1,862 Matching Results

Search Results

Advanced search parameters have been applied.

Particle emission from polycrystalline and monocrystalline niobium under 14.1 MeV neutron impact

Description: A brief review of recent results on particle emission from cold-rolled and annealed niobium with coarse surface finishes, and from monocrystalline niobium surfaces under 14.1-MeV neutron impact is given. For the cold-rolled and annealed samples two types of deposits were discovered on collector surfaces facing the irradiated targets. One type appeared as a fractional atom layer covering the surface, the other in the form of chunks. No chunk emission was observed for the monocrystalline niobium surface. The chunk deposits on the collectors facing the cold-rolled samples are not uniformly distributed over the collector area but are clustered along certain streaks or appear in patches. Furthermore, the number of chunks per unit collector area and the average chunk size were larger for the rough, cold-rolled samples than for the annealed samples. These observations suggest that the release of stresses (e.g. stresses caused by cold-rolling) in the surface regions by the energy deposited by primary recoils, and surface roughness affect the chunk emission significantly. Previously reported crude estimates of sputtering yields were improved on the basis of information which has become available from additional neutron irradiation runs. (auth)
Date: January 1, 1975
Creator: Kaminsky, M. & Das, S.K.
Partner: UNT Libraries Government Documents Department

Analysis of (n,2n) cross-section measurements for nuclei up to mass 238

Description: All suitable measurements of the energy dependence of (n,2n) cross sections of all isotopes up to mass 238 have been analyzed. The objectives were to display the quality of the measured data for each isotope and to examine the systematic dependence of the (n,2n) cross section upon N, Z, and A. Graphs and tables are presented of the ratio of the asymptotic (n,2n) and nonelastic cross section to the neutron-asymmetry parameter (N--Z)/A. Similar data are presented for the derived nuclear temperature, T, and level-density parameter, $alpha$, as a function of N, Z, and A. This analysis of the results of over 145 experiments on 61 isotopes is essentially a complete review of the current status of (n,2n) cross-section measurements. (auth)
Date: June 1, 1975
Creator: Davey, W.G.; Goin, R.W. & Ross, J.R.
Partner: UNT Libraries Government Documents Department

Neutron cross section covariances in the resolved resonance region.

Description: We present a detailed analysis of the impact of resonance parameter uncertainties on covariances for neutron capture and fission cross sections in the resolved resonance region. Our analysis uses the uncertainties available in the recently published Atlas of Neutron Resonances employing the Multi-Level Breit-Wigner formalism. We consider uncertainties on resonance energies along with those on neutron-, radiative-, and fission-widths and examine their impact on cross section uncertainties and correlations. We also study the effect of the resonance parameter correlations deduced from capture and fission kernels and illustrate our approach on several practical examples. We show that uncertainties of neutron-, radiative- and fission-widths are important, while the uncertainties of resonance energies can be effectively neglected. We conclude that the correlations between neutron and radiative (fission) widths should be taken into account. The multi-group cross section uncertainties can be properly generated from both the resonance parameter covariance format MF32 and the cross section covariance format MF33, though the use of MF32 is more straightforward and hence preferable.
Date: April 1, 2008
Creator: Herman,M.; Mughabghab, S.F.; Oblozinsky, P.; Pigni, M.T. & Rochman, D.
Partner: UNT Libraries Government Documents Department

Neutron Capture Cross Section Measurement on $^{238}$Pu at DANCE

Description: The proposed neutron capture measurement for {sup 238}Pu was carried out in Nov-Dec, 2010, using the DANCE array at LANSCE, LANL. The total beam-on-target time is about 14 days plus additional 5 days for the background measurement. The target was prepared at LLNL with the new electrplating cell capable of plating the {sup 238}Pu isotope simultaneously on both sides of the 3-{micro}m thick Ti backing foil. A total mass of 395 {micro}g with an activity of 6.8 mCi was deposited onto the area of 7 mm in diameter. The {sup 238}Pu sample was enriched to 99.35%. The target was covered by 1.4 {micro}m double-side aluminized mylar and then inserted into a specially designed vacuum-tight container, shown in Fig. 1, for the {sup 238}Pu containment. The container was tested for leaks in the vacuum chamber at LLNL. An identical container without {sup 238}Pu was made as well and used as a blank for the background measurement.
Date: February 14, 2011
Creator: Chyzh, A & Wu, C Y
Partner: UNT Libraries Government Documents Department

