Fission ofU238by 2.2-GeV Protons

Abstract

Angular distributions and differential range curves at angles of 15\ifmmode^\circ\else\textdegree\fi{}, 90\ifmmode^\circ\else\textdegree\fi{}, and 165\ifmmode^\circ\else\textdegree\fi{} have been obtained by radiochemical methods for several representative products which recoil out of thin uranium targets irradiated with 2.2-GeV protons. Mean momenta observed at 90\ifmmode^\circ\else\textdegree\fi{} to the beam direction are 133, 125, and 125 ${(\mathrm{MeV}\mathrm{amu})}^{\mathrm{\textonehalf{}}}$ for the neutron-excess isotopes ${\mathrm{Ba}}^{140}$, ${\mathrm{Mo}}^{99}$, and ${\mathrm{Sr}}^{91}$, but only 96 and 104 ${(\mathrm{MeV}\mathrm{amu})}^{\mathrm{\textonehalf{}}}$ for the neutron-deficient products ${\mathrm{Ba}}^{131}$ and ${\mathrm{Pd}}^{103}$. The neutron-deficient isotopes have broader momentum distributions and more forward-peaked angular distributions than do the neutron-excess products. Results are discussed in terms of a two-step vector model, i.e., a fast nucleonic cascade followed by fission or another slower de-excitation step. The observed spectra and angular distributions for all the isotopes are consistent with such a model. Observed mean velocities are compared to those predicted by a liquid-drop theory of fission. It is concluded that products in the mass-90-to-140 range are produced primarily by a conventional fission mechanism. However, some contribution of a spallationlike process appears necessary to account for low-velocity fragments in the case of the neutron-deficient products (particularly ${\mathrm{Ba}}^{131}$).