First Ionization Potentials of Th, Np, and Pu by Surface Ionization

The surface ionization comparison technique was used to measure the first ionization potentials of Th, Np, and Pu relative to U. The differences in ionization potentials (ΔI.P.) obtained were: ΔI.P.(U–Th) = −1.29 ± 0.26 eV; ΔI.P.(U–Np) = 0.03 ± 0.01; and ΔI.P.(U–Pu) = 0.48 ± 0.02. The I.P. of U had previously been determined in this laboratory to be 6.19 ± 0.06 eV. Based on this value, the I.P. of Th is 7.5 ± 0.3 eV; Np, 6.16 ± 0.06; and Pu, 5.71 ± 0.06.