ABSTRACT
To give a simple illustration of the approach described above, we consider Zr isotopes (proton number Z = 40) with valence neutrons beyond the closed shells of N = 50. The 90Zr nucleus does not have closed proton shells and yet it exhibits some properties of a doubly magic nucleus. Its first excited state has J π = 0+ and lies 1.76 MeV above the ground state. The next higher state, 2+, lies at 2.19 MeV. We can thus tentatively consider states with neutron numbers beyond 50 as states of valence neutrons.
