Ion Tracks in Apatite and Quartz and Their Behaviour with Temperature and Pressure

Abstract

Interaction between high energetic particles and matter typically leads to structural damage of the irradiated material. Swift heavy ions predominantly interact with a solid by exciting its electrons. The energy transfer from the electrons to the atoms can lead to the formation of so-called ion tracks. These represent damage regions of cylindrical shape, which surround the entire length of the ion trajectory with a radial size of several nanometres. In materials science, ion tracks are utilised for a wide range of applications, from the detection of radiation to the fabrication of nano-pore filters or nano-electronic devices.

In geology, similar tracks occur naturally in minerals from the spontaneous fission of radioactive impurities. These fission tracks can partially anneal and shrink in length when exposed to elevated temperatures. In this way, the thermal history of a rock can be determined. The length distributions are routinely studied in the mineral apatite with optical microscopy after the tracks have been dissolved and enlarged by chemical etching. Fission tracks within rocks from thousands of metres below the Earth’s surface have inevitably experienced high pressures of several thousand atmospheres. However, pressure is generally not taken into account when studying formation or annealing rates of fission tracks.

The present work shows a detailed investigation of ion tracks in apatite and quartz, specifically a characterisation of their structure, formation, and thermal stability under ambient and high pressure conditions. All tracks were created under controlled conditions by irradiation with ions of energies between 100 MeV to 37.2 GeV in Canberra, Australia, and Darmstadt, Germany. The tracks were subsequently characterised at the Australian Synchrotron in Melbourne through small angle x-ray scattering (SAXS). This allows assessing the track diameter with sub-nanometre precision and without altering their structure.

Combining the ability to create tracks under well-controlled conditions with SAXS characterisation, track formation in high-pressure and high-temperature environments was studied. This includes a temperature range between -250 and 640 deg C and elevated pressure conditions up to 4.4 GPa. The size of the track radii showed a positive correlation with temperature as well as pressure. This work further presents an anisotropy for the track radii along different crystallographic axes and a characterisation of the track’s cross-section and longitudinal shape. In situ SAXS was used to monitor the size of the ion tracks, while these were undergoing thermal annealing. For tracks in quartz, an anisotropic annealing behaviour was found, depending on the direction of the tracks within the crystal lattice. To study thermal track annealing at high pressures, apatite samples were annealed in heatable diamond anvil cells. An increase in annealing rate was demonstrated and attributed to the high-pressure environment. The effects of pressure on track annealing were demonstrated to be negligible when extrapolated to geological values. Thus, the present results confirm the validity of current fission track annealing models. Moreover, the present findings contribute to the field of radiation materials under extreme conditions and the theoretical modelling of such effects.