Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Neutron Capture”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

The calculation of neutron capture gamma-ray yields for space shielding applications

The application of nuclear models to the calculation of neutron capture and inelastic scattering gamma yields is discussed. The gamma ray cascade model describes the cascade process in terms of parameters which either: (1) embody statistical assumptions regarding electric and magnetic multipole transition strengths, level densities, and spin and parity distributions or (2) are fixed by experiment such as measured energies, spin and parity values, and transition probabilities for low lying states.

Yost, K. J.↗
NASA NTRS2001

Neutron-Capture Kr-80 and Ar-36 in the Martian Atmosphere and Regolith

We calculate that approximately 10% of martian atmospheric Kr-80 formed by neutron capture on Mars in approx. 0.5 Ga. The regolith contains even larger amounts of n-capture Kr-80 and Ar-36, which may provide clues to the evolution of the martian regolith and atmosphere. Additional information is contained in the original extended abstract.

Rao, M. N.↗
NASA NTRS1972

Neutron capture on Gd and Sm in the Luna 16, G-2 soil.

Measurement of the Gd and Sm isotopic compositions in the Luna 16, G-2 soil, a sample which has the largest low energy neutron fluence yet observed in a lunar sample. The ratio of the number of neutrons captured per atom by Sm-149 to Gd-157 is 0.76, which is distinct from the value of 0.86 observed at the Apollo 11, 12, and 14 sites. This indicates a softer neutron energy spectrum at the Sea of Plenty.

Russ, G. P.↗
NASA NTRS1972

Exposure ages and neutron capture record in lunar samples from Fra Mauro.

Cosmic-ray exposure ages of Apollo 14 rocks and rock fragments obtained by the Kr81-Kr83 method range from 27 to 700 m.y. Rock 14321, collected near the Cone crater rim, is one of the many approximately 27 m.y. old ejecta which were reported at the Third Lunar Science Conference. All the other rocks have considerably higher exposure ages. Isotopic anomalies from neutron capture in gadolinium, bromine, and barium are used to obtain information on the lunar neutron spectrum at various depths below the lunar surface. The flux ratio of resonance and slow (less than 0.3 eV) neutrons is found to be nearly constant in the topmost approximately 100 g/sq cm.

Lugmair, G. W.↗
NASA NTRS1990

A new gamma-ray diagnostic for energetic ion distributions - The Compton tail on the neutron capture line

This paper presents a new radiation diagnostic for assaying the energy spectrum and the angular distribution of energetic ions incident on thick hydrogen-rich thermal targets. This diagnostic compares the number of emergent photons in the narrow neutron capture line at 2.223 MeV to the number of Compton scattered photons that form a low-energy tail on the line. It is shown that the relative strength of the tail can be used as a measure of the hardness of the incident ion-energy spectrum. Application of this diagnostic to solar flare conditions is the main thrust of the work presented here. It is examined how the strength of the Compton tail varies with flare viewing angle and the angular distribution of the flare-accelerated particles. Application to compact X-ray binary systems is also briefly discussed.

Vestrand, W. Thomas↗
NASA NTRS1980

Neutron capture time scale of the s-process, estimated from s-process krypton in a meteorite

A krypton fraction enriched in s-process isotopes was extracted from a mineral fraction of the Murchison C2 chondrite. The (Kr-86)/(Kr-84) ratio is enhanced by 6 standard deviations, showing that significant amounts of Kr-86 were made in the s-process, despite the short, 10.8 yr beta-decay half-life of its precursor, Kr-85. Judging from this sample, the mean neutron capture time in the s-process was on the order of 5-100 yr for nuclei with cross sections of 125 mb.

Matsuda, J.-I.↗
NASA NTRS2011

Atomic Data for Neutron-capture Elements I. Photoionization and Recombination Properties of Low-charge Selenium Ions

We present multi-configuration Breit-Pauli AUTOSTRUCTURE calculations of distorted-wave photoionization (PI) cross sections. and total and partial final-state resolved radiative recombination (RR) and dielectronic recombination (DR) rate coefficients for the first six ions of the trans-iron element Se. These calculations were motivated by the recent detection of Se emission lines in a large number of planetary nebulae. Se is a potentially useful tracer of neutron-capture nucleosynthesis. but accurate determinations of its abundance in photoionized nebulae have been hindered by the lack of atomic data governing its ionization balance. Our calculations were carried out in intermediate coupling with semi re1ativistic radial wavefunctions. PI and recombination data were determined for levels within the ground configuration of each ion, and experimental PI cross-section measurements were used to benchmark our results. For DR, we allowed (Delta)n = 0 core excitations, which are important at photoionized plasma temperatures. We find that DR is the dominant recombination process for each of these Se ions at temperatures representative of photoionized nebulae (approx.10(exp 4) K). In order to estimate the uncertainties of these data, we compared results from three different configuration-interaction expansions for each ion, and also tested the sensitivity of the results to the radial scaling factors in the structure calculations. We find that the internal uncertainties are typically 30-50% for the direct PI cross sections and approx.10% for the computed RR rate coefficients, while those for low-temperature DR can be considerably larger (from 15-30% up to two orders of magnitude) due to the unknown energies of near-threshold autoionization resonances. These data are available at the CDS, and fitting coefficients to the total RR and DR rate coefficients are presented. The results are suitable for incorporation into photoionization codes used to numerically simulate astrophysical nebulae, and will enable robust determinations of nebular Se abundances.

