A many-body model study of fragment formation in fission

The mechanism of fragment formation in fission has been studied in 32S as calculable many-body model for similar effects in the fission of actinides. The calculations have been performed for soft-core two-body forces with alpha cluster model (ACM) states, and these wave functions have been compared with two-centre shell-model (TCSM) states. The results show that, already at a small deformation, a correlation of the nucleons into two closed-shell clusters (16O+16O) is energetically favoured over the many-body states without such clustering. No cluster structure of that kind is found in the ACM ground-state of the compound nucleus. The (16O+16O) clustering sets in at the barrier between the first and the second minimum. In fact, the gain in correlation energy associated with the clustering turns out to be the very reason behind the occurrence of a shape isomeric state. This explains the paradox of 'fragment pre-formation', as these fragment-like correlations are noticeable long before the actual break-up into fragments. A comparison of the minimum energy ACM states with the lowest TCSM 1p-1h states yield very high overlaps at any deformation. This demonstrates that the description of fission in terms of cluster formation results in the same consequences as those from the usual shell-model picture of fission. (author)