A method to analyse fine-structure resonances in subthreshold fission for235U,238Np,241Pu and243Pu nuclei

Abstract

In a previous paper (1) the general theory of intermediate structures (2) in nuclear reactions has been applied to the study of narrow resonances in subthreshold fission cross-section induced by a neutron beam. In the scheme of the double-humped fission barrier (3) the well-separated groups of fine-structure resonances at low energy have been interpreted as being due to an effect of single class-II states in the second well acting as doorways in the exit channel. Successively the same forin'dism has been extended (4) to the region of overlapping class-II states to interpret the smooth-energy behaviour and some large bumps (fl-vibrations) observed in the near threshold neutron fission cross-section. In these works (4,~) a n,ethod has been also introduced to compute heights and curvatures of the double fission barrier, for odd-neu?ron compound nuclei, from the knowledge of the cross-seetion slope ne:~r the threshold and from the Fr and Fr parameters of isolated resonances, using also information on half-lives and excitation energies of shape isomers. Iu this letter we show an alternative method 1o analyse the fine structure observed in subthreshold (n, f) cross-section. This approach, similar to that shown in ref. (6,v) in the tlne-structurc analysis around tile almlogue-state anomaly, consists in a fit of the cumulative sum of fission widths instead of the width distribution averaged on an energy interval (1,5). The method has the advantage of employing directly the experimental fission widths FfQj and then it furnishes a test of the independence from tile averaging energy intervM in the previous results (a,5).