Manifestation of the Berry phase in the atomic nucleus $^{213}$Pb

The neutron-rich 213 Pb isotope was produced in the fragmentation of a primary 1 GeV A 238 U beam, separated in FRS in mass and atomic number, and then implanted for isomer decay γ -ray spectroscopy with the RISING setup at GSI. A newly observed isomer and its measured decay properties indicate that states in 213 Pb are characterized by the seniority quantum number that counts the nucleons not in pairs coupled to angular momentum J=0 . The conservation of seniority is a consequence of a geometric phase associated with particle-hole conjugation, which becomes observable in semi-magic nuclei where nucleons half-fill the valence shell. The γ -ray spectroscopic observables in 213 Pb are thus found to be driven by two mechanisms, particle-hole conjugation and seniority conservation, which are intertwined through a Berry phase.