We present a measurement of the half-life of ${}^{121}{\mathrm{Sn}}^m,$ obtaining a value of $43.9\pm 0.5\mathrm{yr}.$ In the same experiment, we measured a half-life of $21.8\pm 0.3\mathrm{yr}$ for ${}^{210}\mathrm{Pb}$ to verify the reliable performance of our data acquisition system and analysis methods. Our experimental ${}^{121}{\mathrm{Sn}}^m$ half-life is consistent with 49 yr, a value predicted on the basis of the systematic trend of the M4 isomeric γ-ray transition in ${}^{117}\mathrm{Sn}$ and ${}^{119}\mathrm{Sn}.$

• Received 4 February 2002

DOI:https://doi.org/10.1103/PhysRevC.65.057302