Large-angle correlations betweenHe4and1,2,3H orHe4in the reactions 247 and 337 MeVAr40+natAg: Unexpected properties of the nuclear stratosphere

We have measured coincidences between ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^1{}_{}{}^{}$,2,3H or a second ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ (large-angle correlations) in the reactions 247 and 337 MeV ${}_{}{}^{40}{}_{}{}^{}$Ar${+}^{\mathrm{nat}}$Ag. These coincidences are dominated by reactions leading to evaporation residues with spins of about 0–70ħ. 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 ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ that are strongly reduced compared to empirical fusion barriers. The barrier reduction required for ${}_{}{}^2{}_{}{}^{}\mathrm{H}$ is rather small, and for ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ 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 ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ reflect the radial extent of this transitory, diffuse surface.