Nuclear Charge Distributions in the Interaction of 2.9-GeV Protons with Heavy Elements

Cross sections for the formation of a number of nuclides in the mass range 65-74 in the bombardment of In, Au, and U with 2.9-GeV protons have been measured. Four isobars of mass 72, three of mass 67, and three of mass 66 are included. Charge distribution curves have been constructed from the data, using $\frac{N}{Z}$ the ratio of neutrons to protons in the nucleus, as abscissa. The curves are not symmetric about the peak, falling less steeply toward large $\frac{N}{Z}$, with U having the most asymmetry. The peak position shifts to larger $\frac{N}{Z}$ as the target mass increases. The data for In are consistent with a cascade-evaporation mechanism involving a long evaporation sequence, while the U data show the importance of low-excitation-energy fission in the formation of neutron-excess nuclides.