Precision measurements on the hyperfine structure of the transitions corresponding to $\Delta M=4\mathrm{}$ of the cubic-field paramagnetic resonance spectrum of gadolinium in single crystals of thorium oxide yield the following values: isotopic abundance: even isotopes, 69.45%; ${\mathrm{Gd}}^{155}$, 15.05±0.2%; ${\mathrm{Gd}}^{157}$, 15.5±0.2%. Ratio of magnetic moments: $\frac{{\mu }^{155}}{{\mu }^{157}}=0.7495\mathrm{}\pm 0.0045$.

Irradiation of crystals at the Harwell pile with a thermal neutron flux of 1.2×${10}^{12}$ n/${\mathrm{cm}}^2$ sec, and a fast flux of 2.3×${10}^{11}$ n/${\mathrm{cm}}^2$ sec, yields the ratio of nuclear capture cross sections $\frac{{\sigma }^{157}}{{\sigma }^{155}}=2.82\mathrm{}$, using the above abundance values.

No $F$-center spectrum was detected in crystals irradiated with a total neutron flux of about ${10}^{18}$ n/${\mathrm{cm}}^2$. The paramagnetic resonance spectrum of gadolinium in irradiated crystals is unchanged, indicating negligible radiation damage in the neighborhood of the paramagnetic ions.

• Received 20 November 1958

DOI:https://doi.org/10.1103/PhysRev.115.424