STRETCHED PROTON-NEUTRON CONFIGURATIONS IN FP-SHELL NUCLEI .2. SYSTEMATICS

The systematics of the binding energies of stretched proton-neutron configurations (f7/2, g9/2)8-, (P3/2, g9/2)6-, (g9/2, p3/2)6- and (g9/2)9+(2) are studied over a wide range of fp-shell nuclei. The effective proton-neutron interaction energies deduced from the data are nearly constant for (P3/2, g9/2)6- and g9/2)9+(2) states while the (f7/2,g9/2)8- configuration reveals an additional repulsive term proportional to the partial filling of the f7/2 orbit in the target ground state. Two-body matrix elements are extracted. A crude shell model, which predicts that the ''citation energy of a stretched state is equal to the sum of the single-particle energies, works well for the 6- and 9+ states, but fails for the 8- levels due to neglect of the additional interactions described above. The physics underlying the empirically introduced basic assumptions of the crude shell model is discussed. The binding energies are found to be linearly dependent on the mass number A and the isospin T(z) component and are well described by the weak-coupling model of Bansal and French. The derived parameters agree with averaged values of a similar analysis for the single-particle states in the corresponding odd-even neighbours. The data indicate a significant change of the particle-hole energies with closure of the proton f7/2 shell.