Properties of Low-Lying Levels inSb121

Resonance fluorescence in ${\mathrm{Sb}}^{121}$ has been observed with ${\mathrm{Te}}^{121}$ as the source of the exciting gamma radiation. A comparison of the pulse-height distribution of the resonance scattered radiation with that of the incident radiation showed unambiguously that the 506- and the 576-keV gamma rays emitted in the decay of 17-day ${\mathrm{Te}}^{121}$ are the ground-state transitions from two separate levels in ${\mathrm{Sb}}^{121}$. The angular distribution of the resonance radiation was studied with the resonant gamma rays from a gaseous source of ${\mathrm{Te}}^{121}$. On the basis of this angular distribution, the most probable spin assignments are $\frac{3}{2}$ for the 506-keV level, and ½ for the 576-keV level. The transition probabilities of these levels were determined using the centrifuge method of compensating for the recoil energy losses. Assuming a branching of $\frac{{\Gamma }_0}{\Gamma }=0.96\mathrm{}$, the partial width for the 576-keV $E2\mathrm{}$ transition was found to be ${\Gamma }_0\mathrm{}\left(576\right)=(5.4\mathrm{}\pm 0.6)\times {10}^{-5\mathrm{}}$ eV. For the mixed $M1+E2\mathrm{}$ 506-keV transition, using $\frac{{\Gamma }_0}{\Gamma }=0.90\mathrm{}$, the partial width was determined to be ${\Gamma }_0\mathrm{}\left(506\right)=(2.2\mathrm{}\pm 0.2)\times {10}^{-4\mathrm{}}$ eV. While the single-particle model predicts the observed spin sequence, it fails to account for the observed transition probabilities. Wave functions obtained recently by Kisslinger and Sorensen, and representing a combination of quasiparticle and phonon excitations, predict the observed transition probabilities rather well.