Small angle correlations in low energy heavy ion reactions: Emission time scales

Light particles (p, d, α) have been studied (both singles and two-fold coincidences) from the reactions 80, 140, and 215 MeV 18O + 27Al. Singles data indicate evaporation-like emission for laboratory angles greater than about 50 degree (lab). Small angle correlations measured here of p-p (at the lower two energies), p-d, and d-d suggest long-range Coulomb forces dominate the interactions. Comparison to a trajectory model implies particle emission times of a few x 10-21 s at 140 MeV. In contrast, p-α, d-α, and α-α correlations have peaks associated with the short-range nuclear force. By comparing coincidence yields for detector pairs having Δ φ ∼ O and ∼ 180 degree azimuthal separation, and by exploiting compound nucleus decay symmetries, we argue these latter correlations are due to decay of preformed, metastable resonances and are therefore insensitive to lifetime effects. All correlations become stronger with increasing energy