Large-angle correlations between He4 and 1,2,3H or He4 in the reactions 247 and 337 MeV Ar40+natAg: Unexpected properties of the nuclear stratosphere

We have measured coincidences between He4 and 1,2,3H or a second He4 (large-angle correlations) in the reactions 247 and 337 MeV 40Ar+natAg. These coincidences are dominated by reactions leading to evaporation residues with spins of about 070Latin small letter h with stroke. Large values of the angle-integrated double and triple coincidence cross sections indicate that many particle evaporation chains involve 2 or 3 protons and/or 2 or 3 alphas. The angular and energy distributions are well described by an evaporation model with Hauser-Feshbach coupling. However, one must employ effective evaporation barriers for H1 and He4 that are strongly reduced compared to empirical fusion barriers. The barrier reduction required for H2 is rather small, and for H3 no barrier reduction is needed. Evaporation calculations that include emitter deformation do not give a satisfactory explanation for these results. We suggest a new aspect of composite nucleus excitation and evaporation-like decay. A hot, very diffuse nuclear surface may be formed that promotes evaporative decay prior to its relaxation. The low barriers for H1 and He4 reflect the radial extent of this transitory, diffuse surface.