Less than 12% of 27.5-h ${\mathrm{Sn}}^{121}$ is formed independently in thermal-neutron fission of ${\mathrm{U}}^{235}$; 24±3% of it is formed by beta decay of 3.1-min ${\mathrm{In}}^{121}$ and 74±10% by beta decay of 30-sec ${\mathrm{In}}^{121}$. The small independent yield of 27.5-h ${\mathrm{Sn}}^{121}$ and the very small total fission yield of ∼25-yr ${\mathrm{Sn}}^{121m}$ show that "the most probable charge" ${\mathrm{Z}}_{\mathrm{p}}$ for fission products with $A=121\mathrm{}$ must be less than 50. This result indicates that the large decrease in the total kinetic energy of fission fragments that has been observed as the fragment mass ratio approaches unity cannot be explained entirely by a distribution of nuclear charge in which heavy fragments retain, on the average, 50 or more protons.

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