Controlling subsurface radiation tolerance in swift heavy ion irradiated ceramics

Swift heavy ion irradiation induces complex, depth-dependent damage in nuclear ceramics, challenging their radiation tolerance. Using Brillouin spectroscopy and optical reflectometry, we resolve micrometer-scale elastic, photoelastic, and strain profiles in MgAl2O4 spinel irradiated with 710 MeV Bi ions at fluences of 6 × (1010–1012) ions/cm2. We discover a subsurface reversal of radiation damage characterized by a swelling-to-compression transition: while near-surface elastic (C11, C22, C44) and photoelastic (P12, P21) constants decrease with fluence, they recover at deeper regions following the tapering of ion tracks and reduction of electronic stopping power (Se). At the highest fluence, overlapping tapered tracks form a composite of acoustically mismatched nano-crystalline and nano-amorphous phases, evidenced by the emergence of confined GHz longitudinal acoustic modes. This study reveals a self-healing densification mechanism driven by tapering track morphology and electronic stopping power which provides a new strategy for engineering depth-resolved tolerance in ceramics to extreme radiation.