Fission ofU238by 2.2-GeV Protons. II

Differential range curves (at an angle of 90°) have been obtained by radiochemical methods for 17 isotopes of Cu, Sr, Pd, and Ba which recoil out of thin uranium targets irradiated with 2.2-GeV protons. Neutron-rich isotopes of Ba, Pd, and Sr all have sharply peaked spectra with mean momenta >120 ${\mathrm{}\left(\mathrm{MeV}\mathrm{amu}\right)\mathrm{}}^{1/2}$, consistent with their production by binary fission. Neutron-deficient isotopes of these elements are characterized by lower mean momenta [<106 ${\mathrm{}\left(\mathrm{MeV}\mathrm{amu}\right)\mathrm{}}^{1/2}$] and broader spectra which include significant low-momentum components [<50 ${\mathrm{}\left(\mathrm{MeV}\mathrm{amu}\right)\mathrm{}}^{1/2}$]. The absolute contribution of these low-momentum recoils decreases from the order of a few mb/amu at mass 128 or 131 to ≅0.1 mb/amu at mass 83. Fission and some contribution from a spallationlike process are suggested as the probable formation mechanisms for the neutron-deficient products. The formation mechanism for copper isotopes is not clear. Spectra of ${}_{}{}^{61}{}_{}{}^{}\mathrm{Cu}$, ${}_{}{}^{64}{}_{}{}^{}\mathrm{Cu}$, and ${}_{}{}^{67}{}_{}{}^{}\mathrm{Cu}$ have widths comparable to those observed for binary fission and contain few low-momentum recoils; however, mean momenta are lower than those observed in semiconductor measurements of fission or predicted by a liquid-drop theory.