POLARIZATION AND ISOTOPE SHIFT EFFECTS IN URANIUM ISOTOPE RATIO MEASUREMENTS BY RESONANCE IONIZATION MASS-SPECTROMETRY

Many strongly resonant transitions and their corresponding isotope shifts were identified by two-colour, three-photon, pulsed photoionization spectroscopy and time-of-flight mass spectrometry. Steep wavelength sensitivity of the 235/238 ratio limited the precision of ratio measurements to 4% (1 std. dev.), and was caused by the inability to obtain uniform excitation of the isotopically shifted atomic lines within the bandwidth of the laser, despite the use of the lasers in the broadband mode. In addition, changes in the relative polarization of the lasers caused up to 20% bias in the observed 235/238 ratio, due to the preparation of aligned states in each resonant transition.