Recoil Properties of Rare-Earth Nuclides Produced by the Interaction of 28-GeV Protons with Uranium and Gold

The thick-target-thick-catcher technique was used to determine the recoil properties of some rare-earth nuclides produced by the interaction of 28-GeV protons with uranium and gold. Mean momenta derived from this and other experiments are discussed in terms of fission and spallationlike processes. It is concluded that, while fission can account for neutron-rich products from uranium, the momenta of neutron-deficient rare-earth nuclides from both uranium and gold are too low to arise from any conventional fission process. In the mass region below the rare earths, many neutron-deficient isotopes appear to be formed by mixed mechanisms. Only the most neutron-deficient products have momenta as low as those predicted for a spallationlike process. Mean cascade deposition energies, $E^{*}$, derived from the forward to backward ratios average 145 MeV for the neutron-deficient rare earths from uranium and 199 MeV for these products from gold. Such low values for gold and the even lower values for uranium cannot be understood in terms of a model for the spallationlike process involving a nucleonic cascade followed by conventional evaporation. Either there is a break-down of the two-step model, e.g. as might be expected if fragment emission is involved, or it is not valid to extrapolate to 28 GeV the momentum-transfer excitation-energy relationship inferred from Monte Carlo calculations at much lower energies.