Intermediate structure studies ofU234cross sections

Abstract

Neutron induced fission and total cross sections of "-"U have been measured over the neutron energy range from a few eV to 8.9 MeV. Neutron and fission widths for 118 cross section resonances below 1500 eV have been determined and give a class I level spacing of ]0.6+0.5 eV and an s-wave strength function of (0.86+ 0.11) g 10 '. These fine structure resonances form two narrow intermediate structure resonances in the subthreshold fission cross section of '"U. Parameters for the Lorentzian energy dependence of the mean fission width are deduced by maximum likelihood analysis on the assumption that, relative to this mean, the observed fission widths have a Porter-Thomas distribution. Two large fission widths measured for resonances at 1092.5 and 1134 eV prompted a likelihood ratio test which indicates the presence of a narrow irtermediate structure resonances at 580 and at 1227 eV. The class II level spacing derived from the observation of seven intermediate structure resonances below 14 keV is 2. 1 + 0.3 keV. Broad structures in the fission cross section at 310, 550, and 770 keV are assumed to be due to P-vibrational levels in the second minimum of the Strutinsky potential. Fluctuations due to the presence of class II resonances are strongly evident for each of these vibrational levels. It is shown that the fluctuations near 310 keV are consistent with parameters deduced from the!ow energy data and this allows parameters for the double humped fission barrier potential to be obtained.

Neutron-induced fission cross section ofU234andNp237Paradela1, Tassan‐Got2, Audouin3 et al. 2010Phys. Rev. C789301View full textAdd to dashboardCite

“…5 by comparison with the data of Refs. [39,40] and with evaluated data [35][36][37]. In general, there is fair agreement with the ENDF/B-VII evaluation except that resonance multiplets could now be resolved by the present data and a slight displacement of the resonance position is observed with respect to evaluated resonances.…”

“…[42] 034601-6 In Tables IV and V the averaged cross section in wider energy intervals is compared with the evaluations and the previous data. In the resolved resonance region, our values for 234 U are about 27% above the results obtained by James et al [39]. Beyond 1.5 keV, the differences are greater up to [42], and Plattard [43].…”

“…Apart from the results of James et al [39], a recent experiment at Geel [40] has been reported, but only in a reduced-energy region. The high resolution achieved at the n TOF facility allowed us to resolve the subthreshold fission resonances with unprecedented accuracy.…”

High accuracy 234U(n,f) cross section in the resonance energy region

“…The ENDF/B-VII.1 and JENDL-4.0 evaluations are shifted in energy and some resonances are missing in one of the two databases. In the Resolved Resonance Region (RRR), both evaluations are based on the experimental data of James et al [3], but JENDL-4.0 includes also the resonance parameters provided by Dridi et al [4] for the capture reaction and the fission cross section of Heyse et al [5] for the 5.17 eV resonance. Above 1.5 keV, in the Unresolved Resonance Region (URR), the fission data of James et al and the capture data of Pennington were used in ENDF/B-VII.1 whereas JENDL-4.0 used the experimental data sets mentioned in Ref.…”

Recent Advances in Modeling Fission Cross Sections over Intermediate Structures

“…Ever increasing computing capabilities have made possible more exact calculations of the average fission cross section, using a Monte Carlo sampling of energies, fission and coupling widths of the class-II states as well as the characteristics of the class-I states from the statistical distribution of levels and partial widths. A few attempts were made in the past [4,5], but the most significant application of this method was performed in 2002 by Lynn and Hayes [6] over a wide range of actinide fission cross sections. In addition to better predict the magnitude of the cross section, this Monte Carlo approach paves the way for modeling genuine fluctuations in the Unresolved Resonance energy Range (at the right energies and with correct fluctuation envelopes) and provides a more a e-mail:…”