Schauries, Daniel
Description
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...[Show more] 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.
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