Beta-delayed fission and low-lying structures in the beta strength function

Beta-delayed fission (βDF) gives a possibility of investigating the fission barrier for nuclei far off beta stability. However, before any information on the fission barrier can be extracted, the effect of low-lying structures in the beta strength function (Ssub(β)) on the βDF branching ratio has to be considered. This is in general not being done. In this paper, the low-lying structures that occur in Ssub(β) are discussed and microscopic calculations for the Gamov-Teller strength function are presented for 232Th (β--decay) and 232Pu, 240Cm, sup(244,248)Cf and 248Fm (β+-decay). Using the calculated strength functions, β+DF branching ratios are calculated and compared with the experimental ones. The sensitivity of the results to different shapes of Ssub(β) is investigated. It is concluded that, when the expected structures in Ssub(β) are considered, there are, at present, no indications from β+DF measurements that the errors in the fission barrier calculations are larger than the uncertainty given for those calculations. The difference in magnitude between the β-DF and the β+DF branching ratios is also explained by the occurrence of low-lying structures in Ssub(β). (author)