Superconductivity of Thorium and Uranium

The low-temperature susceptibility and specific heat of thorium and uranium have been measured. It is found that thorium becomes a superconductor at Tc=(1.374±0.001)^∘{}K, and has a value of Cₑₛ(Tc)γTc=2.42, in good agreement with BCS theory. (Here Cₑₛ is the superconducting electronic specific heat, and γ is the temperature coefficient of the normal electronic specific heat.) The γ and ΘD for thorium were found to be (4.31±{}0.05) mJ/mole deg² and (163.3±{}0.7)^∘{}K, respectively. Both uranium samples appeared to undergo superconducting transitions when observed magnetically, yet both exhibited only normal-state behavior in their specific heat. Hence it seems likely that the apparent superconductivity of alpha uranium is not characteristic of the bulk metal. The γ and ΘD of the purer uranium sample were found to be (10.03±{}0.02) mJ/mole deg² and (207±{}1)^∘{}K, respectively.