$\frac{E2\mathrm{}}{M1\mathrm{}}$ Ratios in the Isomeric Transitions of ${\mathrm{Te}}^{121}$ and ${\mathrm{Te}}^{123}$, and the Decay of the ${\mathrm{Te}}^{121}$ Isomers to ${\mathrm{Sb}}^{121}$

The decay of ${\mathrm{Te}}^{121}$ isomers and the decay of ${\mathrm{Te}}^{123m}$ have been investigated with scintillation and proportional counter spectrometers, coincidence techniques, and a high resolution beta-ray spectrometer for the measurement of the conversion spectrum. In the decay of the ${\mathrm{Te}}^{121}$ isomers to levels of ${\mathrm{Sb}}^{121}$, transitions of the following energies have been found: 37, 66, 470, 508, 573, and 1110 keV. The 37, 1110, and 66 508 cascades have been established by coincidence measurements. A 37, 470 cascade is indicated by precision energy measurements. Coincidences between the 66- and 1110-keV transitions expected to occur on the basis of reports in the literature, could not be found. By a careful analysis of the conversion spectrum, it was possible to determine, from $L$ subshell ratios, the multipolarities of most of the gamma rays. A decay scheme is proposed which accounts for all of the observed data and in which the first-excited state in ${\mathrm{Sb}}^{121}$ lies at 37 keV. The half-life of the 37-keV level in ${\mathrm{Sb}}^{121}$, measured by the delayed coincidence method, was found to be (3.5±0.2)×${10}^{-9\mathrm{}}$ sec. Furthermore, from the analysis of the $\frac{\frac{L_1}{L_2}}{L_3}$ ratios observed in the conversion spectrum, the $\frac{E2\mathrm{}}{M1\mathrm{}}$ ratio values could be determined for the 212- and 159-keV transitions in ${\mathrm{Te}}^{121m}$ and ${\mathrm{Te}}^{123m}$, respectively. The results are: $\frac{E2\mathrm{}}{M1\mathrm{}}=0.050\mathrm{}$ for the 212-keV transition and $\frac{E2\mathrm{}}{M1\mathrm{}}=0.0067\mathrm{}$ for the 159-keV transition. The $E2\mathrm{}$ speeds of these transitions have been inferred from the known lifetimes of the 212-keV state in ${\mathrm{Te}}^{121}$ and of the 159-keV state in ${\mathrm{Te}}^{123}$; the results are 26 and 4.5 times the single-particle speeds, respectively.