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 systems 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 an 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 fusion calculation. The He-6 data show a regular trend with an enhancement below the barrier compared to He-4 + U-238 or Li-6 + U-238. This enhancement could be due to the fusion process itself or to a transfer component which triggers 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.