Speaker
Description
Reich-Moore (RM) approach has been extensively used in the resolved resonance energy range (RRR) for a wide range of isotopes. The Reich-Moore approximation was suggested late in the fifties for cross section representation of fissile isotopes since experimental fission width distribution according to given resonance spin and parity showed that only a few degrees of freedom (DoF) was involved during the fission process. On this ground, experimental cross section data in RRR were successfully reproduced and the interference in fission channels were well described. The fitting of the fission cross section data was done according to one or two fission channels for a given resonance spin (J) and parity (π). By using the two-fission channel RM representation, channel interference effects observed on cross section data for fissile heavy isotopes were adequately treated but only on a phenomenological basis. Indeed, this approach is physically unsatisfactory since no fission penetrability is involved in reduced fission channel width evaluation and therefore no actual connection between R-matrix fission channel widths and Aage Bohr transition fission channels can be made neither in terms of individual barrier height nor by the shape. Our intent is to address this long-standing model deficiency by including ’fluctuating’ fission barrier penetrability as a function of resonance spin and parity over the RRR for fissile isotopes. Fissile systems are easier to model since we can refer to a statistical representation of the double-humped barrier. In this representation, the neutron-induced fissile system excitation is assumed to be much above the energy of the fission barrier such that a statistical equilibrium among all degrees of freedom in the compound system occurs. The partial transmission coefficient of the whole barrier is calculated as TF (μ) = (TATB (μ))/ (TA + TB ) where TA and TB are the total fission transmission coefficients across the inner barrier transition states sequence and the μ outer barrier transition states sequence, respectively. TA and TB are calculated using the Hill-Wheeler formula. The strength of each Jπ resonance fission channel width depends on the effective number of transitions states of same Jπ available for fission. A list of collective DoF, ideally represented by their spin projection (Kπ) onto the elongation axis of the nucleus under fission, was set up. They serve as band heads to build an adequate discrete sequence of Jπ rotational levels on top of each barrier hump. Application is made to the fissile system (n+235U) to determine a new set of fission widths over the RRR (0 - 2.25 keV) according to the two s-wave j=3- and 4- spins. It leads to truly physical fitted fission widths and requires a four-fission channel RM representation; each of those devoted to a set of (J, K, π) quantum numbers.