Modified Methods of Neutron Cross-Section Calculations

Abstract

Conventional formulas of the neutron cross sections based on the statistical model are modified so as to keep equality between the total cross section and the sum of the partial cross sections by taking into account the effects of the cascade process accompanied by the emission of 7-rays from the compound nuclear states, and of other processes which compete with the neutron scattering and capture. T h e modifications are applied to the cross-section formulas with width-fluctuation correction and resonanceresonance interference, according to the statistical distribution law of the level widths and spacings, respectively. Examples of calculations are shown for 238U, logag, Io3Rh andDiscussions are given concerning the contributions of the cascade process, the competing processes, the width-fluctuation and resonance-interference effects.

Calculations of Maxwellian-averaged Cross Sections and Astrophysical Reaction Rates Using the ENDF/B-VII.0, JEFF-3.1, JENDL-3.3 and ENDF/B-VI.8 Evaluated Nuclear Reaction Data Libraries

“…With these parameters, the σ Maxw γ (30keV ) is calculated as 21.3±2.0 mb, in excellent agreement with Stadler's result [116]. The JENDL-3.3 value, 23.9 mb, is slightly overestimated since the CASTHY calculations [78] on which it was based adopted an average s-and p-wave radiative widths of 130 125 Sn (T 1/2 = 9.64 d): There are no resonance parameter information and cross section measurements for this nucleus. Hence, the determination of the capture cross section relies on systematics.…”

“…One of the main uncertainties in this approach is attributed to the imprecise value of the s-wave level spacings, D 0 = 6.7±4.7 keV [12]. Using the Gilbert-Cameron relation [50] and D 0 = 1.48±0.20 keV for 78 Se, a value of 6.7 keV is confirmed for the level spacing of 82 Se. With the average resonance parameters [12], as well as systematics [47], a σ Maxw γ (30keV ) of 25±7 mb is deduced for 82 Mughabghab [17] calculates 288±19 mb for the σ Maxw γ (30keV ) , compatible with Walter et al's result [105], within the uncertainty limits.…”

“…L. Here, the ENDF/B-VII.0adopted JENDL-3.3 evaluation, undertaken by H. Nakamura [10], is based on CASTHY calculations [78]. Due to lack of experimental data for 40 K, we cannot resolve the discrepancy between the neutronresonance parameter calculation of Mughabghab [17], the values from Bao et al [5], and the evaluated nuclear libraries σ Maxw γ (30keV ) .…”

“…74,76 Se: There is agreement between the Bao et al, and Atlas cross sections [5,12] and the recent measurement of Dillmann et al [102] for 74 Se. The 76 Se unresolved-resonance region, 9 keV -100 keV, in both ENDF/B-VII.0 and JENDL-3.3 are based on CASTHY model calculations [78], yielding σ Maxw γ (30keV ) of 96 mb. Beer et al [103] report a measurement of 164±8 mb that falls readily within the uncertainty limits of the calculation based on the Atlas parameters, and the recently determined systematics of capture widths [17,47], yielding σ Maxw γ (30keV ) = 148±22 mb.…”

“…eV [12], the capture cross section for this isotope above this energy is obtained by nuclear-model calculations, along with dependence on the systematics of average resonance parameters. The evaluated data libraries give σ Maxw γ (30keV ) = 318 mb from the nuclear-model calculations, CASTHY [78]. With systematics, Bao et al [5] derived an estimate of 482±92 mb.…”