Isobaric Yield Distributions in the Interaction of 1.8-GeV Protons with ${\mathrm{Zr}}^{96}$, ${\mathrm{Mo}}^{96}$, and ${\mathrm{Ru}}^{96}$

Cross sections for the formation of 4 isobars with $A=72\mathrm{}$ in the interaction of 1.8-GeV protons with ${\mathrm{Zr}}^{96}$, ${\mathrm{Mo}}^{96}$, and ${\mathrm{Ru}}^{96}$, have been measured. Results have also been obtained for a number of other nuclides with $A=66-74\mathrm{}$. It is found that the charge of the most probable product with $A=72\mathrm{}$ increases by 0.6 units in going from ${\mathrm{Zr}}^{96}$ to ${\mathrm{Ru}}^{96}$, resulting in large differences in isobaric yield ratios for the different targets. These results are in qualitative agreement with cascade-evaporation calculations. A strong correlation between the peak position in the isobaric yield curve and the target $\frac{N}{Z}$ value is indicated both by the present results and previous measurements by Kaufman for a number of heavier targets.