The thorium-229 nucleus has an extremely low-lying excited state, called a nuclear isomer, at an energy of 8.36 eV above its ground state. This energy is comparable to that of valence electrons. A phenomenon unique to this isomer is that its decay mode and lifetime vary depending on the electronic configuration.

Using an ion trap apparatus, we measured the decay lifetimes of the thorium-229 isomer in neutral, singly and triply charged states and estimated the lifetime in the doubly charged state. The isomer ions were obtained as recoil ions from the alpha decay of uranium-233. Our results for neutral and doubly charged isomers are consistent with previous studies, while the lifetimes of the singly and triply charged isomers were newly measured in this work. The observed isomer lifetimes differ by several orders of magnitude between charge states. Specifically, the lifetime of the singly charged isomer indicates the existence of a new decay mode: the electronic bridge (EB) decay. 

Our findings pave the way for the development of a nuclear clock using triply charged thorium-229 and provide deeper insight into EB decay, where nuclear-electron interactions play a crucial role.

Biography

Dr. Atsushi Yamaguchi earned his doctoral degree from Kyoto University. He subsequently worked at National Institute of Communications Technology (NICT) as a post-doctoral fellow. He then moved to Physikalisch-Technische Bundesanstalt (PTB) in Germany as an Alexander von Humbold Fellow. After two and half years at PTB, he joined Quantum Metrology Laboratory at RIKEN. He is now a senior research scientist at Spacetime Engineering Research Team at RIKEN and working on trapped-ion nuclear clock.

Inviter: Wu Wang