Anisotropy of the Superconducting Energy Gap in Th

A value of the mean-square anisotropy constant $〈a^2〉$ for the superconducting energy gap of thorium has been determined from the depression of the critical temperature as carbon impurity is added. The results indicate that $〈a^2〉=0.021\mathrm{}$ and that the valence effect for carbon in thorium is negligible. The effect of anisotropy on the critical-field curves of Th-Gd alloys is discussed in detail.