Interaction of 28-GeV Protons with Uranium: Charge and Mass Distribution of Rare-Earth Products

Independent and cumulative yields of about 50 rare-earth nuclides produced in the interaction of 28-GeV protons with uranium have been measured. Chain yields on the neutron-excess side of $\beta$ stability were found to have the same mass dependence as in low-energy fission. Based on the results obtained in the present study and with the aid of the published Cs isotopic distribution, charge-dispersion curves at $A\approx 147$ and $A\approx 170$ were constructed. The curve at $A\approx 147$ has two maxima, one on the neutron-deficient and one on the neutron-excess side of $\beta$ stability, separated by a deep valley; the curve at $A\approx 170$ has a single peak on the neutron-deficient side of $\beta$ stability. Corrections were made for feeding and depletion of various mass chains by $\alpha$ decay. Total isobaric yields were determined for 11 mass numbers between $A=143\mathrm{}$ and $A=171\mathrm{}$. On the basis of these results and published data at lower masses, a mass-yield curve was constructed. The apparent constancy of the neutron-deficient yields between mass 143 and mass 171 is consistent with the yield pattern of a spallation-like process. Over the mass range $50<A<\mathrm{}160\mathrm{}$, binary fission and spallation cannot account for all the observed yields. Other processes must be partially responsible for the production of these nuclides, especially at the lower masses.