Evaluation of the²³⁸U Neutron Cross Section in the Unresolved Resonance Range

Abstract

This paper presents a new analysis of the 238 U cross sections in the unresolved resonance range, from 20 to 150 keV. Statistical analysis of the resonance parameters in the resolved resonance range with random-matrix theory provides accurate experimental values of strength function, average radiative width and average level spacing for sand p-wave resonances. Above 20 keV, the simultaneous fit of selected experimental data (average transmission and capture) is performed with a statistical model of nuclear reactions as implemented in the SAMMY code. Compared to previous evaluations, such as those described by Fröhner or by Maslov et al., this work benefits from the accurate transmission data measured by Harvey et al. at Oak Ridge Electron Linear Accelerator, which have never been studied before. This new evaluation was written into the current ENDF format for use in practical applications. This work stresses the need for an improved ENDF format to store average resonance parameters and cross sections in the unresolved resonance range.

Evaluation of cross sections for neutron interactions with 238U in the energy region between 5 keV and 150 keV

“…(5), results in D 0 = 22.20 eV and Γ γ ( = 1) = 22.5 meV. The level spacing derived for the two cases are within the uncertainties consistent with the value D 0 = (21.19 ± 0.55) eV that was derived from a statistical analysis of resolved resonance parameters by Derrien et al [43] and Courcelle et al [7]. Both versions have shown almost the same cross sections and very similar behaviour in the interpretation of benchmark experiments, since they retain the fitted capture transmission coefficients practically intact.…”

“…The initial strength functions S 0 and S 1 at zero energy derived from the DCCOM were practically not changed. Derrien et al [43] and Courcelle et al [7] used the same resonance parameter file for their analysis. They are also consistent with the strength functions recommended in the RIPL library [42], i.e.…”

“…In addition, results of high resolution time-of-flight (TOF) total cross section a e-mail: peter.schillebeeckx@ec.europa.eu (corresponding author) measurements carried out by Harvey et al [5] have not been included in previous evaluations for the URR, e.g. in the evaluation performed by Fröhner [2,3], Maslov et al [6] and Courcelle et al [7]. The data of Harvey et al [5] were analysed by Derrien et al [8] to derive average total cross sections between 10 keV and 100 keV.…”

Neutron capture cross section measurements for 238U in the resonance region at GELINA

“…This evaluation is based on an analysis of experimental data using the Hauser-Feshbach theory with width fluctuations. A similar evaluation was performed by Maslov et al [16] and Courcelle et al [17]. The capture cross section of Fröhner [14,15], which is strongly based on the data of Kazakov et al [18], is in good agreement with the one of Carlson et al [19].…”

“…The capture cross sections derived by de Saussure et al [5] and Macklin et al [6], which were the only capture data used in the evaluation of Derrien et al [8], strongly deviate from the one of Carlson et al [19] and Fröhner [14,15]. Therefore, they were not included in the evaluation of Fröhner [14,15] and Courcelle et al [17]. A more extended discussion on the status of evaluated data files for 238 U in the resonance region is reported by Kopecky et al [24].…”