Filament-induced breakdown spectroscopy (FIBS) is an attractive remote detection approach that is potentially applicable to nuclear materials such as uranium (U). Simultaneous detection of atomic U and U monoxide features is demonstrated in a single laser shot with detection probabilities of 52.2% and 39.0%, respectively, and with a 1% false-alarm probability. It is shown that a nearly 100% detection probability can be reached with 1% false-alarm probability in $\lesssim 1$ second when using an 80 Hz laser and data acquisition system. Atomic U and uranium oxide (UO) signal features are identifiable in the remote measurement with comparable signal-to-background ratios, suggesting that, despite being broader than the U I emission line, the UO band is also suitable for remote detection of U by FIBS.

Spatial segregation of species presents one of the main challenges in quantitative spectroscopy of laser-produced plasmas, as it may lead to overestimation of the concentration of the heavier species. Analytical capabilities can also be affected by excessive Stark broadening at atmospheric pressure, hindering the ability to spectrally resolve closely spaced spectral lines, such as those belonging to isotopes of the same element. We present an experimental and modeling study of the segregation of species and spectral line broadening in D₂O-H₂O plasma produced by single- and double-pulse nanosecond laser ablation in air. The ability to resolve Balmer spectral lines of hydrogen and deuterium is investigated by considering the effects of plume segregation. Transient plasma properties which lead to improvements in spectral line separation are discussed. While the plume segregation is found to be negligible in air regardless of the ablation scheme used, we observe a significant improvement in the separation of isotopic spectral lines by employing the double-pulse excitation. This study may lead to increased reliability of optical emission spectroscopy in deuterium-rich plasma environments and suggests the potential for sensitive detection of tritium in air via laser-induced breakdown spectroscopy.

Optical measurement techniques can address certain important challenges associated with nuclear safety and security. Detection of uranium over long distances presents one such challenge that is difficult to realize with traditional ionizing radiation detection, but may benefit from the use of techniques based on intense femtosecond laser pulses. When a high-power laser pulse propagates in air, it experiences collapse and confinement into filaments over an extended distance even without external focusing. In our experiments, we varied the initial pulse chirp to optimize the emission signal from the laser-produced uranium plasma at an extended distance. While the ablation efficiency of filaments formed by self-focusing is known to be significantly lower when compared to filaments produced by external focusing, we show that filaments formed by self-focusing can still generate luminous spectroscopic signatures of uranium detectable within seconds over a 10-m range. The intensity of uranium emission varies periodically with laser chirp, which is attributed to the interplay among self-focusing, defocusing, and multi-filament fragmentation along the beam propagation axis. Grouping of multi-filaments incident on target is found to be correlated with the uranium emission intensity. The results show promise towards long-range detection, advancing the diagnostics and analytical capabilities in ultrafast laser-based spectroscopy of high-Z elements.

We perform optical emission spectroscopy of ultrafast laser filament-produced air plasmas in the multiple filament regime at driving wavelengths of 400 nm and 800 nm. The spatiotemporal structure of the emission from the plasmas are observed and the emission spectra are used to estimate plasma temperature and density for a range of laser parameters. Plasma temperatures are determined from the molecular nitrogen fluorescence, while the electron densities are estimated from Stark broadening of the oxygen-I 777.19-nm line. Electron temperatures are determined to be in the range of 5000–5200 K and they do not vary significantly along the length of the filament, nor are they sensitive to incident laser energy or wavelength. Electron densities are on order of 10¹⁶ cm⁻³ and show a greater variation with axial position, laser energy, and laser wavelength. We discuss mechanisms responsible for spatial localization of emitting species within the filament. Optical emission spectroscopy offers a simple, non-perturbing method to measure filament properties, that allows the information on the associated molecular transitions and excitation/ionization mechanisms to be extracted.

Uranyl fluoride (UO₂F₂) is a compound which forms in the reaction between water and uranium hexafluoride, a uranium containing gas widely used for uranium enrichment. Uranyl fluoride exhibits negligible natural background in atmosphere; as a result, its observation implies the presence and active operation of nearby enrichment facilities and could be used as a tracer for treaty verification technologies. Additionally, detection of UO₂F₂ has a potential application in guiding remediation efforts around enrichment facilities. Laser-induced fluorescence (LIF) has been proposed in the past as a viable technique for the detection and tracking of UO₂F₂. We demonstrate that ultrafast laser filamentation coupled with LIF extends the capabilities of standard LIF to enable remote detection of UO₂F₂. An intense femtosecond laser pulse propagated in air collapses into a plasma channel, referred to as a laser filament, allowing for the extended delivery of laser energy. We first investigate the luminescence of UO₂F₂ excited by the second harmonic of an ultrafast Ti:sapphire laser and subsequently excite it using the conical emission that accompanies ultrafast laser filamentation in air. We measure the decay rates spanning 4.3–5.6 × 10⁴ s⁻¹ and discuss the characteristics of the luminescence for both ultrafast- and filament-excitation. Larger decay rates than those observed using standard LIF are caused by a saturated component of prompt decay from annihilation of dense excited states upon excitation with an ultrafast source. The reproducibility of such decay rates for the given range of incident laser intensities 1.0–1.6 × 10¹¹ W cm⁻² is promising for the application of this technique in remote sensing.