Near-Symmetric Fission—Identification and Yield ofCd121

Evidence was acquired for the formation of ${\mathrm{Cd}}^{121}$ in the thermal-neutron fission of ${\mathrm{U}}^{235}$. Its half-life and cumulative fission yield are ${12.8}_{-0.3\mathrm{}}^{+0.4\mathrm{}}$ sec and (6.4±0.5)×${10}^{-3\mathrm{}}$%, respectively. From the cumulative fractional chain yield, a value of ≤48.45 was inferred for the most probable nuclear charge ($Z_p$) of the primary fission fragment formed in this chain. The addition of this value to a partially formulated empirical $Z_p$ function suggests that the value of $Z_p$ proceeds toward the point of symmetrical fission by a gentle rather than abrupt approach. The mass-121 chain yield was redetermined and the value obtained was (1.2±0.1)×${10}^{-2\mathrm{}}$%. This value is based on a measured cumulative yield of (1.1±0.01)×${10}^{-2\mathrm{}}$% for ${\mathrm{Sn}}^{121}$ (27.5 h) and a literature value of 0.08×${10}^{-2\mathrm{}}$% for ${\mathrm{Sn}}^{121m}$(25 yr).