Ultrafast laser filament-induced fluorescence for detecting uranium stress in Chlamydomonas reinhardtii.

Plants and other photosynthetic organisms have been suggested as potential pervasive biosensors for nuclear nonproliferation monitoring. We demonstrate that ultrafast laser filament-induced fluorescence of chlorophyll in the green alga Chlamydomonas reinhardtii is a promising method for remote, in-field detection of stress from exposure to nuclear materials. This method holds an advantage over broad-area surveillance, such as solar-induced fluorescence monitoring, when targeting excitation of a specific plant would improve the detectability, for example when local biota density is low. After exposing C. reinhardtii to uranium, we find that the concentration of chlorophyll a, chlorophyll fluorescence lifetime, and carotenoid content increase. The increased fluorescence lifetime signifies a decrease in non-photochemical quenching. The simultaneous increase in carotenoid content implies oxidative stress, further confirmed by the production of radical oxygen species evidence in the steady-state absorption spectrum. This is potentially a unique signature of uranium, as previous work finds that heavy metal stress generally increases non-photochemical quenching. We identify the temporal profile of the chlorophyll fluorescence to be a distinguishing feature between uranium-exposed and unexposed algae. Discrimination of uranium-exposed samples is possible at a distance of [Formula: see text]35 m with a single laser shot and a modest collection system, as determined through a combination of experiment and simulation of distance-scaled uncertainty in discriminating the temporal profiles. Illustrating the potential for remote detection, detection over 125 m would require 100 laser shots, commensurate with the detection time on the order of 1 s.

The effect of oxygen concentration on the speciation of laser ablated uranium.

In order to model the fate and transport of particles following a nuclear explosion, there must first be an understanding of individual physical and chemical processes that affect particle formation. One interaction pertinent to fireball chemistry and resultant debris formation is that between uranium and oxygen. In this study, we use laser ablation of uranium metal in different concentrations of oxygen gas, either 16O2 or 18O2, to determine the influence of oxygen on rapidly cooling uranium. Analysis of recovered particulates using infrared absorption and Raman spectroscopies indicate that the micrometer-sized particulates are predominantly amorphous UOx (am-UOx, where 3 ≤ x ≤ 4) and UO2 after ablation in 1 atm of pure O2 and a 1% O2/Ar mixture, respectively. Energy dispersive X-ray spectroscopy (EDS) of particulates formed in pure O2 suggest an O/U ratio of ~ 3.7, consistent with the vibrational spectroscopy analysis. Both am-UOx and UO2 particulates convert to α-U3O8 when heated. Lastly, experiments performed in 18O2 environments show the formation of 18O-substituted uranium oxides; vibrational frequencies for am-U18Ox are reported for the first time. When compared to literature, this work shows that cooling timescales can affect the structural composition of uranium oxides (i.e., crystalline vs. amorphous). This indicator can be used in current models of nuclear explosions to improve our predicative capabilities of chemical speciation.

Millisecond-long suppression of spectroscopic optical signals using laser filamentation.

Ultrashort laser pulse filamentation in air can extend the delivery of focused laser energy to distances greatly exceeding the Rayleigh length. In this way, remote measurements can be conducted using many standard methods of analytical spectroscopy. The performance of spectroscopic techniques can be enhanced by temporal gating, which rejects the unwanted noise and background. In the present work, we investigate the thermal relaxation of air in the wake of single-filament plasmas using shadowgraphy. We demonstrate that the transient change in refractive index associated with relaxation of the gas can be used to reject both continuous and time-varying spectroscopic signals, including emission from laser-produced plasmas. This method can augment temporal gating of simple optical detectors.