Approach to the limits for massive energy and spin deposition into a composite nucleus

We report measurements of correlated fission-like fragments from the reaction 1080 MeV (E/A=27 MeV) ${}_{}{}^{40}{}_{}{}^{}$Ar${+\mathrm{}}^{238}$U and their further correlations with H/He particles. Fusion-like reactions that form highly excited composite nuclei (500–800 MeV) comprise ≊40% of the reaction cross section. The probability for such fusion-like reactions is dramatically smaller for ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ of E/A of 35–44 MeV. We infer that for this reaction the conditions for high probability of such massive energy and spin containment are near to their limits. The integrated multiplicities for H and He emission are both ≊3 for fission reactions that involve 50–100 % momentum transfer. These multiplicities are about evenly divided between evaporative and forward-peaked emission; comparison with other results indicates that both have increased by about twentyfold for an increase in projectile energy of 340–1080 MeV. The major sources of evaporative He emission are the very highly excited composite nuclei.