Residence in forbidden regions

Modern nuclear physics deals with high energy densities, high spins or high values of isospin. This thesis is concerned with the latter subject, namely the behaviour at the dripline (the limit of particle stability) where nucleons are very weakly bound. The nucleons will here tunnel out from the nuclear core and spend most of their time in a classically forbidden region. Such systems, called halos, were discovered less than ten years ago and can only be produced at rather few accelerators. Most of this thesis is concerned with halos. It starts with a general discussion of when halos can appear and what properties they will have. A general description turns out to be possible, for a nucleus of mass number A and binding energy E halos can only appear for EA 2/3 ≤ 5 MeV. Then experiments with halo nuclei are presented. First subject is nuclear reactions from beam energies of 25 MeV/u to 1 GeV/u. In this range the reactions can be interpreted in a rather simple way, in particular for the cases where only the halo particle is removed that give quite direct information on the original nucleus. An experiment on 11Li at GANIL (Caen, France) is described in detail and a review is given on the present experimental and theoretical status. Next, beta decay experiments, performed at ISOLDE (CERN, Switzerland), are described. Beta decays close to the dripline differ from the ones closer to stability in that particle emission from excited states in the daughter nucleus is possible. This process has been investigated in several light nuclei, and used as background for showing that halos influence the decay of 17Ne. The decay of 6He is especially interesting; here deuterons can be emitted directly (i.e. without involvement of the daughter nucleus), a decay mechanism not seen earlier. Finally some perspectives on the future of this field are offered. (au) (165 refs.)