Test of the applicability of the two-step model to the emission of Sc fragments in the interaction ofU238with 400 GeV protons

Differential ranges of ${}_{}{}^{44}{}_{}{}^{}\mathrm{Sc}_{}^m{}_{}{}^{}$, ${}_{}{}^{46}{}_{}{}^{}\mathrm{Sc}$, ${}_{}{}^{47}{}_{}{}^{}\mathrm{Sc}$, and ${}_{}{}^{48}{}_{}{}^{}\mathrm{Sc}$ emitted at 17° and 163° to the beam in the interaction of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ with 400 GeV protons have been measured. The mean kinetic energies of the fragments decrease from 54.2±1.0 MeV at 17° to 42.0±0.7 MeV at 163°. From this shift, the mean velocity of the emitting nucleus $〈v_{\parallel }〉$ is obtained as 0.10 ${(\mathrm{M}\mathrm{e}\mathrm{V}/\mathit{A})}^{\mathrm{½}}$. The angular distribution of Sc fragments in a system moving with this velocity is found to be asymmetric about 90° with more fragments emitted at backward than at forward angles ($\frac{F}{B}\sim 0.8$). The implications of this result for the reaction mechanism are considered.