More than 250 spectrum shifts have been measured for lines of the neutral thorium atom (Th I) and of the singly-ionized thorium atom (Th II) in a sample of thorium containing 13 percent Th 230 (ionium). The observed isotope shifts between Th 230 and Th 232 spectrum lines range in magnitude up to 0.94 cm-1. The Th 232 line in every case is to longer wavelength, and hence the Th 230 energy levels are more tightly bound than the corresponding levels of Th 232. A summary is given of isotope shift data.
G. L. STUKENBROEKER and J. R. MCNALLY, JR., "Isotope Shifts in Thorium—Th 230 and Th 232," J. Opt. Soc. Am. 43, 36-41 (1953)