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Description
Beta-delayed fission (βDF) is a two-step process where a parent nucleus β-decays into a daughter nucleus that can then fission with a certain probability from an excited state [1]. The experimentally measured βDF probability (P$_{βDF}$) is often very small and has been studied mostly for neutron-deficient isotopes, while only a few cases have been reported on the neutron-rich side of the nuclear chart. To expand the limited information in the neutron-rich region, the LOI216 experiment was performed at ISOLDE to study βDF in $^{230,232,234}$Ac [2]. Since no fission fragments were observed, only upper limits for the P$_{βDF}$ could be deduced [3].
The limited results obtained from the experiment motivated the development of a theoretical framework based on an EDF approach to model the different ingredients required for calculating P$_{βDF}$ values, and the code PyNEB [4] to extract the fission paths. All the inputs are fed into TALYS [5] to calculate the final P$_{βDF}$ for all the experimentally measured cases of βDF. The nuclei for which finite P$_{βDF}$ values are reported were used to calculate a root mean square (RMS) deviation between the calculated probabilities and the experimental ones. After accounting for modelling deficiencies, an RMS deviation of about two orders of magnitude was found. This result is similar to what can be found from the βDF systematics approach based purely on the exponential dependence of the P$_{βDF}$ on the difference between the Q$_β$-value of the mother and the fission barrier of the daughter (see for example [6]). However, the theoretical framework developed using the EDF approach is based on a microscopic description of the nuclei, and, therefore, offers a more realistic description of the process studied. The results of this combined experimental and theoretical characterization of βDF will be discussed in this contribution, along with new cases identified as potential βDF candidates to be studied in the future at ISOLDE or other facilities.
References
[1] A. N. Andreyev, et al., Rev. Mod. Phys., Vol. 85, No. 4 (2013).
[2] A. N. Andreyev, et al., INT-I-216 (2020).
[3] S. Bara, et al., Phys. Rev. C 111, 065803 (2025).
[4] E. Flynn, et al., Phys. Rev. C 105, 054302 (2022).
[5] A. Koning, S. Hilaire, and S. Goriely, EPJ A 59, 1 (2023).
[6] L. Ghys et al., Phys. Rev. C 91, 044314 (2015).
