PNNL

The Science

Understanding the initial reactive species in aqueous systems after exposure to ionizing radiation is an important aspect of predicting long-term radiation damage.  A team of researchers from the Interfacial Dynamics in Radioactive Environments and Materials (IDREAM) Energy Frontier Research Center, a multi-institution partnership led by Pacific Northwest National Laboratory (PNNL), reviewed current and future methods that capture the origins of these reactive species, and explored how new experimental methods using X-ray free electron lasers (XFELs) can be used to provide insight into previously unobserved processes.

The Impact

Pinpointing the origin of reactive species, as well as their respective reaction pathways, helps describe their molecular decomposition, or radiolysis, in both simple and complex systems. This review outlines methods using ultrafast X-ray pulses from XFELs and photon-in/photon-out spectroscopies to track the reactions after ionizing radiation events in aqueous systems at the earliest stages. By linking experiments with coordinated theory, IDREAM contributes to future methodological improvements that will help us better understand radiation-induced reaction pathways in complex environments. This knowledge can ultimately be used to support safer and more affordable nuclear waste processing in Department of Energy (DOE) sites like Hanford.

Summary

Previously, reaction pathways and products have been studied by techniques targeting the chemical stage, which can be used to infer the identity and timescales of the initial species without directly observing the mechanism. With the availability of XFELs, with their intense and tunable X-ray pulses, examination of processes occurring during the sub-picosecond timescale domain—the so-called physicochemical stage—is now possible.

IDREAM researchers reviewed the state of photon-in/photon-out XFEL studies in the sub-picosecond regime targeting the origin of reactive species after samples are subjected to ionizing radiation. Leveraging earlier work using optical pump/X-ray probe studies of radiolysis in pure water, they plan to use tunable X-ray pump/X-ray probe experiments to directly probe species of interest after ionization. This new technique will allow for comprehensive analysis of radiation effects at faster timescales and with better resolution, including in complex systems containing hydroxide and aluminum ions.

In establishing the concept of using XFELs to perform ultrafast photon-in/photon-out studies to probe physicochemical stages, IDREAM can now pursue this technology to perform first-time studies at the DOE Office of Science’s Linac Coherent Light Source user facility, located at the Stanford Linear Accelerator Center. Development of these spectroscopies to target the physiochemical stage will enable further examination of radiation-induced processes in complex systems, like highly alkaline concentrated salt solutions representative of high-level nuclear waste.

Contact

Carolyn Pearce
Pacific Northwest National Laboratory
carolyn.pearce@pnnl.gov

Funding

This research was supported by IDREAM, an Energy Frontier Research Center funded by DOE Office of Science, Basic Energy Sciences Program. Non-IDREAM researchers were supported by the DOE Office of Science, Basic Energy Sciences Program, Chemical Sciences, Geosciences, and Biosciences Division.