Abstract
A nuclear-decay spectroscopy study of the neutron-deficient isotope $^{196}\mathrm{At}$ is reported where an isotopically pure beam was produced using the selective Resonance Ionization Laser Ion Source and On-Line Isotope Mass Separator (CERN). The fine-structure $\ensuremath{\alpha}$ decay of $^{196}\mathrm{At}$ allowed the low-energy excited states in the daughter nucleus $^{192}\mathrm{Bi}$ to be investigated. A $\ensuremath{\beta}$-delayed fission study of $^{196}\mathrm{At}$ was also performed. A mixture of symmetric and asymmetric fission-fragment mass distributions of the daughter isotope $^{196}\mathrm{Po}$ (populated by $\ensuremath{\beta}$ decay of $^{196}\mathrm{At}$) was deduced based on the measured fission-fragment energies. A $\ensuremath{\beta}\text{DF}$ probability ${P}_{\ensuremath{\beta}\mathrm{DF}}(^{196}\mathrm{At})=9(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ was determined.
Highlights
The region of very neutron-deficient astatine isotopes provides a rich variety of interesting nuclear structure and decay phenomena
Over the past two decades, extensive nuclear-decay investigations have been performed for a long chain of the lightest astatine isotopes, both by particle decay and by γ -ray spectroscopy
One of the most important conclusions from the previous studies was the evidence for an onset of deformation at low excitation energy, demonstrated, e.g., by the behavior of excited states in 197–201At [6,7,8,9]
Summary
The region of very neutron-deficient astatine isotopes provides a rich variety of interesting nuclear structure and decay phenomena. For βDF of 192At, a rather large and positive energy difference of QEC(192At) − Bf (192Po) = 2.08 MeV is predicted within the FRDM/FRLDM model, giving a possibility of β-decay feeding to excited states well above the fission barrier in the daughter 192Po. Even a larger QEC-Bf difference is expected if one uses the TF model. Within the TF framework, the QEC(196At) and Bf (196Po) are nearly the same; only the subbarrier fission is expected within this model Based on these reasons, a smaller PβDF value is expected for 196At in comparison with the lighter isotopes 192,194At. Separator (ISOLDE), almost all previous investigations produced 196At in complete fusion reactions with heavy ions. Data from a number of irradiations with different projectiletarget combinations further established the candidate precursor as 196At
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