Pu-239/Pu-240 isotope ratios determined using high resolution emission spectroscopy in a laser-induced plasma

Laser-induced breakdown spectroscopy (LIBS) has been applied for the determination of plutonium isotope ratios through direct observation of atomic emission from laser-induced plasmas at high resolution. The Pu-239/Pu-240 isotope shift of −0.355 cm−1 from the plutonium atomic line at 594.52202 nm (Blaise et al., The Atomic Spectrum of Plutonium, Argonne National Laboratory Report ANL-83-95, 1984) is clearly resolved in our plasma conditions. Atomic emission is dispersed through a 2-m spectrometer in double pass mode and collected on an electronically gated, intensified charge-coupled device (ICCD) camera. The integrated peak areas obtained from curve-fitting closely match the Pu-239/Pu-240 isotopic ratios obtained from standard methods of thermal ionization mass spectrometry and gamma spectrometry. The observed plutonium linewidths were 0.19 cm−1 (0.0067 nm). These linewidths are within the experimental error of the ideal instrument-limited linewidth, which is calculated to be 0.15 cm−1 (0.0052 nm) based upon the known modulation transfer function for the ICCD system. This linewidth should allow LIBS to be applicable for isotopic ratio measurements for all of the light actinides.