Sr-Cu-O and Y-Ba-Cu-O systems is of primary importance for analysis of high-T(sub c) superconductivity in these compounds. Neutron diffraction experiments showed the antiferromagnetic ground state for nonsuperconducting La2CuO4 and YBa2Cu3O6 with the strongest antiferromagnetic superexchange being in the ab plane. The nonsuperconducting '1-2-3' system has two Neel temperatures T(sub N1) and T(sub N2). The first one corresponds to the ordering of Cu atoms in the CuO2 planes; T(sub N2) reflects the antiferromagnetic ordering of magnetic moments in CuO chains relatively to the moments in the planes T(sub N1) and T(sub N2) which depend strongly on the oxygen content. Researchers describe magnetic interactions in high-T superconductors based on the Linear Muffin-Tin Orbitals (LMTO) band structure calculations. Exchange interaction parameters can be defined from the effective Heisenberg Hamiltonian. When the magnetic moments are not too large, as copper magnetic moments in superconducting oxides, J(sub ij) parameters can be defined through the non-local magnetic susceptibility of spin restricted solution for the crystal. The results of nonlocal magnetic susceptibility calculations and the values of exchange interaction parameters for La CuO and YBa2Cu3O7 systems are given in tabular form. Strong anisotropy of exchange interactions in the ab plane and along the c axis in La2CuO4 is obviously seen. The value of Neel temperature found agrees well with the experimental data available. In the planes of '1-2-3' system there are quite strong antiferromagnetic Cu-O and O-O interaction which appear due to holes in oxygen subbands. These results are in line with the magnetic model of oxygen holes pairing in high-T(sub c) superconductors.

Turshevski, S. A.↗
NASA NTRS1994

Equilibrium and Dynamics Properties of Poly(oxyethylene) Melts and Related Poly(alkylethers) from Simulations and Ab Initio Calculations

Molecular dynamics simulations of POE melts have been performed utilizing a potential force field parameterized to reproduce conformer energies and rotational energy barriers in dimethoxyethane as determined from ab initio electronic structure calculations. Chain conformations and dimensions of POE from the simulations were found to be in good agreement with predictions of a rotational isomeric state (RIS) model based upon the ab initio conformational. energies. The melt chains were found to be somewhat extended relative to chains at theta conditions. This effect will be discussed in light of neutron scattering experiments which indicate that POE chains are extended in the melt relative to theta solutions. The conformational characteristics of POE chains will also be compared with those of other poly (alkylethers), namely poly(oxymethylene), poly(oxytrimethylene) and poly(oxytetramethylene). Local conformational dynamics were found to be more rapid than in polymethylene. Calculated C-H vector correlation times were found to be in reasonable agreement with experimental values from C-13 NMR spin-lattice relaxation times. The influence of ionic salts on local conformations and dynamics will also be discussed.

Smith, Grant D.↗
NASA NTRS1972

Calculation of the absorbed dose and dose equivalent induced by medium energy neutrons and protons and comparison with experiment

Monte Carlo calculations have been carried out to determine the absorbed dose and dose equivalent for 592-MeV protons incident on a cylindrical phantom and for neutrons from 580-MeV proton-Be collisions incident on a semi-infinite phantom. For both configurations, the calculated depth dependence of the absorbed dose is in good agreement with experimental data.

Armstrong, T. W.↗
NASA NTRS1995

Fission foil measurements of neutron and proton fluences in the A0015 experiment

Results are given from sets of fission foil detectors (FFD's) (Ta-181, Bi-209, Th-232, U-238) which were included in the A0015 experiment to measure combined proton/neutron fluences. Use has been made of recent FFD high energy proton calibrations for improved accuracy of response. Comparisons of track density measurements have been made with the predictions of environmental modeling based on simple 1-D (slab) geometry. At 1 g/cm(exp 2) (trailing edge) the calculations were approximately 25 percent lower than measurements; at 13 g/cm(exp 2) (Earthside) calculations were more than a factor of 2 lower. A future 3-D modeling of the experiment is needed for a more meaningful comparison. Approximate mission proton doses and neutron dose equivalents were found. At Earthside (13 g/cm(exp 2) the dose was 171 rad and dose equivalent was 82 rem. At the trailing edge (1 g/cm(exp 2) dose was 315 rad and dose equivalent was 33 rem. The proton doses are less than expected from TLD doses by 16 percent and 37 percent, respectively. These differences can be explained by uncertainties in the proton and neutron spectra and in the method used to separate proton and neutron contributions to the measurements.

