Secondary beams of He-4 and He-6 were produced through the transfer re
actions with the 18.5 Me V/u B-11 and 35.5 Me V/u Li-7 primary beams.
The fusion-fission cross sections have been measured for the He-4,He-6
+ Bi-209 reactions at energies E(cm) > 1.5B-fus. In the present expe
riment it was found that the fission excitation function for the neutr
on rich nuclei He-6 is significantly higher than for the He-4 nuclei.