Nuclei exhibit many types of collective behavior. One of the most illusive is the quadrupole vibrational type. One of the signatures of this type of collectivity is the observation of the two-phonon triplet of states 0+2, 2+2 and 4+1 at an excitation energy of about twice the energy of the basic one-phonon 2+1 state (the subscript indicates the ordering of the states for the same J value). One of the nuclei that exhibit this feature for the energies is 62Ni. A important test of the vibrational model is to find the three-phonon states with spin 6+1, 4+2, 3+1, 2+3 and 0+3 and to see if their decay pattern is as expected for a vibration. This has now been done with an experiment at the University of Kentucky, and the results have been compared with large-scale configuration interaction calculations [1]. The figure shows the theoretical "E2-map" for 62Ni. This shows the excitation energy for states of different J values (the black dots). The dots are connected by red lines whose width is proportional to the size of the reduced E2 transition strength, the B(E2) (transitions below a value of about 5% of the strongest are not shown). Collective decay features show up in this figure are wide red bands. In this figure a triplet of states near 2.2 MeV with strong E2 transitions to the "one-phonon" state at 1.1 MeV. But the predicted properties of the state in the region of "three-phonon" expected around 3.3 MeV are much different from that expected in the phonon model. The calculated properties of 62Ni are in good agreement with experiment and very different from that expected for three-phonons. For example, the strongest three-phonon to two-phonon E2 transition is the 3+1 to 2+2. In contrast, the calculated B(E2) for this transition is very small (too small to show up as a red line in the figure), and this is in agreement with experiment. Thus, we conclude that the properties of 62Ni do show evidence for quadrupole vibrations, but do show the success of the more general shell-model configuration interaction theory. The collective feature associated with the wide red bands for the 6+3 to 5+2 to 4+4 transitions are not presently understood. It would be interesting to confirm these in experiments.


[1] A. Chakraborty, J. N. Orce, S. F. Ashley, B. A. Brown, B. P. Crider, E. Elhami, M. T. McEllistrem, S. Mukhopadhyay, E. Peters, S. Singh and S. W. Yates, Phys. Rev. C 83, 034316 (2011). link to paper