Study of phonons in irradiated epitaxial thin films of UO$_2$

TL;DR

This study investigates how radiation damage affects phonon spectra in UO$_2$ thin films, revealing unchanged phonon energies but increased linewidths and reduced lifetimes, which imply decreased thermal conductivity.

Contribution

It provides the first direct measurement of phonon spectra in irradiated UO$_2$ thin films, highlighting the impact of radiation on phonon lifetimes and thermal transport.

Findings

Phonon energies remain unchanged after irradiation.

Phonon linewidths increase significantly with irradiation.

Thermal conductivity decreases by approximately 50%.

Abstract

We report experiments to determine the effect of radiation damage on the phonon spectra of the most common nuclear fuel, UO$_2$. We have irradiated thin ($\sim$ 300 nm) epitaxial films of UO$_2$ with 2.1 MeV He$^{2+}$ ions to 0.15 dpa and a lattice swelling of $\Delta$a/a $\sim$ 0.6 %, and then used grazing-incidence inelastic X-ray scattering to measure the phonon spectrum. We succeeded to observe the acoustic modes, both transverse and longitudinal, across the Brillouin zone. The phonon energies, in both the pristine and irradiated samples, are unchanged from those observed in bulk material. On the other hand, the phonon linewidths (inversely proportional to the phonon lifetimes), show a significant broadening when comparing the pristine and irradiated samples. This effect is shown to increase with phonon energy across the Brillouin zone. The decreases in the phonon lifetimes of the acoustic modes are roughly consistent with a 50 % reduction in the thermal conductivity.