EXAFS/XANES Studies of Metamict Materials

Abstract

Radiation damage effects and the kinetics of annealing in crystalline materials are important in the evaluation of the long-term stability of crystalline radioactive waste forms. The selection of a radioactive waste form which will maintain its chemical and physical integrity for periods of 104–106 years requires a thorough understanding of the structural and bonding controls on radiation damage, as well as the thermal conditions under which the alpha, alpha-recoil and fission fragment damage will be annealed. One approach in evaluating these effects is to study the transition from the crystalline to the metamict state in naturally-occurring minerals which contain uranium, thorium and their daughter products. Metamict minerals are a special class of “amorphous” materials which were initially crystalline, but due to damage caused by alpha particles and their recoil nuclei have become “X-ray diffraction amorphous.” Recent simulations of the metamictization process have included a variety of techniques but these simulation experiments generally have not been validated by comparative studies of the properties of irradiated materials with those observed in natural metamict minerals of great age. This paper summarizes the results of EXAFS and XANES spectroscopy on the evaluation of the coordination geometries of Ti [1] and Ca [2] in selected natural metamict minerals.