Fragment Production in the Interaction of 5.5-GeV Protons with Uranium

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Abstract

The energy spectra of nuclear fragments produced by the interaction of 5.5-GeV protons with uranium have been determined at several laboratory angles by means of $\frac{\mathrm{dE}}{\mathrm{dx}}\ensuremath{-}E$ measurements with semiconductor-detector telescopes. Individual isotopes of the elements from hydrogen to carbon were resolved, and from nitrogen to argon the study was continued for the elements without isotopic separation. The evaporation-like energy spectra were integrated to obtain angular distributions and total cross sections for the isotopes of helium through carbon. Total cross sections for many rare isotopes were also estimated. The yield surface constructed from these cross sections has a ridge-like shape positioned one neutron in excess of the line of $\ensuremath{\beta}$ stability. The yields fall off more steeply on the neutron-deficient side and exhibit odd-even effects reflecting those of the mass surface. The energy spectra of the neutron-deficient isotopes differ from the others in that the high-energy parts of the spectra are more pronounced and flatter, and the angular distributions are more forward peaked.Some of the energy spectra were fitted with calculated curves based on the isotropic evaporation of fragments from a system moving along the beam axis. The apparent Coulomb barriers obtained from this analysis were about one half the nominal Coulomb barriers, and the apparent nuclear temperatures fell in the 10- to 13-MeV range. For the highest-energy fragments observed at 90\ifmmode^\circ\else\textdegree\fi{} the apparent temperatures rose to 20 MeV. From the forward-backward shifts in energy it was deduced that the average velocity of the moving system is about $0.006c$ and that there is a positive correlation between this velocity and the velocity of the fragments in the moving system. However, all of the data are more peaked forward in intensity than can be explained by this simple two-step model.Radiochemical cross sections are also presented for the production of $^{7}\mathrm{Be}$ from uranium, silver, and aluminum, and for $^{22}\mathrm{Na}$ from aluminum.

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