INVESTIGATION OF THE 2(1)(+) LEVEL ENERGY ANOMALY IN VIBRATIONAL NUCLEI

Recently, a method for the identification of multiphonon states in vib rational nuclei was successfully developed. A survey was performed and it was found, surprisingly, that the energies of the first 2(+) state s are systematically low by about 15% with respect to the values expec ted from the global nuclear properties. This appears to be in contradi ction with the general belief that these states have a high purity. It can be questioned whether such results are not simply an artefact due to the weights used in the fits. One way to answer this question is t o compare the experimental results with the predictions of the Brink m odel. To this end, the equations for four and five phonons were explic itly calculated. It will be shown that the predictions are quite good, provided the one phonon energy be renormalized, i.e. increased by abo ut 15%. Since the modified Brink method involves only the use of a ''v irtual'' 2(1)(+) energy and no level fit, a problem of weights cannot be invoked. Possible explanations of the anomaly are examined. (C) 199 7 Elsevier Science B.V.