Effects of Atmospheric Conditions on Optical Signatures of Cerium Laser-produced Plasmas

To detect undeclared nuclear fuel cycle activities and monitor nuclear weapons tests, optical detection methods targeting electromagnetic radiation signatures can be used. Optical spectroscopy techniques such as laser-induced breakdown spectroscopy (LIBS) offer robust, field-deployable analytical methods to detect and characterize multi-elemental samples at standoff distances. Using laser ablation (LA) coupled with spectroscopic techniques, these methods generate an optical spectrum of characteristic atomic, ionic, and molecular transition lines that serve as fingerprints for the irradiated sample. Laser-produced plasmas (LPPs) generated during the LA process are known to be representative of the highly complex and sensitive spatio-temporal conditions of an explosive fireball following a chemical detonation. Additionally, LPPs have commonly been used as surrogates for uranium nuclear fireballs, where a significant effort has been conducted in the literature over the past decade to study the complex plasma properties and environmental effects driving the evolution of laser-produced uranium plasmas and their associated optical signatures. It has repeatedly been shown that the plasma response to reactive species such as oxygen plays a significant role in the formation of molecular species in the fireball, which can considerably impact particle formation and debris distribution following a detonation. However, equivalent investigations into plutonium nuclear fireballs using LPPs are severely limited despite marked differences in chemistry between the elements. Therefore, to improve capabilities in the wide area environmental sampling of nuclear materials, optical signatures of plutonium must be developed by studying the fundamental physics and high-temperature gas-phase chemistry of plutonium LPPs. This work will present the start of a campaign into the characterization of optical plutonium signatures using cerium laser-produced plasma surrogates. Experimental techniques using LIBS and time-resolved fast-gated imaging will be applied to measure the breakdown mechanisms of cerium metal as well as the oxidation chemistry and expansion dynamics of cerium LA plasma plumes. Cerium LPPs will be generated under inert and ambient atmospheric conditions to study the effects of environmental factors on cerium optical signatures.