Nuclear spectroscopy, nuclear clustering and exotic shapes

Nuclear clustering has been a subject of intense study since the advent of heavy ion accelerators. Looking back more than 30 years, we are able today to see the connection between resonances in heavy ion reactions and extreme states in nuclei. In particular, excited states close to the decay thresho lds into substructures show pronounced cluster structures. The ever repeate d question about the possibility to describe such cluster phenomena in an e xtended (to a very large basis) and deformed shell model, has recently been revived in view of improved computational facilities. The new experimental facilities and the powerful new detector arrays for gamma-spectroscopy and for charged particles have increased the sensitivity to study clustering p henomena by orders of magnitude. Today we are able to study nuclear structu re at the limits of the mean held concept. With the advent of radioactive b eams and the study of nuclei, which are weakly bound in their ground states , the question of the limits of mean held concepts has reached the low exci tation energy region of nuclei.