Two-photon laser isotope separation of atomic uranium: spectroscopic studies, excited-state lifetimes, and photoionization cross sections

Experimental studies are described wherein an atomic beam of uranium vapor produced by electron-beam evaporation is selectively excited and ionized by light from two short-pulse narrow-bandwidth, tuned dye lasers, and detected and analyzed by a mass spectrometer. The total number of ions per pulse produced is small; however, the time of production is known precisely. By counting single ions, using digital logic, and multiplexing the mass spectrometer between 238U and 235U, it is possible to measure isotope ratios as a function of exciter wavelength and to correct for background effects and spurious ions. These results demonstrated 50 percent enrichment of 235U/238U. Excited-state lifetimes were measured by observing yields as a function of the delay between the two laser pulses. In addition, for an excitation wavelength of 4266.325 A, the variation of two-step photoionization efficiency was measured as a function of the wavelength of the ionizing laser. The maximum yield at an ionizing wavelength of 3609 A corresponds to a cross section of 2 x 10-17 cm2 for matched linewidths. Also the ionization potential of uranium was determined to be 6.187 +- 0.002 eV. 11 fig