68 Matching Results

Search Results

Advanced search parameters have been applied.

High-resolution neutron capture and transmission measurements and the stellar neutron capture cross sections of {sup 116,120}Sn

Description: Improved astrophysical reaction rates for {sup 116,120}Sn(n, {gamma}) are of interest because nucleosynthesis models have not been able to reproduce the observed abundances in this mass region. For example, previous s-process calculations have consistently underproduced the s-only isotope {sup 116}Sn. Also, these studies have resulted in residual reprocess abundances for the tin isotopes which are systematically larger than predicted by reprocess calculations. It has been suggested that these problems could be solved by reducing the solar tin abundance by 10-20%, but there is no experimental evidence to justify this renormalization. Instead, it is possible that the problem lies in the (n,T) cross sections used in the reaction network calculations or in the s-process models. One reason to suspect the (n, {gamma}) data is that previous measurements did not extend to low enough energies to determine accurately the Maxwellian-averaged capture cross sections at the low temperatures (kT=6-8 keV) favored by the most recent stellar models of the s process. Also, the two most recent high-precision measurements of the {sup 120}Sn(n, {gamma}) cross section are in serious disagreement. Because of its small size, this cross section could affect (via the s-process branching at {sup 121}Sn) the relative abundances of the three s-only isotopes of Te.
Date: June 1, 1997
Creator: Koehler, P.E.; Spencer, R.R. & Guber, K.H.
Partner: UNT Libraries Government Documents Department

Science with soft X-rays

Description: Synchrotron radiation with photon energies at or below 1keV is giving new insights into such areas as wet cell biology, condensed matter physics and extreme ultraviolet optics technology.
Date: July 7, 2000
Creator: Smith, Neville V.
Partner: UNT Libraries Government Documents Department

Neutron Capture and Neutron Total Cross Sections Measurements for {sup 27}Al at the Oak Ridge Electron Linear Accelerator

Description: We have used the Oak Ridge Electron Linear Accelerator (ORELA) to measure neutron total and capture cross sections of {sup 27}Al in the energy range from 100 eV to {approximately}400 keV. We report the resonance parameters as well as the Maxwellian average capture cross sections.
Date: August 30, 1999
Creator: Guber, K.H.; Harvey, J.A.; Hill, N.W.; Koehler, P.E.; Leal, L.C.; Sayer, R.O. et al.
Partner: UNT Libraries Government Documents Department


Description: The R-matrix resonance analysis of experimental neutron transmission and cross sections of {sup 233}U, with the Reich-Moore Bayesian code SAMMY, was extended up to the neutron energy of 600 eV by taking advantage of new high resolution neutron transmission and fission cross section measurements performed at the Oak Ridge Electron Linear Accelerator (ORELA). The experimental data base is described. In addition to the microscopic data (time-of-flight measurements of transmission and cross sections), some experimental and evaluated integral quantities were included in the data base. Tabulated and graphical comparisons between the experimental data and the SAMMY calculated cross sections are given. The ability of the calculated cross sections to reproduce the effective multiplication factors k{sub eff} for various thermal, intermediate, and fast systems was tested. The statistical properties of the resonance parameters were examined and recommended values of the average s-wave resonance parameters are given.
Date: February 27, 2001
Creator: Leal, L.C.
Partner: UNT Libraries Government Documents Department

Generalized oscillator strengths. Progress report, July 1, 1973--July 1, 1974

Description: Progress is described on research under Contract AT(11-1)3247 and research still to be completed in the period July 1, 1973 to July 1, 1974. The research objectives defined in the original proposal were closely followed. The principal accomplishments during the period were: The introduction of a computer controlled counting system with provision for automatic control and operation of the electron spectrometer currently in use. Electron scattering with excitation of singlet-triplet transitions for helium was studied during the year both to acquire information on collision cross sections and to test a general theory of the abnormally high cross sections for forward scattering found for certain types of transitions. A theoretically predicted minimum in the cross section (at zero scattering angle) was found in a study over the range 100 to 500 eV of the 1/sup 1/S yields 2/sup 3/S transition of helium. Abnormally high cross sections for singlet-triplet transitions at high kinetic energy are predicted when the orbital term symbol is unchanged on excitation. As a test of the theory the X/sup 1/ SIGMA /sup +/ yields b/sup 3/ SIGMA /su p +/ transition in CO was looked for and found at THETA = 0 deg at both 200 and 300 eV thus confirming the theory. New electron scattering studies on both CO and CO/sub 2/ are described. A new method for the calculation of singlet-triplet energy differences from generalized oscillator strengths is described. (auth)
Date: January 1, 1974
Creator: Lassettre, Edwin N.
Partner: UNT Libraries Government Documents Department

New neutron capture and transmission measurements for {sup 134,136}Ba at ORELA and their impact on s-process nucleosynthesis calculations

