[en] The paper reviews some current approximations used in R-matrix theory for calculating angular integrated nuclear cross sections. In particular, it distinguishes the SLBW and MLBW approximations of their practical applications ENDF-oriented. This paper also focuses on the problem of prior resonance parameter determination compulsory for any experimental data adjustment in the resolved resonance range. The major contribution of this paper concerns R-matrix calculations made with no approximations using the SAMMY program which is developed at the Oak Ridge National Laboratory. These new R-matrix calculations are applied to real cases which experimental data are extracted from radiative capture gamma rays measurements on ²³Na, ¹⁹F and ²³⁸U isotopes. Small but significant R-matrix effects show up in the wings of the resonances and especially at thermal energies when calculating the capture cross section without the classic Reich-Moore approximation. (author)

[en] In calculations of resonance shielding and Doppler coefficients the effect of thermal motion on the energy distribution of scattered neutrons is neglected. Recent publications by Ouisloumen and Sanchez, and also by Kurchenkov and Laletin, have shown that secondary energy distributions are strongly perturbed by Doppler broadening in resonances. In the presence paper calculations of resonance absorption and Doppler effects are described. These show that in a thermal reactor spectrum the resonance absorption in U-238 is increased by about 1% at reactor operating temperatures and the Doppler effect is increased by about 9%, when the exact secondary energy distribution is used instead of the asymptotic distribution. The effect of a temperature distribution across the fuel pin is also calculated and it is shown that the usual definition of equivalent temperature is valid

[en] This article describes an original approach to simultaneously analyzing cross sections and data obtained with surrogate reactions, using an efficient Monte Carlo extended R-matrix theory algorithm based on an unique set of nuclear structure parameters. The alternative analytical path based on the manifold Hauser-Feshbach equation was intensively used in this work to gauge the errors carried by the surrogate-reaction method commonly taken to infer neutron-induced cross sections from observed decay probabilities. The present paper emphasizes in particular a dedicated way to treat ingoing direct reaction and outgoing channels widths correlations and to recall the common absence of class-II states width fluctuation factor in standard codes for calculating average fission cross sections. The present approach opens interesting perspectives on the matter of neutron cross section inference as simultaneously measured fission and gamma-emission probabilities become widely available. (authors)

[en] Recent advances in the modeling of the nuclear fission process for data evaluation purposes are reviewed. In particular, it is stressed that a more comprehensive approach to fission data is needed if predictive capability is to be achieved. The link between pre- and post-scission data is clarified and a path forward to evaluate those data in a consistent and comprehensive manner is presented. Two examples are given: (i) the modeling of fission cross-sections in the R-matrix formalism, for which results for Pu isotopes from A = 239 to 242 are presented; (ii) the modeling of prompt fission neutrons in the Monte Carlo Hauser-Feshbach framework. Results for neutron-induced fission on ²³⁵U are discussed. (authors)

[en] This paper is willing to illustrate and quantify the impact of a double fission barrier in terms of average fission cross-section and in particular, to status on both the additional class-II width correction factor and the degree of freedom of the overall double fission barrier which is a key parameter in the calculation of the standard W_n,f factor of Hauser-Feshbach theory. (author)

[en] Since its inception, the so-called surrogate-reaction method (SRM) has motivated the development and improvement of theories in connection to direct reactions. This paper reassesses some of the developments carried out in previous decades to deal with the representation of direct reaction probability data. It is believed that the experimental probabilities assimilation in the neutron cross section evaluation process can be better estimated using tools resulting from the efforts made over the years. This paper provides a new perspective on this issue both in terms of fission and gamma-ray emission probabilities. In addition to the 'natural' assimilation path that considers analyzing probabilities jointly with cross sections to extract nuclei structural properties, this article puts forward a prescription to convert, with a good level of confidence, measured direct-reaction induced probabilities to pseudo-experimental neutron-induced cross sections. This approach is named after the SRM as extended SRM (ESRM). (authors)

