Observations of the Mössbauer Effect inCs133

In an extension of measurements on the Mössbauer effect of the 81-keV transition in ${\mathrm{Cs}}^{133}$ we have measured the isomer shifts of a variety of cesium chemicals, including the halides and the metal. The measurements were done in transmission at 4.2°K using a source whose nominal composition was ${\mathrm{Ba}}^{133}$ ${\mathrm{Al}}_4$. The isomer shifts of the halides are shown to correlate linearly with the chemical shifts of their NMR frequencies. A fractional effect of (1.86±0.26)×${10}^{-4\mathrm{}}$ was observed for the cesium metal, and the isomer shift with respect to Ba${\mathrm{Al}}_4$ was -0.158±0.057 mm/sec. When the latter is combined with the halogen shifts (which lead to an isomer shift of ∼0.3 mm/sec for ${\mathrm{Cs}}^{+}$), it is found that the cesium decay product of ${\mathrm{Ba}}^{133}$ in Ba${\mathrm{Al}}_4$ has the electron configuration $6s^2$. The existence of this species is supported by a plausibility argument in terms of the band structure of aluminum. The observed linewidth of the resonance in metallic cesium is 0.75±0.18 mm/sec. In view of the source contribution, this is consistent with the natural linewidth. A spin-density-wave (SDW) ground state for Cs would lead to a hyperfine structure that was not observed. We conclude that for a linear SDW such as postulated for potassium by Overhauser, the linewidth indicates that the amplitude must be less than ∼1% or the relaxation time less than 6×${10}^{-10\mathrm{}}$ sec. An area analysis of the absorption in the metal leads to a recoilless fraction $f_{\mathrm{Cs}}=5.5\mathrm{}\times {10}^{-5\mathrm{}}$ and to the characteristic temperature ${\ominus }_M=49\mathrm{}\pm 1°$K. In comparison, the Debye temperature at 0°K is ${\ominus }_D\mathrm{}\left(0\right)=40.5\mathrm{}°$. From the shape of the function ${\ominus }_D\mathrm{}\left(T\right)\mathrm{}$ and the result ${\ominus }_M>{\ominus }_D\mathrm{}\left(0\right)\mathrm{}$, it is concluded that the phonon spectrum cuts off substantially higher than the frequency corresponding to ${\ominus }_D\mathrm{}\left(0\right)\mathrm{}$. A numerical calculation is made for the case of sodium where the spectrum is known and exhibits the predicted features. For this case a cancellation of two effects gives ${\ominus }_M={\ominus }_D\mathrm{}\left(0\right)\mathrm{}$ within the error.