Description: We have made high-resolution neutron capture and transmission measurements on isotopically enriched samples of {sup 134}Ba and {sup 136}Ba at the Oak Ridge Electron Linear Accelerator (ORELA) in the energy range from 20 eV to 500 keV. Previous measurements had a lower energy limit of 3 - 5 keV, which is too high to determine accurately the Maxwellian-averaged capture cross section at the low temperatures (kT {approx} 6 - 12 keV) favored by the most recent stellar models of the {ital s}-process. Our results for the astrophysical reaction rates are in good agreement with the most recent previous measurement at the classical {ital s}-process temperature, kT = 30 keV, but show significant differences at lower 40 temperatures. We discuss the astrophysical implications of these differences.
Date: October 1, 1996
Creator: Koehler, P.E.; Spencer, R.R.; Winters, R.R.; Guber, K.H.; Harvey, J.A.; Hill, N.W. et al.
Partner: UNT Libraries Government Documents Department

Final report for the Sept. 1997 - Jan. 2000 period of Contract No. DE-FG02-97ER54445. Fundamental science of high-density fluorocarbon plasmas

Description: This report describes the results obtained during the two and a half years of work on the contract ''Fundamental Science of High-Density Fluorocarbon Plasmas.'' This program established critical elements of the scientific knowledge base of high-density fluorocarbon plasmas used for SiO{sub 2} patterning. To this end the authors characterized the species that exist in the gas phase and the processes that occur at relevant surfaces in contact with the plasma using complementary diagnostic instrumentation for plasma and surface characterization at two universities. By moving diagnostics from one university to the other, the full spectrum of diagnostics was applied to a single plasma reactor and fluorocarbon plasma etching process. The results of these measurements were correlated with data obtained when patterning SiO{sub 2} using identical conditions. In parallel, a reactive beam scattering system was employed to establish the consequences of the interaction of mass- and energy-resolved low-energy (20 to 300 eV) ions with SiO{sub 2} and resist surfaces.
Date: February 1, 2001
Creator: Oehrlein, G.S.; Anderson, H.M. & Cecchi, J.L.
Partner: UNT Libraries Government Documents Department

Surface and adsorbate structural studies by photoemission in the h. nu. = 50-500 eV range

Description: The present status of photoelectron spectroscopy in the 50-500 eV range is discussed in relation to its application to surface science. Instrumentation aspects of synchrotron radiation sources are reviewed. The direct transition model is shown to be applicable in this range with some limitations. Cooper minima and adsorbate sensitivity enhancement for h..nu.. > 100 eV are reviewed. A new effect--condensed phase photoelectron asymmetry--is noted. Finally, photoelectron diffraction - another new effect - is described and evaluated.
Date: August 1, 1979
Creator: Shirley, D.A.
Partner: UNT Libraries Government Documents Department

Indirect processes in electron-impact ionization of multiply-charged ions

Description: Recent research efforts have assembled a steadily increasing array of measurements and calculations of electron impact ionization of multiply-charged ions. Significant disagreement is often found, however, between experimental results and theory due to the influence of a number of indirect processes observed in the experiments but not included in the calculations. Some of the most important of these indirect ionization processes are described and examples are discussed. New data for electron impact single-ionization of Kr/sup 2 +/ are presented in the energy range from below threshold (36 eV) to 1500 eV, and compared to related measurements and to theory.
Date: January 1, 1984
Creator: Gregory, D.C.
Partner: UNT Libraries Government Documents Department

Developing a 500-eV proton beam

Description: Tests have been carried out which indicate that H- intensity limitations in lamb-shift polarized ion sources result from large diveregence in the 500-eV H+ ion beam. Intensity limitations due to other effects are not yet measurable.
Date: January 1, 1983
Creator: Chamberlin, E.P.; Benage, J.F. Jr. & Williams, H.E.
Partner: UNT Libraries Government Documents Department

Neutron resonance spectroscopy: $sup 209$Bi

Description: Neutron time-of-flight transmission measurements were made on several samples of $sup 209$Bi by using the Nevis Synchrocyclotron of Columbia University. The resonance parameters are given for 29 levels to 75 keV. Out of the 29 observed levels 10 were l = 0 and 19 were l = 1 levels. The implied s and p strength functions are 10$sup 4$S$sub 0$ = (0.60$sup +0$.$sup 39$/sub -0.21/) and 10$sup 4$S$sub 1$ + (0.19$sup +0$.$sup 08$/sub -0.05/). 1 figure, 2 tables. (auth)
Date: January 1, 1975
Creator: Singh, U.N.; Rainwater, J.; Liout, H.I.; Hacken, G. & Garg, J.B.
Partner: UNT Libraries Government Documents Department

TART calculations using the new 2021 energy group cross section library compared with B. Czirr's resonance self-shielding experiments

Description: Results from calculations using a 2021-energy-group cross section library with sufficient energy definition in the lower energy range to resolve the known resorances in /sup 239/Pu and /sup 235/U are compared with flat-plate resorance self-shielding experimental data. The results are somewhat ambiguous. The lower energy ranges appear to agree better with experimental data for small areal densities, but disagree more as the areal density increases. (20 figures) (RWR)
Date: November 13, 1973
Creator: Bacon, L.
Partner: UNT Libraries Government Documents Department

