The $\frac{K}{(L+M+\cdots )}$ internal-conversion ratio for the 355-keV transition in ${}_{}{}^{133}{}_{}{}^{}\mathrm{Cs}$ has been measured as 4.27±0.13. Interference of $K$-shell electrons from the 382-keV transition was eliminated by standard fast-slow coincidence techniques using pulses from a Si(Li) detector at 77°K and NaI(Tl) scintillator. The experimental result indicates the multipolarity of the 355-keV transition to be (97.5% $E2\mathrm{}$, 2.5% Max $M3\mathrm{}$) when compared with existing tabulations of internal-conversion coefficients (ICC) compared with predictions made using Coulomb-screened ICC calculations, a predominantly $M3\mathrm{}$ multipolarity is implied. In either case no $M1\mathrm{}$ contribution is evidenced and the spin of the 436-keV level is verified as ${\mathrm{½}}^{+}$, allowing this level to be interpreted in terms of the shell model, as a single-particle excitation to an $s_{\frac{1}{2}}$ state.

• Received 28 October 1966

DOI:https://doi.org/10.1103/PhysRev.158.1181