MSA PRESIDENTIAL ADDRESS: THE METAMICT STATE - POEMS AND PLUTONIUM

The metamict state, first defined over 100 years ago, is a crystalline material that has become amorphous. By 1914, the metamict state was understood to be the result of a radiation-induced transformation from a periodic-to-aperiodic structure caused by the decay of uranium, thorium and their daughter nuclides. Although initially considered a mineralogical curiosity, metamict minerals have provided the basis for systematic studies of particle-solid interactions in complex structures. The alpha-decay event from actinides provides two energetic projectiles (an alpha-particle with an energy of 4.5 MeV and a recoil nucleus with an energy of 70-90 keV) that transfer energy by electronic and ballistic interactions with the target atoms of the lattice. A single alpha-decay event can displace several thousand atoms in a volume of several nanometers “on megavolt missions of chance destruction.” Although the collision cascade lasts only a few tens of femtoseconds, “cannon balls awry through through a crowded dance floor”, the damage microstructure can persist for hundreds of millions of years. Well-controlled studies of the damage and annealing processes are now performed by ion-beam irradiations that can both probe and modify the properties of materials. A special aspect of this research has been the investigation of metamict minerals combined with studies of actinide-doped phases and ion beam irradiations. The dose rates (atomic displacements/second) vary over 15 orders of magnitude for these three types of experiments. A variety of analytical techniques must be used in order to investigate damage structures over length-scales that vary by 6 orders of magnitude. The most important result has been to develop an understanding of the structural controls on radiation damage, and this has led to the design of radiation-resistant materials. A notable example has been the development of radiation-resistant, chemically durable phases for the disposition of plutonium from dismantled nuclear weapons. Quotes from “The Metamict State” poems by Roald Hoffmann.