The positive parity spectrum of 90Zr

A shell model calculation using a two-particle-two-hole basis is introduced for the examination of the positive parity spectrum of 90Zr. Only proton excitations are considered, with the additional restriction that the two-proton particles are excited from the Of-1p shell to the Og9/2 orbital. Two realistic interactions are used, and second-order corrections are found to be important for the construction of the 'effective' interaction. The results of the calculation give a good account for the spacings of the excited states, but predict a too depressed ground state. This indicates that additional configurations may contribute significantly for a better interpretation of the positive parity spectrum of 90Zr. (author)