It is well established that the optical isotope shifts for pairs of isotopes with 88 and 90 neutrons in 60Nd, 62Sm and 63Eu are anomalously large.(1) This is directly related to a change in the neutron-shell configuration. The sudden onset of permanent deformation occurs at a neutron number of 90, as predicted by the nuclear model of Nilsson. It is the increase in nuclear deformation, coupled with the usual increase in nuclear volume, that gives origin to such anomalous isotope shifts.
