Energy and angular correlation of He ions and heavy fragments in 11.5-GeV proton bombardment of uranium

Measurements of the number and kinetic energy distribution of He ions formed in the 11.5-GeV proton bombardment of uranium were made with a particle telescope as a function of the angle between the He ion and the direction of a heavy fragment. The mass of the latter was determined in a time-of-flight system. No correlation in the intensity or energy of He ions with the direction of the heavy fragment was observed. This is interpreted as an indication that the He ions are formed prior to or simultaneously with the formation of the heavy fragments rather than being evaporated from an excited, fully accelerated fragment. In the case of fission fragments these results provide direct evidence that fission does not occur as a prominent process at high excitation energy ($E^{*}$), but rather, is essentially a low energy phenomenon. Current formulations of $\frac{{\Gamma }_f}{{\Gamma }_n}$ predict a large proportion of fission at high $E^{*}$ for the nuclides involved here, in disagreement with the results of the present experiment. An estimate is made of the relative deposition energy involved in the reactions as a function of the energy and mass of the heavy fragment formed. This clearly shows that the region of $30\lesssim A\lesssim 70$ is associated with the highest deposition energy. This is just the region of the so-called "fragmentation" products whose recoil properties provide evidence for the existence of a specific high-energy reaction mechanism.