Gert H. Rohr

Commission of the European Communities, Joint Research Centre - Geel-Establishment
Central Bureau for Nuclear Measurements, 2440 Geel, Belgium

Neutron resonances of the 4n target nuclei 28Si, 32S, 40Ca, 52Cr and 96Zr are studied. According to the independent particle model the excited states of these nuclei are created in one or maximum two collisions and are the best candidates for studying the properties of nucleon-nucleon interaction in nuclei. It is shown that the doorway resonances and resonances originating from the same doorway state result in a Gaussian-like nearest level spacing distribution. The strong energy correlation between resonances is assigned to the nucleon-nucleon interaction and contradicts the independent particle picture. The almost equally spaced doorway states are important, since they fulfil in place of equidistant single particle states the rather simple assumption of the Bethe level density formula. Furthermore, they explain the steps in the level density systematics, which is a pre-assumption for the prediction of level spacings for nuclei which are not measurable.

KEYWORDS: Level spacing distribution, nucleon-nucleon interaction in nuclei, doorway states, nuclear models