Investigation of ion-track morphology and annealing behavior using small-angle X-ray scattering

Abstract

When heavy ions with energies in the range of hundreds of MeV to GeV penetrate a solid, they lose their energy through inelastic interactions with the target electrons and can leave narrow cylindrical trails of permanent damage along their path, known as ion tracks. Ion tracks are typically a few nanometers in diameter and can be up to tens of micrometers long. When these tracks are annealed at elevated temperatures, they shrink in size and eventually the damage inside the material can recover. In this project ion tracks are studied in Durango apatite, San Carlos olive and synthetic quartz. In minerals such as apatite track formation can result from spontaneous fission of naturally occurring uranium inclusions that produces high energetic fragments. These so called “fission tracks” are used for dating and constraining the thermal history of geological samples. The current dating methods, however, utilize chemical etching, which destroys the primary damage such that essential information on the actual scale of the underlying radiation damage is irrevocably lost. A detailed understanding of the un-etched track damage in minerals and its dependence on geologically relevant conditions is of fundamental importance for the application of etched tracks in geo- and thermochronology. Tracks in olivine are used for identification of cosmic rays in meteorites. Many meteorites contain different amounts of olivine which is a crystalline mineral susceptible to ion track formation from high energetic cosmic particles. Studying the annealing behavior of ion tracks in meteorites can lead to estimation of the temperature of the mineral during track formation. In quartz, swift heavy ion irradiation leads to a change in the refractive index of the material inside the narrow tracks and provides new possibilities for fabrication and micromachining of optical devices. Annealing of ion tracks in quartz is of interest as for example device fabrication processes often involve elevated temperatures. In this work synchrotron based small angle x-ray scattering in combination with in situ and ex situ annealing experiments is used to study the morphology and damage recovery of un-etched ion tracks. It is demonstrated that SAXS is a powerful tool for studying ion track damage in a variety of materials as it is sensitive to small density changes at the nanometer scale that often occur in the damaged regions. It is a non-destructive technique and can be used to determine changes in the track radii with sub-nanometer precision. Short acquisition times make it well suited for studying track annealing kinetics in situ. The work presented in this thesis has aided in developing the possibilities that small angle x-ray scattering can provide for studying the morphology and annealing behavior of nano sized damage structures. The high accuracy with which the track radii can be determined using SAXS, the nondestructive, artifact-free measurement methodology, as well as the data analysis models introduced in this work provide an effective means for in-depth studies of ion-track morphology and annealing behavior in a variety of materials.