Radiochemical Studies of the Interaction of Lead with Protons in the Energy Range 0.6 to 3.0 Bev

Abstract

Lead targets have been bombarded with protons of several energies between 0.6 and 3.0 Bev and formation cross sections have been determined for about 30 nuclides of $A<\mathrm{}140\mathrm{}$ produced in these bombardments. The excitation functions, both for the lightest products studied ($A<\mathrm{}35\mathrm{}$) and for neutron-deficient barium isotopes, rise steeply with increasing energy. For intermediatemass products ($50<A<\mathrm{}120\mathrm{}$), the changes in formation cross sections with increasing energy are much smaller and may largely be interpreted as a shift towards more neutron-deficient products and as a broadening of the yield-mass distribution. At 3 Bev the spallation and fission product regions have merged, and the cross sections for forming all mass numbers below the target mass are equal within an order of magnitude. The changes in yield pattern above about 0.4 Bev are shown to be associated with increasing probabilities of very large transfers (of the order of 1 Bev) from the incident proton to the struck nucleus, and this trend is explained in terms of an energy transfer mechanism involving the production, scattering, and reabsorption of pions. Besides the well-known modes of de-excitation—particle evaporation (spallation) and fission—a new mode termed fragmentation is postulated to account for some of the observed products, especially the light fragments. Fragmentation is thought to be associated with the short mean free paths of pions in nuclear matter which cause local heating and can thus lead to dissociation of the nucleus into fragments in a time short compared to that required for equipartition of energy.