Within the field of high-explosives (HE) science, the need to better understand HE chemistry drives research into interactions involving isotope substitution and other species of interest (e.g. tritium or uranium). This feasibility study was designed to investigate the potential of using laser ablation and optical emission spectroscopy to understand the high-temperature chemistry of an HE analogue with deuterium gas and, separately, with depleted uranium. The combination of laser ablation with optical emission spectroscopy is ideally suited for studying these interactions for two reasons: (1) emission spectra are highly sensitive to isotope substitution, making it easy to distinguish between chemical reactions involving 1H or 2H (deuterium), and (2) laser ablation emission spectroscopy is a rapid and cost-effective platform for studying the chemistry of HE.
In order to accurately interpret the early chemistry of high-explosive detonations, scientists require an improved understanding of the chemistry between dissociated HE products and relevant species, such as tritium or uranium. One current topic of interest is in developing validated models of HE chemistry to understand how detonation products form. While tests using actual HE detonations provide model validation and valuable insight into these chemical processes, HE tests can often be technically challenging and expensive to execute. Laser ablation emission spectroscopy (see schematic below) provides a simple and cost-effective means by which to understand high-temperature chemistry and develop emission signatures relevant to actual HE detonations.
Laser ablation produces an initial plasma in excess of 10,000 K (~1 eV) (De Giacomo and Hermann 2017). In the condensing plasma, ions, atoms, and molecular species are initially in an excited state. The molecular species subsequently undergo rovibronic (rotational-vibrational-electronic) de-excitation. One can easily measure light from this de-excitation process using a dispersive grating and a detector to acquire characteristic spectra of these species (Bol'shakov et al. 2016, De Giacomo and Hermann 2017). Molecular emission spectroscopy has been shown to be effective for studying chemistry subsequent to the laser ablation of chemical compounds or pure elements reacting with surrounding materials (Harilal et al. 2016, Weisz et al. 2018, Serrano et al. 2015). It is also beginning to be applied to the chemistry of HE detonations (Glumac, N. 2005, 2017, Gottfried 2017, Kalam et al. 2017).
Prior to this study, we developed the laser ablation and emission spectroscopy technique to understand chemistry relevant to post-detonation nuclear forensics for the Defense Threat Reduction Agency (Finko et al. 2017, Koroglu et al. 2017, 2018, Weisz et al. 2017, 2018). This capability was used to observe the formation of a previously reported uranium oxide subsequent to laser ablation (Mao et al. 2017, Hartig et al. 2017), and we were able to confirm that this species was uranium monoxide (UO) by measuring the spectral shift in an 18O and 16O isotopic substitution study (Weisz et al. 2017). We have also used the laser ablation and emission spectroscopy technique to calibrate a simple kinetics model for oxide formation in a Sr, Zr, and O system, subsequent to the laser ablation of a SrZrO3 target. Further, we were able to deduce the timescales of chemistry between oxygen from the sample and oxygen from the surrounding atmosphere, again using the spectral shift in an 18O isotopic substitution experiment (Weisz et al. 2018). We are concurrently developing time-resolved IR spectroscopy and laser-induced fluorescence to track more complex molecular formation to continue applying the laser ablation technique to the study of post-detonation chemistry.
In order to understand the fundamental effects of low and high atomic number species (e.g. H and depleted U) on HE chemistry and relevant spectral interpretations, we proposed and accomplished the following research objectives:
Objective 1: To develop a signature of isotopic substitution with 2H with HE-derived species (or a relevant surrogate) using laser ablation and emission spectroscopy in a deuterium-rich environment.
Objective 2: Develop a preliminary dataset illustrating the influence of uranium on the formation of diatomic HE-relevant intermediates by spectrally monitoring the formation of UO.
This study expands the field of HE science and supports the NNSA mission in stockpile stewardship.
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