Neutron Capture gamma ENDF libraries for modeling and identification of neutron sources

Description: There are a number of inaccuracies and data omissions with respect to gammas from neutron capture in the ENDF libraries used as field reference information and by modeling codes used in JTOT. As the use of Active Neutron interrogation methods is expanded, these shortfalls become more acute. A new, more accurate and complete evaluated experimental database of gamma rays (over 35,000 lines for 262 isotopes up to U so far) from thermal neutron capture has recently become available from the IAEA. To my knowledge, none of this new data has been installed in ENDF libraries and disseminated. I propose to upgrade libraries of {sup 184,186}W, {sup 56}Fe, {sup 204,206,207}Pb, {sup 104}Pd, and {sup 19}F the 1st year. This will involve collaboration with Richard Firestone at LBL in evaluating the data and installing it in the libraries. I will test them with the transport code MCNP5.
Date: October 29, 2007
Creator: Sleaford, B
Partner: UNT Libraries Government Documents Department

Benchmarking the External Surrogate Ratio Method using the (alpha,alpha' f) reaction at STARS

Description: We measured the ratio of the fission probabilities of {sup 234}U* relative to {sup 236}U* formed via an ({alpha},{alpha}{prime}) direct reactions using the STARS array at the 88-inch cyclotron at the Lawrence Berkeley National Laboratory. This ratio has a shape similar to the ratio of neutron capture probabilities from {sup 233}U(n; f) and {sup 235}U(n; f), indicating the alpha reactions likely formed a compound nucleus. This result indicates that the ratios of fission exit channel probabilities for two actinide nuclei populated via ({alpha}, {alpha}{prime}) can be used to determine an unknown fission cross section relative to a known one. The validity of the External Surrogate Ratio Method (ESRM) is tested and the results support the conclusions of Burke et al. [1].
Date: January 9, 2008
Creator: Lesher, S R; Bernstein, L A; Ai, H; Beausang, C W; Bleuel, D; Burke, J T et al.
Partner: UNT Libraries Government Documents Department

Compound-Nucleus Formation Following Direct Interactions to Highly-Excited Final States

Description: When direct reactions populate highly excited, unbound configurations in the residual nucleus, the nucleus may further evolve into a compound nucleus. Alternatively, the residual system may decay by emitting particles into the continuum. Understanding the relative weights of these two processes as a function of the angular momentum and parity deposited in the nucleus is important for the surrogate-reaction technique. A particularly interesting case is compound-nucleus formation via the (d, p) reaction, which may be a useful tool for forming compound nuclei off the valley of stability in inverse-kinematics experiments. We present here a study of the compound formation probability for a closely-related direct reaction, direct-semidirect radiative neutron capture.
Date: February 5, 2008
Creator: Dietrich, F S
Partner: UNT Libraries Government Documents Department

Neutron-capture Cross Sections from Indirect Measurements

Description: Cross sections for compound-nuclear reactions play an important role in models of astrophysical environments and simulations of the nuclear fuel cycle. Providing reliable cross section data remains a formidable task, and direct measurements have to be complemented by theoretical predictions and indirect methods. The surrogate nuclear reactions method provides an indirect approach for determining cross sections for reactions on unstable isotopes, which are difficult or impossible to measure otherwise. Current implementations of the method provide useful cross sections for (n,f) reactions, but need to be improved upon for applications to capture reactions.
Date: October 18, 2011
Creator: Escher, J E; Burke, J T; Dietrich, F S; Ressler, J J; Scielzo, N D & Thompson, I J
Partner: UNT Libraries Government Documents Department

