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Title: Origin of asymmetric fission in neutron-deficient mercury isotopes
Plain: Mercury-180 was expected to split into two equal halves, zirconium-90 nuclei, but a 2010 experiment found it splits unevenly. Which nuclear structure makes the split uneven in this region, far from the uranium-like nuclei where uneven fission is common, is only partly understood.
Precise: Explain the asymmetric mass distribution observed in beta-delayed fission of $180\mathrm{Hg}$ (peaks near $A \sim 80$ and 100) and the evolution toward symmetric fission in heavier Hg and Pb isotopes. The answer is the shell structure of the pre-scission configuration or of the nascent fragments (for example octupole-deformed proton or neutron numbers) that controls the asymmetry, with quantitative yield predictions for neighbouring isotopes.
Settled by: A microscopic calculation that reproduces the measured mass yields of 178,180,182Hg and correctly predicts the asymmetric-to-symmetric transition across the region before it is measured.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: partially-resolved
Status note: Calculations attribute the asymmetry to deformed shell effects in the nascent fragments, and 2025 work studied how excitation energy and angular momentum move the transition in 180Hg.
Posed since: 2010