Pairing-excitation versus intruder states in 68Ni and 90Zr

A discussion on the nature of the 0(+) states in Ni-68 (Z = 28, N = 40) is presented and a comparison is made with its valence counterpart Zr-90 (Z = 40, N = 50). Evidence is given for a 0(+) proton-intruder state at only similar to 2.2-MeV excitation energy in Ni-68, while the analogous neutron-intruder states in Zr-90 reside at 4126 and 5441 keV. The application of a shell-model description of 0(+) intruder states reveals that many pair-scattered neutrons across N = 40 have to be involved to explain the low excitation energy of the proton-intruder configuration in Ni-68.