Superconducting energy gap of thorium determined by electron tunneling

Electron-tunneling measurements have been performed over the temperature range 0.4-1.4 K on Th-ThO₂-Au tunnel junctions fabricated on the surface of polycrystalline thorium substrates. Both the temperature dependence of the energy gap and the voltage dependence of the tunneling density of states agree rather well with the theoretical predictions for a weak-coupling superconductor. No significant phonon-induced structure was observed in the tunneling density of states.