Reaching the Capacity of a Composite Nucleus for Energy and Spin Containment

The reaction 1080-MeV ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$+${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ has been studied via correlated fission fragments and He particles. Between $\frac{1}{3}$ and ½ of the reaction cross section goes into fusionlike events to form composite nuclei excited to 500-800 MeV. The probability for such high-energy containment drops dramatically for ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ projectiles between 27 and 44 MeV/u. Evaporative He particles for fusionlike events (multiplicity ≈ 1.5) have been found to arise predominantly from the massively excited composite nuclei.