Gamma decoration at FRM II: recent optimisations and parametric studies for microstructural investigations of halite

Microstructural investigations of halite are essential for understanding deformation mechanisms relevant to salt tectonics and underground storage applications, including radioactive waste disposal and salt caverns. However, the identification of subgrain boundaries, dislocation structures, acting creep mechanisms and fluid-related features remains challenging due to the optical transparency and inherently low defect contrast of halite. Gamma decoration provides a powerful solution by inducing radiation-related colour centers that selectively highlight lattice defects and deformation structures.

At the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II, TUM), gamma decoration has been implemented since over a decade, recently we re-established and systematically optimized it using older spent fuel elements characterized by comparatively low dose rates. This contribution focuses on methodological developments and parametric studies that enable reliable gamma decoration under these conditions, extending the applicability of the technique beyond high-dose irradiation facilities.

We present results from controlled irradiation experiments on halite thin sections covering a wide range of total doses, irradiation times, and temperatures, combined with post-irradiation optical microscopy, spectroscopy, and digital colorimetry to quantify and optimize suitable optical contrast. Our experimental results from long-term irradiations are compared with theoretical models describing dose-rate-dependent radiation effects on defect formation in natural rock salt. This parametric approach allows identification of threshold conditions required for effective defect visualization, as well as optimization strategies to compensate for reduced dose rates, including extended irradiation times and temperature control.

These results establish gamma decoration at FRM II as a robust and versatile experimental method for salt-rock research, providing a valuable link between laboratory testing, microstructural analysis, and mechanical modelling, and ensuring continued applicability of this technique with ageing irradiation infrastructure.