Argonne National Laboratory Reports

This supplement to the Proceedings of the NEANDC/NEACRP Specialists Meeting on Fast Fission Cross Sections summarizes the data and graphical material presented for consideration by the Working Groups on absolute cross section values and cross section ratios

Argonne National Laboratory Reports

Compilation of papers presented during the meeting of NEANDC/NEACRP specialists to discuss fast neutron fission cross sections of U-233, U-235, U-238, and Pu-239

Optical model with multiple band couplings using soft rotator structure

A new dispersive coupled-channel optical model (DCCOM) is derived that describes nucleon scattering on 238U and 232Th targets using a soft-rotator-model (SRM) description of the collective levels of the target nucleus. SRM Hamiltonian parameters are adjusted to the observed collective levels of the target nucleus. SRM nuclear wave functions (mixed in K quantum number) have been used to calculate coupling matrix elements of the generalized optical model. Five rotational bands are coupled: the ground-state band, β-, γ-, non-axial- bands, and a negative parity band. Such coupling scheme includes almost all levels below 1.2 MeV of excitation energy of targets. The "effective" deformations that define inter-band couplings are derived from SRM Hamiltonian parameters. Conservation of nuclear volume is enforced by introducing a monopolar deformed potential leading to additional couplings between rotational bands. The present DCCOM describes the total cross section differences between 238U and 232Th targets within experimental uncertainty from 50 keV up to 200 MeV of neutron incident energy. SRM couplings and volume conservation allow a precise calculation of the compound-nucleus (CN) formation cross sections, which is significantly different from the one calculated with rigid-rotor potentials with any number of coupled levels.Ministerio de Economía y Competitividad FPA2014-53290-C2-2-

Updated Nucleosynthesis Constraints on Unstable Relic Particles

We revisit the upper limits on the abundance of unstable massive relic particles provided by the success of Big-Bang Nucleosynthesis calculations. We use the cosmic microwave background data to constrain the baryon-to-photon ratio, and incorporate an extensively updated compilation of cross sections into a new calculation of the network of reactions induced by electromagnetic showers that create and destroy the light elements deuterium, he3, he4, li6 and li7. We derive analytic approximations that complement and check the full numerical calculations. Considerations of the abundances of he4 and li6 exclude exceptional regions of parameter space that would otherwise have been permitted by deuterium alone. We illustrate our results by applying them to massive gravitinos. If they weigh ~100 GeV, their primordial abundance should have been below about 10^{-13} of the total entropy. This would imply an upper limit on the reheating temperature of a few times 10^7 GeV, which could be a potential difficulty for some models of inflation. We discuss possible ways of evading this problem.Comment: 40 pages LaTeX, 18 eps figure

U-235 sample-mass determinations and intercomparisons

The neutron-induced fission cross section of U-235 is not only one of the most-frequently used references but is also of direct importance in reactor applications. As a consequence, knowledge of this cross section is required with approx. 1% uncertainty as reflected in corresponding entries in request lists, which have persisted since the last 10 to 15 years. Measurements to that level of accuracy require the investigation of the contributing components, one of which is the fission mass. The latter is most often determined by others than the experimenter who measures the differential cross sections or integral reaction-rate ratios in a reactor test facility. The isotopic composition and the sample mass are usually obtained from associated chemistry departments or standard laboratories, however, the experimenter has still the responsibility to assure that the values he uses are adequately described by the quoted uncertainties. This can be achieved by comparing samples from different origins. It was in this spirit that an intercomparison of fission samples obtained from different US laboratories, which were involved in cross section measurements, was carried out in 1979. The notable outcome of this effort was that a bias of approx. 0.7% was found between the standard laboratory and other contributing laboratories (which was, however, within the stated uncertainty). The National Bureau of Standards (NBS) has since then worked on a redefinition of the mass assignments of its reference samples, has revised its mass scale by 0.8%, and has reduced its uncertainty by a factor of two (to about +-0.5%). However, this new mass scale includes values relative to others. In the present work these have been removed in order to compare mass scales as independent from one another as possible. Independence already appears hard to come by. Results are presented. (WHK

Covariances of evaluated nuclear data based upon uncertainty information of experimental data and nuclear models

A straightforward derivation is presented for the covariance matrix of evaluated cross sections based on the covariance matrix of the experimental data and propagation through nuclear model parameters. 10 refs

Argonne National Laboratory Reports

This proceedings contains the summaries and contents of a number of papers presented at the 1982 NEANDC/NEACRP specialists meeting on fast-neutron capture cross sections. The table of contents has been summarized to provide a sketch of the topical matter

Fast-neutron capture cross sections

SEparate abstracts were prepared for 30 of the 39 papers presented. Nine papers wre previously included in the data base. (WHK

Evaluation of /sup 235/U(n,f) above 100 keV for ENDF/B-V and the implications of a unified /sup 235/U mass scale. [Above 100 keV]

A previously reported evaluation of /sup 235/U(n,f) in the fast neutron energy range was updated to include data published up to the 1978 Harwell Conference on Neutron Physics. The shape of the cross section resulting from this evaluation and a normalization factor extracted from data provided within the framework of this evaluation were used by the Subcommittee on Standards and Normalizations of the Cross Section Evaluation Working Group to establish /sup 235/U(n,f) for ENDF/B-V above 100 keV. The /sup 235/U sample mass comparisons made between different laboratories were compiled in order to investigate the implications of different sample masses on recent /sup 235/U(n,f) data. A new intercomparison of several such samples was carried out using absolute and relative alpha-counting and relative fast neutron fission counting. The result of this work is a unified /sup 235/U mass scale with an uncertainty of 0.6%. 39 references

Interpretation and normalization of experimental data for total, scattering, and reaction cross sections

Problem areas in the interpretation of fast-neutron data are discussed. Their impact on experimental uncertainties and hence the evaluation process are reviewed in the context of user needs. Contributions of supplementary information such as nuclear models and applications tests are explored. Specific means for resolving difficulties cited are proposed and illustrated