Oral Contribution :

Colloque GANIL 2009 The XVIth "colloque GANIL" will be held from September 6th to 11th, 2009 in Giens (France). (Webpage)

[HAL]

 Title :

Interplay between structure and reaction mechanisms around the Coulomb barrier probed using 8He

 Abstract:

The availability of low energy Radioactive Ion Beams, has provided the possibility to study the influence of exotic structures and/or weak binding on reaction dynamics far from stability around the Coulomb barrier. Fusion reactions around the Coulomb barrier are not only a unique laboratory for probing various aspects of multidimensional tunneling but are also relevant to the synthesis of Super Heavy Elements and reactions of astrophysical interest. The SPIRAL facility at GANIL provides the most intense beams of 6,8He presently available, giving a unique opportunity to study neutron correlations [1] and the effect of Borromean structure on the reaction processes. The low energy and relatively low intensities ( 10^5 p/s) of the Radioactive Ion Beams make measurements challenging thus requiring the development of innovative methods to each new detections limits [2].

In this talk we will present recent results obtained with low intensity beams of Borromean isotopes of Helium. The complementarity of in-beam and off-beam methods will be discussed. In-beam inclusive and exclusive measurements of gamma-rays, charged particles and neutrons performed in the 8He+65Cu system to obtain fusion cross-sections and the angular distributions of direct reactions at energies above the barrier will be presented (Expt. E403aS). The first sub-barrier cross-sections measurements for fusion and nucleon transfer reactions involving the 8He beams on a 197Au target will also be presented (Expt. E543S). These results were obtained using a new activation technique [3] involving coincidences between characteristic X-rays and gamma-rays. The precision and accuracy reached in this experiment are at the same level as those obtained using stable beams having a million times larger intensity.

Results of these experiments will be discussed within a coupled channel framework using FRESCO [4].The role of channel coupling between the elastic and direct reaction channels, its influence on fusion at energies around the Coulomb barrier arising as a result of the exotic structure of 8He will be illustrated.