The talk will give a review of recent low-energy fission experiments in very proton-rich nuclei in the lead region, with an emphasis on the exotic phenomenon of beta-delayed fission (bDF). This decay mode, which ideally suits to study low-energy fission, is experimentally accessible only in the well-studied uranium region and in the scarcely-studied (by fission) neutron-deficient lead region. Our pioneering bDF study of 180Tl at ISOLDE(CERN) showed an unexpected asymmetric fission-fragment mass split for 180Hg [1], despite a symmetric split in two semi-magic 90Zr nuclei was expected. This result also identified a new region of asymmetric fission in addition to the well-known asymmetry in trans-uranium nuclei.

More recently, new bDF data on the neutron-deficient nuclides 194,196At and 200,202Fr were obtained at ISOLDE. A mixture of asymmetric and symmetric fission is clearly seen in respective daughter isotopes (after precursor’s beta decay) 194,196Po and 202Rn [2]. This establishes a phenomenon of multimodal fission in this region of nuclei. The results will be compared with predictions from several current fission models.

As a conclusion, further perspective of low-energy fission studies with radioactive beams will be presented.

1. A. N. Andreyev et al., “New type of asymmetric fission in proton-rich nuclei”, Phys. Rev. Lett. 105, 252502 (2010)

2. L. Ghys et al., “Evolution of fission-fragment mass distributions in the neutron-deficient lead region”, Phys. Rev. C90, 041301(R) (2014)

*On behalf of the IS466/IS534 collaboration : RILIS-ISOLDE(CERN) – University of York (UK) – IKS, KU Leuven (Belgium) – Comenius University, Bratislava (Slovakia), JAEA, Tokai (Japan) – SCK•CEN, Mol (Belgium)

Contact : tellier@ill.fr