Accurate determination of uranium isotope abundances by wavelength modulation spectroscopy in atomic beams

The design and demonstration of an optical analysis system based on wavelength modulation spectroscopy in an atomic beam for uranium isotope abundance determinations is presented. This system probes the uranium 5f 3 6d7s 2 ( 5 L 6 ) → 5f 2 6d 2 7s 2 ( 5 K 5 ) transition at 861.031 nm, which is considered to be the most suitable transition for uranium isotopic analysis . A new laser characterization strategy was developed for the conditions where optimum laser wavelength modulation depth was small compared to the free spectral range (FSR) of etalons . Two capabilities enabled the higher-precision determination of isotope abundances of atomic beams: (1) reduction of low-frequency additive noise , especially the noise caused by black-body radiation and (2) suppression of non-absorption transmission losses. The performance of this system was validated with uranium samples of various isotopic compositions . By comparing the measurements using natural uranium samples between the direct absorption and the wavelength modulation approaches, a 21-fold decrease in uncertainty of the integrated absorbance and a 6.8-fold improvement in the 1-σ precision of the number density were achieved. In addition, by comparing the results using uranium oxide samples, a 6.1-fold decrease in the uncertainty of inferred isotope abundance was obtained. These results demonstrate that the 1f-normalized 2f wavelength modulation spectroscopy (WMS-2f/1f) technique enables higher-precision analysis of atomic beams. • Wavelength modulation spectroscopy (WMS) is employed to the isotopic analysis of atomic beams of uranium. • WMS allows the reduction of low-frequency of additive noise and the supression of non-absorptive transmission losses. • A 21-fold decrease in uncertainty of uranium absorption measurements was demonstrated.