We summarize a computational and experimental collaboration between the University of Memphis and Argonne National Laboratory to study UF6 interactions on self-assembled monolayer (SAM) surfaces. The program aim was to investigate whether Passivated Implanted Planar Silicon (PIPS) alpha particle detectors could be functionalized to allow direct detection of UF6 enrichment ratios from process streams. It is possible that SAMs on gold could be used as effective passivation layers for PIPS detectors used in UF6 detection, if judicious choice of functional group in the organic layer is made. Fluorinated surfaces (long chain perfluorocarbon SAMs and AlF3) are the best choices of examined surfaces. Beyond the technical determination, much fundamental chemistry is revealed about UF6 reactions with organic surfaces. Computational efforts include quantum chemical calculations to screen potential functional groups, molecular dynamics simulations of UF6 interacting with various SAMs, and alpha spectrometry (Monte Carlo) simulations to establish that sufficient signal ratios could be achieved with functionalized PIPS detectors. Analytical intermolecular potential energy surfaces were developed between UF6 and the best of &gt;40 candidate groups. Experiments required constructing a custom exposure chamber operating from atmosphere to high-vacuum pressures. SAM surfaces are installed within the chamber, and UF6 pressure is controlled above the surface. Surface changes from UF6 exposure are tracked with Reflection-Absorption InfraRed Spectroscopy (RAIRS) and Quartz Crystal Microbalance (QCM). Results of exposure experiments reveal details about the kinetics and mechanism of UF6 reactions with hydrocarbon, fluorocarbon, and amine-functionalized organic surfaces. Functionalized PIPS detectors were also used in studying interactions of UF6 with AlF3 surfaces.

Studies of uranium hexafluoride interactions with organic self-assembled monolayer surfaces in a low-to-high vacuum environment.

ACS Fall 2021

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