Frank, A. L.↗
NASA NTRS1979

CW nuclear pumped lasing of 3He-Ne

The results of a study on the CW nuclear pumped lasing of the He-Ne system are presented. Preliminary experiments measured single pass gain for the 6328.2A laser transition at neutron fluxes up to 1 x 10 to the 14th power neutrons/sq cm sec. Peak gain for a 300 torr (He-3)-Ne (5:1 mixture) was found to be 8.84 dB/meter at a 2 x 10 to the 12th power neutrons/sq cm sec neutron flux. Further experiments measured the gain in the flux region from one million to 2 x 10 to the 12th power neutrons/sq cm sec. In addition, gain vs. probe laser intensity at various neutron fluxes was measured. These measurements resulted in the construction of a laser cavity and the subsequent CW nuclear pumped lasing at 6328.2A in (He-3)-Ne. Experiments were also carried out on the 3.39 micrometer superradiant line in (He-3)-Ne. On three separate occasions, at two different facilities, lasing was observed. Although (He-3)-Ne was found to have the lowest threshold for lasing found so far, it also had the lowest output power on the order of tens of microwatts.

Carter, B. D.↗
NASA NTRS1997

Assay of the Martian Regolith with Neutrons

The purpose of the research is to combine experiments and Monte Carlo transport of neutrons through volume of soil in an attempt to model neutron leakage from planetary surfaces. Emphasis is given to the change of neutron spectra as a function of water content and location. During the first stage of effort, two experiments were conducted in which leakage of neutrons from a Pu-Be source through about 30 g/cm(exp 2) of soil were measured with several counters. A Monte Carlo code, MCNP, has been used to model many of the 100 individual runs of the experiment. Hydrogen is the element that has the most dramatic effect on the neutron spectrum and its effect on the neutron spectrum is almost the same whether it is in the form of water or polyethylene. In order to simulate various water configurations, sheets of polyethylene have been used between layers of soil as well as water in several concentrations up to 18%. Comparison of experimental results to theoretical predictions made with the MCNP code were disappointing for low concentrations of water. We have made extensive calculations to see if room return could be the cause of the discrepancies. Water concentrations of the 'dry' soil were measured by two different laboratories and differed only by 0.5%. We have made calculations to optimize the next experiment and are investigating other methods of determining the water content of 'dry' soil.

Drake, Darrell M.↗
NASA NTRS1993

Redistribution of subsurface neutrons caused by ground ice on Mars

As part of an ongoing effort to understand data that will be returned by the Mars Observer Gamma Ray Spectrometer (MOGRS) experiment, we calculate neutron and gamma ray fluxes for simple models describing near-surface ground ice on Mars. Our goal is to determine the limits in information content of remotely sensed neutrons and gamma rays is delineating the abundance of ground ice and its distribution with depth. Such ice is expected based on simple theoretical calculations of the H2O environment of the martian surface and atmosphere. We find it produces a marked redistribution of neutrons with depth. Neutron-capture gamma ray production closely follows the flux of thermal neutrons and is likewise redistributed with depth. Calculated effects at Mars mapping orbit are sufficiently large that, if ice is present at the depths and in the quantities expected, the MOGRS should provide a robust detection of its presence. Although in the information content of gamma rays and neutrons alone is not sufficient to define the many parameters needed to specify its abundance and emplacement below the surface, their combined use might allow separation of the ice abundance from its vertical distribution within the regolith.

Feldman, W. C.↗