When radioactive atoms inside minerals decay, they fire out high energy particles, that leave trails of destruction through the mineral. Weirdly, researchers have found, it turns out these trails can have quite unexpected cross sectional shapes.

The new knowledge will help with understanding ancient geological events, and also give insights into choosing materials for safer storage of radioactive waste. 

It’s the first time that measurements have been made with high enough resolution to reveal the shape of the tracks. To their surprise the researchers found previous assumptions were wrong - tracks did not always have round cross-sections, instead showing up as elliptical.

The effect stems from the way crystal structure interacts with the particles, explained Ms Jessica Wierbik, PhD student in the Department of Materials Physics.

“The host material is important – people have previously focused only on how the irradiation parameters affect the tracks.”

“The crystals are anisotropic, so the track shapes are too,” she said.

Ms Wierbik’s background in crystallography gave her the perfect skills to unravel the puzzle and understand the link between track shapes and crystal orientation.

She is the lead author on the paper reporting the work, in Physical Review B. The work was featured by the journal as a Physics Synopsis.

“We were seeing strange streaks in the scattering images, the sizes were jumping around, and we couldn’t explain it,” said leader of the research group, Professor Patrick Kluth.

“I started wondering what exactly we were measuring, and decided to measure a couple of different angles – and then we saw this cool correlation. But we didn’t understand what it meant.”

The high-resolution study was done using small-angle x-ray scattering (SAXS) at the Australian Synchrotron, with a specially adapted sample stage.

“The crystals have to be carefully aligned so they can be rotated around their crystallographic axes,” Ms Wierbik said.

“It’s a very challenging measurement to make!” she said.

First, the team created ion tracks in crystals by bombarding them with 185 MeV gold ions, using the Heavy Ion Accelerator Facility at ANU.

The damage tracks created by these ions were then studied with the SAXS set up, and their shape showed a clear dependency on the alignment between the track and the crystal axes.

The first measurements were made with apatite, a mineral which often contains uranium and thorium, radioactive elements that create natural damage trails. Apatite has a hexagonal crystal structure, and its properties are anisotropic: they vary with crystallographic direction.

As expected, the damage cross sections were round when the tracks were aligned parallel to the principle crystallographic direction, or c-axis. But perpendicular to that, the track cross-sections were elliptical, flattened in the c-axis direction.

Similar results were found with a-quartz crystals. Ms Wierbik’s theory was that the effect was due to the different stiffness of the lattice in different directions – both crystals have a higher Young’s Modulus in the c-axis.

To test her hunch, she performed the same experiment with tourmaline, a trigonal crystal that also has anisotropic properties. However, unlike the other two crystals, the stiffness properties are reversed: the direction along the c-axis has a lower Young’s modulus.

Sure enough, the tracks were again round when parallel to the c-axis. However, when perpendicular to it, the track cross-sections were anisotropic, exhibiting the opposite behaviour to that observed in apatite and quartz, with tracks widened in the dimension of the c-axis. As she had predicted, the higher the Young’s modulus of the crystal axis, the smaller the radius of the track.

“Jessica’s choice of tourmaline to demonstrate this effect was inspired – when she showed the link to Young’s Modulus it was super exciting,” Professor Kluth said.

The effect results because the colliding ion creates a thermal spike of more than 10,000 K along the track. Lasting only fractions of a nanosecond, this spike cools quickly, leaving a trail of disordered atoms. This amorphous arrangement has a lower density than the ordered crystal, and pushes against the crystal surrounding the track – hence the importance of the stiffness in each direction.

The deeper understanding will help with the choice of minerals in which to encapsulate radioactive waste, to ensure the damage from high energy particles doesn’t break down the storage medium.

The team also hope that this information will help earth scientists: these tiny ion tracks are also created by natural radioactivity in rocks, and can be erased again by subsequent melting. If these new insights can be extended to this geological setting it could help to develop better models of the thermal history in the Earth’s crust.