Standard thermal energy group structure for generation of thermal group constants from ENDF/B data

Description: The final specifications of a standard energy group structure for the generation of thermal group constants from ENDF/B data are presented. The report represents the work of a committee appointed by the Codes and Formats Subcommittee of the Cross Section Evaluation Working Group and is a parallel effort to work being done in the epithermal energy range. The thermal energy group structure specified in this report was accepted November 10, 1972, by the Cross Section Evaluation Working Group as the standard for generation of thermal group constants from ENDF/B data. The standard thermal group structure specified in this report is consistent with past design experience and thermal spectrum codes, and incorporates specific features for effects known to be important in nuclear design applications in the thermal energy range. Specific recommendations are made as to methods to be used for generation of thermal group constants. (auth)
Date: March 1, 1974
Creator: Finch, D.R.
Partner: UNT Libraries Government Documents Department

Lawrence Livermore Laboratory Evaluated Nuclear Data Library (ENDL) translated into the ENDF/B format

Description: The LLL Evaluated Nuclear Data Library (ENDL) was translated into the ENDF/B Format. This translation is for the convenience of those people who wish to use the ENDL data, but whose main familiarity is with the ENDF/B formats and procedures. Only the portion of ENDL which deals with neutioninduced interactions (including photon production from neutroninduced reactions) was translated. Those parts of the ENDL library which deal with charged-particle- induced reactions were not translated, since they cannot be accommodated within the ENDF/B system. In order to provide a basis ior judging the validity of the ENDL data calculations were made of the output neutron time spectra from the LLL pulsed-sphere experiments, by using for the calculations both the ENDL cross sections and the ENDF/B-III cross sections. Comparisons between these two sets of calculations and the experimental output neutron time spectra provide a basis for judging the relative merits of the ENDL and ENDF-/B-III data sets in the 5 to 20 MeV neutronenergy regime. (auth)
Date: October 1, 1973
Creator: Howerton, R. J.
Partner: UNT Libraries Government Documents Department

The status of nuclear data for transmutation calculations

Description: At this point, the accurate description of transmutation products in a radiation environment is more a nuclear data problem than a code development effort. We have used versions of the CINDER code for over three decades to describe the transmutation of nuclear reactor fuels in radiation environments. The need for the accurate description of reactor neutron-absorption, decay-power, and decay-spectra properties have driven many AEC, ERDA, and DOE supported nuclear data development efforts in this period. The level of cross-section, decay, and fission-yield data has evolved from rudimentary to a comprehensive ENDF/B-VI library permitting great precision in reactor calculations. The precision of the data supporting reactor simulations provides a sturdy foundation for the data base required for the wide range of transmutation problems currently studied. However, such reactor problems are typically limited to neutron energies below 10 MeV or so; reaction and decay data are required for actinides of, say, 90 {le} Z {le} 96 neutron-rich fission products of 22 {le} Z {le} 72. The expansion into reactor structural materials and fusion systems extends these ranges in energy and Z somewhat. The library of nuclear data, constantly growing in breadth and quality with international cooperation, is now described in the following table.
Date: December 1, 1995
Creator: Wilson, W.B.; England, T.R.; MacFarlane, R.E.; Muir, D.W. & Young, P.G.
Partner: UNT Libraries Government Documents Department

Study of parity nonconservation with epithermal neutrons

Description: The authors describe a new generation of experiments studying the weak interaction between nucleons. Measurements of the effect of this interaction are few in number and the significance of the observed effects are generally small. It is well known that the weak interaction violates parity. This was first experimentally established by C.S. Wu through measurement of an asymmetry of electrons emitted in the beta-decay of polarized {sup 60}Co. The measured asymmetry was large because beta decay is a weak interaction process. For a process in which the - strong interaction can contribute, we expect much smaller asymmetries, of order 10{sup -7}.
Date: September 1996
Creator: Seestrom, S. J.; Bowman, J. D.; Knudson, J. N. & Yuan, V.
Partner: UNT Libraries Government Documents Department