Sterling, N. C.↗
NASA NTRS1982

The heavy element yields of neutron capture nucleosynthesis

Consideration of the contribution made to the abundances of the heavy element isotopes by the S- and R-processes of nucleosynthesis has led to the determination that the previous assumption concerning the exclusive alignment of isobars to one or the other of these processes is probably in error. If the relatively small odd and even mass number abundance fluctuations characterizing R-process abundances are always the case, as assumed by this study, S-process contributions to the abundances of R-process isobars are substantial, consistent with transient flashing episodes in the S-process neutron production processes. A smooth and monotonically-decreasing curve of the abundance of the S-process yields times the neutron capture cross-section versus mass number is therefore the primary tool for the separation of the abundances due to the two processes.

Cameron, A. G. W.↗
NASA NTRS1986

Neutron capture production rates of cosmogenic 60Co, 59Ni and 36Cl in stony meteorites

Results for neutron flux calculations in stony meteoroids (of various radii and compositions) and production rates for Cl-36, Ni-59, and Co-60 are reported. The Ni-59/Co-60 ratio is nearly constant with depth in most meteorites: this effect is consistent with the neutron flux and capture cross section properties. The shape of the neutron flux energy spectrum, varies little with depth in a meteorite. The size of the parent meteorite can be determined from one of its fragments, using the Ni-59/Co-60 ratios, if the parent meteorite was less than 75 g/cm(2) in radius. If the parent meteorite was larger, a lower limit on the size of the parent meteorite can be determined from a fragment. In C3 chondrites this is not possible. In stony meteorites with R less than 50 g/cm(2) the calculated Co-60 production rates (mass less than 4 kg), are below 1 atom/min g-Co. The highest Co-60 production rates occur in stony meteorites with radius about 250 g/cm(2) (1.4 m across). In meteorites with radii greater than 400 g/cm(2), the maximum Co-60 production rate occurs at a depth of about 175 g/cm(2) in L-chondrite, 125 g/cm(2) in C3 chrondrite, and 190 g/cm(2) in aubrites.

Spergel, M. S.↗
NASA NTRS1972

Californium-252 neutron capture and decay methods for elemental analysis

The feasibility of using a Cf-252 neutron source in conjunction with a capture and/or decay gamma ray method for elemental analysis on lunar or planetary missions was tested. The general problems of using a Cf-252 neutron source for both decay and capture gamma ray analysis in terrestrial environments included the determination of the capture gamma ray spectra by neutron absorption in various metals used for the space hardware, Cf-252 source encapsulation materials, shielding, geometry, and optimum source size for a space mission. Computer data reduction and data transmission techniques were also investigated.

Source record↗
NASA NTRS1991

A search for the 2.223 MeV neutron capture gamma-ray line from the directions of Cygnus and the Galactic center

Free neutrons produced by nuclear reactions in high-temperature plasmas may undergo proton capture in ambient material, giving rise to gamma-ray emisison in a line at 2.223 MeV. Plausible settings for this process include accretion disks around black holes, material accreting onto neutron stars, and the binary companions of TeV gamma-ray sources. Data accumulated by the Solar Maximum Mission Gamma Ray Spectrometer (GRS) between 1980 and 1989 have been searched for evidence of this line, during periods when the black hole candidates Cygnus X-1 and the Galactic center were in transit across the GRS aperture. During these periods the neutron-star X-ray binaries Cygnus X-3 and Scorpius X-1 were also in the respective GRS fields of view. A 3-sigma upper limit of 0.0001/sq cm s has been placed on the steady emission in the 2.223 MeV line from the Galactic center and Scorpius X-1. Upper limits in the range 0.00012-0.00022 gamma/sq cm s have been set for Cygnus X-1 according to different models of the origin of the emission. The 3-sigma upper limit to the phase-averaged steady emission from Cygnus X-3 was found to be 0.00012 gamma/sq cm s.

Harris, Michael J.↗