New Type of Asymmetric Fission in Proton-Rich Nuclei

TLDR

In neutron‑deficient 180Hg, a symmetric split would produce two magic 90Zr fragments, making the observed asymmetry unexpected. The study uses resonant laser ionization and mass separation at ISOLDE to investigate β‑delayed fission of 180Tl. The β‑delayed fission of 180Tl produces an asymmetric fragment distribution in 180Hg, a new type of asymmetry unrelated to shell effects, with a branching ratio of 3.6 × 10⁻³, about two orders of magnitude higher than previously reported.

Abstract

A very exotic process of $\ensuremath{\beta}$-delayed fission of $^{180}\mathrm{Tl}$ is studied in detail by using resonant laser ionization with subsequent mass separation at ISOLDE (CERN). In contrast to common expectations, the fission-fragment mass distribution of the post-$\ensuremath{\beta}$-decay daughter nucleus $^{180}\mathrm{Hg}$ ($N/Z=1.25$) is asymmetric. This asymmetry is more surprising since a mass-symmetric split of this extremely neutron-deficient nucleus would lead to two $^{90}\mathrm{Zr}$ fragments, with magic $N=50$ and semimagic $Z=40$. This is a new type of asymmetric fission, not caused by large shell effects related to fragment magic proton and neutron numbers, as observed in the actinide region. The newly measured branching ratio for $\ensuremath{\beta}$-delayed fission of $^{180}\mathrm{Tl}$ is $3.6(7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}%, approximately 2 orders of magnitude larger than in an earlier study.