Speaker
Description
In the process of $\beta$-delayed fission ($\beta$DF), an excited state populated in the daughter nucleus after $\beta$ decay undergoes fission. The achievable excitation energy is limited by $Q_\beta$ value, which is typically $\lesssim$ 10 MeV. Therefore, $\beta$DF allows us to access so called low-energy fission, which is sensitive to nuclear structure, and to study its properties, such as fission fragment mass distributions, fission barriers, etc. [1,2]. Moreover, $\beta$DF plays an important role in the nucleosynthesis as it contributes to the termination and fission recycling in the $r$ process [3].
More than a decade ago, our collaboration started a unique experimental campaign dedicated to $\beta$DF studies at ISOLDE (CERN), in which several isotopes in the neutron-deficient lead region were measured [1,4-6]. In this contribution, our follow-up $\beta$DF experiments, which aimed at extending the studies to $^{178}$Au and to neutron-rich isotopes $^{230,232,234}$Ac, will be reported.
Isotope $^{178}$Au has two $\beta$-decaying states, which we studied individually by employing selective power of RILIS to obtain isomerically pure beams. Such measurement gives an additional possibility to explore spin dependence of fission, because of the strong spin selectivity of $\beta$ decay. No $\beta$DF events were observed for isotopes of interest despite collection of high statistics. Therefore, upper limits of $\beta$DF probabilities were determined. The limit for $^{230}$Ac was almost two orders of magnitude lower than the literature value [7], thus, questioning the observation of $\beta$DF for this isotope. The results will be discussed in the context of experimental systematics of $\beta$DF probabilities and partial half-lives.
[1] A. N. Andreyev, M. Huyse and P. Van Duppen, Rev. Mod. Phys. 85, 1541 (2013).
[2] A. N. Andreyev, K. Nishio, and K.-H. Schmidt, Rep. Prog. Phys. 81, 016301 (2018).
[3] S. Goriely et al., Phys. Rev. Lett. 111, 242502 (2013).
[4] L. Ghys et al., Phys. Rev. C 90, 041301(R) (2014).
[5] V. L. Truesdale et al., Phys. Rev. C 94, 034308 (2016).
[6] B. Andel et al., Phys. Rev. C 102, 014319 (2020).
[7] Y. Shuanggui et al., Eur. Phys. J A 10, 1 (2001).