[en] The description of the decay of the compound nucleus by fission is a multi-dimensional problem, which requires very many parameters. The elucidation and physical understanding of these parameters is an ongoing research process, and in this paper we report on our progress on the study of the fission cross-sections of the full sequence of plutonium isotopes. There are two purposes here: one is to further our understanding of the fission process, the other is for application to evaluating the cross-sections of nuclides for which measurements are unavailable or very poor. Fission barrier heights are the a priori parameters for the evaluation of cross-sections. Several decades of theoretical work using Strutinsky macroscopic-microscopic or HFB methods, have revealed many of the important trends in the fission barrier topography, but an accurate quantitative evaluation of the heights remains elusive. At this stage of our understanding of fission, we depend on analysis of data to extract these parameters. If this can be done accurately enough, we can interpolate or extrapolate to poorly known nuclei with some reliability. In doing this analysis it is most important to include as detailed a knowledge as we have of the physics of fission, complicated greatly by the double-humped nature of the fission barrier in most of the actinide nuclides. In our AVXSF code we do a full Monte Carlo simulation of the intermediate and fine structure coupling and sample from all the width and spacing distributions to obtain the average cross-sections. When the number of open channels becomes large we use approximation formulae to determine the fluctuation averaging factors. The correct modeling of the intermediate structure and its associated fluctuation properties can change the estimate of barrier heights by some hundreds of keV from values deduced using the simple statistical factor. For the fissile odd-even targets the barrier height of the even compound nucleus is deduced by similar analysis of experimental data on the (t,pf) reaction. Over the range of Pu isotopes from 239Pu to 245Pu (compound nuclei) we find a pronounced odd-even difference (lower barriers in the even nuclides). Our analysis of (n,f) cross-section data extends over the full range to 5.5 MeV, i.e. just below the threshold to second-chance fission. At energies above the barrier the densities of the transition states at both barriers are the crucial factors in determining cross-sections. We use a quasiparticle-vibration-rotation model for both the barrier states and the level density of the target nucleus. Our analysis reveals that the pairing gap parameters for barrier densities must be significantly larger than those at the ground state deformation. This correlates with odd-even difference in barrier heights. In this paper we shall also discuss the use of our results in evaluating unknown cross-sections of other nuclides

[en] This paper reviews the current status of the re-evaluation work performed at Cadarache on the ²⁴⁰Pu neutron cross sections in the so-called 'unresolved resonance energy range'. This study was the consequence of both the conclusions of the recent re-evaluation of the neutron cross sections in the resolved range and the trends given by the JEF-2.2 general purpose file validation. The work involves the calculation of average neutron cross sections by using Hauser-Feshbach theory and Moldauer prescription for overlapping resonances and a specific treatment for sub-threshold fission, respectively achieved by using the FITACS code coupled with an updated version of the AVXSF program. Posterior to this stage, the determination of a priori average parameter values was made by employing the ESTIMA code which fits the theoretical integral Porter-Thomas distribution on the cumulated number of experimental resonances after selection of the most confident s-wave resonances. These a priori average values: resonance spacing, D₀ = (13.43±0.25)eV; strength function, S₀ = (1.064±0.125).10^-4 must be compared to the corresponding values before purification of the resonance sample for p-waves contamination and missed small s-waves, being respectively: D₀ = (12.06 ± 0.60)eV; S₀ = (1.032 ± 0.71).10^-4, and to the posterior values resulting from a FITACS/AVXSF fit on the measured average cross sections in the unresolved range. The final values are D₀ = (13.43 ± 0.25)eV; S₀ = (1.102 ± 0.052).10^-4 and S₁ = (1.842 ± 0.083).10^-4. This work leads to a much better agreement between the microscopic cross section measurements and the integral experiments in the so-called unresolved resonance energy range (5.7-40 keV). The upper boundary is currently chosen so as to do not take care to much about the inelastic cross section since the first inelastic threshold is at 43 keV. The average capture cross section has been decreased significantly (of about -12%) in this (5.7-40 keV) energy range consistently with the resolved resonance energy range and this is mainly achieved by reducing the value of the average capture widths such as gΓ-bar⁰_γ = (30.7 ± 2.5) meV and gΓ-bar¹_γ = (22.53 ± 5) meV. Concerning the calculation of the average fission cross section performed in this work, this latter leads to an accurate physical calculation of the sub-threshold fission cross section which is adequate for the calculation of the other partial cross sections but is not well-suited for reproducing the none statistical fluctuations observed in the fission cross section of the ²⁴¹Pu fissioning nucleus. That explains why this work is still under progress for a finer description of the fission cross section. (author)

[en] The essence of this paper is to enlighten the consistency achieved nowadays in nuclear data and uncertainties assessments in terms of compound nucleus reaction theory from neutron separation energy to continuum. Making the continuity of theories used in resolved (R-matrix theory), unresolved resonance (average R-matrix theory) and continuum (optical model) ranges by the generalization of the so-called SPRT method, consistent average parameters are extracted from observed measurements and associated covariances are therefore calculated over the whole energy range. This paper recalls, in particular, recent advances on fission cross section calculations and is willing to suggest, some hints for future developments. (authors)

[en] Full text of publication follows: A critical review is given of subthreshold fission measurements with emphasis on the resonance region. Uncertainties concerning the experiments and their interpretation are discussed. In parallel, the content of the major evaluated data files (Jeff.1, Endf/B-VII and JENDL3.3), in terms of subthreshold fission cross sections, is also carefully examined. In the unresolved resonance region, the use of resulting average resonance parameters for evaluations is addressed. Recommendations are finally drawn on resolved and unresolved subthreshold fission parameters and cross sections. Some remarks on fission fragment mass distributions in the subthreshold region are added. (authors)