Calculation of resonance self shielding in $sup 239$Pu

Description: The 2000-group TART code and the ENDL cross section library were used to calculate the fissions as a function of depth in the flat Pu plate experiment of Czirr and Bramblett (Nucl. Sci. Eng., fissions with the Pu target in to the number with the target out; secondary fissions in the target do not contribute. Results from experiment and Monte Carlo calculations are given for thicknesses from 0.06 to 3.13 g/cm/sup 2/ in nine energy groups from 2.15 eV to 10 keV. It was found that the TART 2000-group structure was it adequate since vast increases in the number of groups did not change the results. Where the total and fission cross section data are well known, the calculations and experiment were in reasonably good agreement. The only region of disagreement was in the unresolved resonance region above 300 eV. (11 figures, 1 table) (RWR)
Date: January 15, 1974
Creator: Plechaty, E. F. & Cullen, D. E.
Partner: UNT Libraries Government Documents Department

2D ACAR momentum density study of the nature of the positron surface state on Al(100)

Description: The two-dimensional angular correlation of the 2..gamma.. annihilation radiation (2D ACAR) has been measured from an Al(100) surface bombarded by 200-eV positrons. After removing the contribution of fast para-positronium annihilation, the spectrum from positrons annihilating at the surface exhibits a nearly isotropic conical shape with a (7.1 +- 0.5) mrad FWHM. 5 refs., 6 figs.
Date: January 1, 1985
Creator: Berko, S.; Canter, K.F.; Lynn, K.G.; Mills, A.P.; Roellig, L.O. & West, R.N.
Partner: UNT Libraries Government Documents Department

Angular distribution and atomic effects in condensed phase photoelectron spectroscopy

Description: A general concept of condensed phase photoelectron spectroscopy is that angular distribution and atomic effects in the photoemission intensity are determined by different mechanisms, the former being determined largely by ordering phenomena such as crystal momentum conservation and photoelectron diffraction while the latter are manifested in the total (angle-integrated) cross section. In this work, the physics of the photoemission process is investigated in several very different experiments to elucidate the mechanisms of, and correlation between, atomic and angular distribution effects. Theoretical models are discussed and the connection betweeen the two effects is clearly established. The remainder of this thesis, which describes experiments utilizing both angle-resolved and angle-integrated photoemission in conjunction with synchrotron radiation in the energy range 6 eV less than or equal to h ..nu.. less than or equal to 360 eV and laboratory sources, is divided into three parts.
Date: November 1, 1981
Creator: Davis, R.F.
Partner: UNT Libraries Government Documents Department

Final technical report: DE FG02-98ER45688

Description: Research using the Advanced Light Source Spectro-microscopy facility is described. Three closely related techniques, x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, and x-ray fluorescence spectroscopy, have become widely accepted as important tools for the study of the chemical composition and electronic properties of surfaces, overlayers, and interfaces. There is now a major effort to push these spectroscopic techniques into a new realm of applications with very high spatial resolution, at and below 1 micron. This results in a new set of probes which can create images of chemical composition with great subtlety. The field is growing rapidly as high brightness sources of x-rays become available. This 6 month project was used to initiate research applications of soft x-ray spectro-microscopes at the Advanced Light Source. Due to its short duration, only preliminary results were obtained. The term ''spectromicroscopy'' is an ugly and unwieldy word to impose on an experimental endeavor, but it has been adopted by a number of disciplines and is likely to remain in use for some time. The word is obviously a contraction of the phrase ''spectroscopic microscopy,'' but there is also a distinction sometimes made between this, and the reverse combination, ''microscopic spectroscopy'', or ''microspectroscopy''. Microspectroscopy is a spectroscopic measurement with a small probe beam, which is usually fixed in position on the sample. Spectromicroscopy refers to the case where an image can be formed from the spatial dependence of the features in some spectrum.
Date: November 30, 2000
Creator: Tonner, Brian P.
Partner: UNT Libraries Government Documents Department

High energy resolution measurement of the /sup 238/U neutron capture yield in the energy region between 1 and 100 keV

Description: A measurement of the /sup 238/U neutron capture yield was performed at the 150 meter flight-path of the ORELA facility on two /sup 238/U samples (0.01224 and 0.0031 atomsbarn). The capture yeild data were normalized by Moxon's small resonance method. The energy resolution achieved in this measurement frequently resulted in doublet and triplet splittings of what appeared to be single resonance in previous measurements. This resolution should allow extension of the resolved resonance energy region in /sup 238/U from the present 4-keV limit up to 15 or 20 keV incident neutron energy. Some 200 small resonances of the (/sup 238/U /plus/ n) compound nucleus have been observed which had not been detected in transmission measurement, in the energy range from 250 eV to 10 keV.
Date: January 1, 1988
Creator: Machlin, R.L.; Perez, R.B.; de Saussure, G. & Ingle, R.W.
Partner: UNT Libraries Government Documents Department