Sub-barrier fusion with the He-6 halo nucleus

The fusion-fission cross sections of the He-4 + U-238 and He-6 + U-238 syst ems have been measured, at Louvain-la-Neuve (Belgium), for energies around and below the barrier. The helium projectiles impinges on a UF4 target. The fission fragments and the possible projectile residue are detected using a n array of Si detectors for angles ranging from 30 to 150 degrees. The data taken with He-4 are in good agreement with previous data and with the fusi on calculation. The He-6 data show a regular trend with an enhancement belo w the barrier compared to He-4 + U-238 or Li-6 + U-238. This enhancement co uld be due to the fusion process itself or to a transfer component which tr iggers the fission without being detected in our set-up. The data for the A =4 and A=6 isobars are compared to a simple barrier penetration model.