Isomers in Fission Fragments

The structure of neutron‐rich nuclei produced as secondary fission fragments was investigated using the EUROGAM and GAMMASPHERE ACS arrays, the LOHENGRIN fission‐fragment mass separator and the FIFI fission‐fragment identifier. Fission products were populated in spontaneous fission of 248Cm and 252Cf and in thermal neutron‐induced fission of 233U, 235U and 241Pu at ILL Grenoble. Particularly useful in such studies are isomeric states, well populated in fission due to their yrast character, easy to detect due to their long half lives and easy to interpret because of their relatively simple composition. We discuss their role in studies of neutron‐rich nuclei, giving examples of isomers found in our recent experiments. A special type of K‐isomers, resulting from ‘crossing’ of extruder and intruder orbitals plays a role in the mechanism of a sudden onset of deformation in the A = 100 and A = 150 regions. We present evidence for these isomers in both regions. Possible further studies in this field are proposed.