Manifestation of the Berry phase in the atomic nucleus ²¹³Pb

The neutron-rich ²¹³Pb isotope was produced in the fragmentation of a primary 1 GeV A ²³⁸U 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 ²¹³Pb are characterized by the seniority quantum number that counts the nucleons not in pairs coupled to angular momentum . 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 ²¹³Pb are thus found to be driven by two mechanisms, particle-hole conjugation and seniority conservation, which are intertwined through a Berry phase.