Noncollective nucleon pairs in even-even Sn 124-128
The effects of possible noncollective pairs in even-evenSn124-128 are studied in the nucleon-pair shell model, in which a few noncollective neutron pairs originating from the alignment of two neutrons in the νh11/2 orbit are considered. From the low-lying-level energies, B(E2) ratios, and excited states obtained from the model, it is shown that the yrast band structure can be explained as the evolution from vibrational to rotational type as a function of spin. The mechanism of the yrast band can be explained as band crossing between the ground-state band and the S band constructed from the neutron alignment in the νh11/2 orbit. The noncollective configurations may be crucial for describing the yrast states in even-evenSn124,126,128 in the SD-pair shell model.