Quantifying Uranium Isotope Ratios Using Resonance Ionization Mass Spectrometry: The Influence of Laser Parameters on Relative Ionization Probability

Abstract

Resonance Ionization Mass Spectrometry (RIMS) has been developed as a method to measure relative uranium isotope abundances. In this approach, RIMS is used as an elementselective ionization process to provide a distinction between uranium atoms and potential isobars without the aid of chemical purification and separation. We explore the laser parameters critical to the ionization process and their effects on the measured isotope ratio. Specifically, the use of broad bandwidth lasers with automated feedback control of wavelength was applied to the measurement of 235 U/ 238 U ratios to decrease laser-induced isotopic fractionation. By broadening the bandwidth of the first laser in a 3-color, 3-photon ionization process from a bandwidth of 1.8 GHz to about 10 GHz , the variation in sequential relative isotope abundance measurements decreased from >10% to less than 0.5%. This procedure was demonstrated for the direct interrogation of uranium oxide targets with essentially no sample preparation. A rate equation model for predicting the relative ionization probability has been developed to study the effect of variation in laser parameters on the measured isotope ratio. This work demonstrates that RIMS can be used for the robust measurement of uranium isotope ratios. Professor Stanley G. Prussin Dissertation Committee Chair iThis dissertation is dedicated to all those who have invested in me, especially my wife and son who have given the most.ii

Rate equation model of laser induced bias in uranium isotope ratios measured by resonance ionization mass spectrometry

“…This approach of calculating ionization probabilities for discrete laser wavelengths, representing individual laser pulses, and then averaging the results based on the variation expected during experiments provides a greatly improved in contrast to the results reported in Isselhardt, 2011. 10 Table 3 gives the range of laser wavelengths, laser pulse energies, and bandwidths studied and also includes the measured values for the pulse-to-pulse uctuations in wavelength and pulse timing of the lasers.…”

“…Recent advancements in the application of RIMS to quantify uranium isotope ratios in nuclear materials have demonstrated the use of broad bandwidth ($10 GHz) laser beams to mitigate the inuence of the isotope shi between atomic levels in 235 U and 238 U. [10][11][12][13] While not completely eliminating the inuence of laser-induced isotope fractionation during ionization, this approach decreases the requirements of laser system performance necessary for reproducible measurement of uranium isotope abundances. The required laser beam spectral distributions and irradiance are achievable practically, allowing uranium and plutonium isotope ratios of unknown materials to be quantied by comparison with standards to account for isotopic fractionation in the measurement system.…”

“…We have developed a rate-equation model for calculating the relative ionization probability of 234 U, 235 U, and 238 U under the conditions of resonance ionization used in our work [10][11][12][13]15 in order to predict the inuence of variations in laser beam parameters on the measured 234 U/ 238 U and 235 U/ 238 U isotope ratios, and thus, to better understand and quantify the sources of laser-induced fractionation. There are two potential types of isotopic fractionation in RIMS.…”

Systems considerations for Resonance-Ionization Mass Spectroscopy in the application of Nuclear Forensic Analysis by Ray Mariella Jr

“…If either of those beams struck the surface, directly, it would likely cause further ablation of the target, which could confound the step of timed removal of undesired ions, could cause the loss of some remaining atoms of interest from the main detection process, and could introduce offnormal or difficult-to-characterize signals. Choosing the areas and shapes of the exciting and ionizing laser beams above the surface is part of the process of optimization, based on the expansion of the ablated/sputtered material and the dimensions and throughput of the ion optics and mass spectrometer 10 [see p70 of B. Isselhardt thesis]. Immediately at the surface, the nascent cloud of ablated/sputtered matter has the density of the solid target [n ≈ 2.3 X 10 22 molecules/cm 3 for UO 2 , n ≈ 2.5 X 10 22 molecules/cm 3 for SiO 2 ] and is a location in which collisions between atoms of interest and all other constituents can be frequent, [mean free path ≈ 1/nσ, where σ is the collision cross section, ≈ 6 X 10 -15 cm 2 , gives m.f.p.…”

“…The presence of a resonant, autoionizing state significantly reduces the power requirements for the photoionization laser and the reduced power reduces the very undesirable, non-specific 19 photoionization. The kinetics of the coupled steps has been discussed by B. Isselhardt in his thesis 10 , but, in short, if the rate of photoionization exceeds the duration of the excitation and photoionization pulses, and the Rabi frequency of the excitation process between the ground state and the upper excited state is rapid enough to replenish the upper state population that is depleted by the photoionization, then most, if not all, of the ground-state atoms that are illuminated by the laser beams can be ionized for extraction into the mass spectrometer.…”