Remote Optical Detection of Uranium and Plant Response to Uranium Exposure

Abstract

Ultrafast laser-based spectroscopy is of interest for nuclear nonproliferation monitoring because it holds promise for in-field, rapid, and remote detection of nuclear materials. One of its features is the ability to elicit atomic and molecular signatures from solids, liquids, and gases. Because optical spectroscopy is not reliant on radioactive decay, both the radioactive and nonradioactive materials can be detected. The existence of atomic and molecular isotope shift can also offer isotopic sensitivity. Lastly, at high peak powers, ultrafast lasers can undergo filamentation, which can enable optical excitation at long distances. One challenge with generation of filaments at high peak powers is the formation of multiple competing filaments that can be seeded by beam aberrations or, in general, its amplitude and phase structure. This work investigates the properties of filaments in the multi-filament regime via non-invasive optical emission spectroscopy (OES). The spatiotemporal molecular transitions and excitation/ionization mechanisms that occur are described. A genetic algorithm (GA) is used to manipulate the beam spatial phase and, therefore, the resulting multi-filament structure such that the filament-induced breakdown spectroscopy (FIBS) signal generation is optimized. FIBS is demonstrated for single-shot detection of uranium atomic and molecular spectral features that could also be suitable for measurements with isotopic discrimination.

While direct detection of uranium is indispensable, there has been recent interest in using plants’ response to their environment as an indirect sensor of nuclear activity. Current techniques used in-field to monitor plant health are either limited in their measurement distance or are significantly inhibited by large solar background. The use of pulsed laser-based methods provides a distinct time structure that accompanies optical signatures and also allows for powerful background rejection when used in conjunction with gated detectors. Filament-induced fluorescence (F-IF) is used for the first time to excite chlorophyll fluorescence (ChlF) in green algae, and the temporal profile is found to be a distinguishing characteristic between healthy and uranium-exposed samples. It is extrapolated from the experimental results that remote discrimination of uranium-exposed algae can be achieved up to 125 m within a ∼1-s measurement time. Ultrashort pulse duration lasers enable resolution of rapid molecular dynamics which influence the fluorescence lifetime. Specifically, two primary nonphotochemical quenching (NPQ) mechanisms that alter the fluorescence lifetime can be interrogated via pump-probe transient absorption spectroscopy. The design, construction, and characterization of the system is discussed, and the predicted observation is presented based on the previous results from F-IF of uranium-exposed algae. This work further strengthens the case for the use laser-based spectroscopy, including the special case of filament-induced breakdown spectroscopy, for nuclear nonproliferation measurements and more broadly in environmental monitoring.