Ultrafast Laser Filament-induced Fluorescence Spectroscopy of Uranyl Fluoride

Abstract

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.

Progress in Uranium Chemistry: Driving Advances in Front-End Nuclear Fuel Cycle Forensics

“…Last, Skrodzki et al considered coupling laser-induced fluorescence (LIF) spectroscopy to ultrafast laser filamentation to enable remote detection of UO 2 F 2 . 75 Filamentation has been shown to enable energetic femtosecond laser pulses to propagate up to 1 km. Their study investigated the excitation of UO 2 F 2 (0.05 M UO 2 F 2 in 0.05 M HF/0.05 M KF) using an ultrafast laser to determine its fluorescence signature under such excitation conditions.…”

“…Last, Skrodzki et al considered coupling laser-induced fluorescence (LIF) spectroscopy to ultrafast laser filamentation to enable remote detection of UO 2 F 2 . Filamentation has been shown to enable energetic femtosecond laser pulses to propagate up to 1 km.…”

“…Other research has focused on understanding the degradation of uranium compounds, particularly under environmental conditions. − These efforts are important for nuclear forensics investigations of aged materials or samples exposed to the environment. For instance, various UOCs stored in controlled relative humidity (RH) and temperature conditions have undergone chemical reactions, converting them to different uranium compounds. ,− One final focus area has been developing improved or new analytical techniques for probing various key uranium compounds. − For example, laser-induced breakdown spectroscopy (LIBS), which can be packaged in a field-deployable form factor, has been studied as a possible technique to rapidly identify various uranium oxides and UO 2 F 2 . , Overall, the significance of the recent research and the growing importance of nuclear forensics is evident in the proposal and establishment of Nuclear Proliferomics, a field of study focused on building comprehensive databases of nuclear materials’ properties that could be leveraged through various data analytics and machine-learning techniques to advance nuclear forensics analyses …”

Challenges and Opportunities in Scientific and Technological Support for Monitoring in the Non-proliferation Regime

“…In addition to drones or mobile platforms, new detection and analyses techniques have also been advancing including especially laser spectroscopy. 3 Promising work has been done to accurately detect small amounts of Uranyl Fluoride using ultrafast laser filament-induced fluorescence spectroscopy, but not yet solving the cost issue. Techniques like this could become less expensive but also could be selectively used if triggered by mobile platforms, satellite imaging, or civil society reporting.…”

“…Schoppner et al[2] 3. Skrodzki et al[3] 4. Passell, H. D. et al, "The Navurz Project: Cooperative, transboundary monitoring, data sharing and modeling of water resources in Central Asia" SAND2006-6673 5.…”