Keeping track of the damage
Scientists resolve long-standing mystery of ion-solid interactions
Silica (silicon dioxide) is an abundant mineral in the earth's crust and a core component in many rocks. It's quite common for such rocks to also contain natural traces of materials like uranium that undergo slow radioactive decay. This radioactivity produces energetic particles that smash through the surrounding silica creating tracks of localized damage in their wake.
The tracks are too small to see directly but because the damage changes the local structure of the material, such tracks serve as a seed point for certain chemical etches. Suitably etched samples show tiny cone shaped pits in the surface that are visible in a powerful light microscope. Geologists have used this etch pit technique for many years to study the density of tracks. Their interest stems from the fact that knowing the number of tracks in a material and the amount of radioactive material present, you can gain information about the age and thermal history of the rocks. High temperature anneals out the damage so a rock with high uranium content and few pits must have been heated in the relatively recent past.
However, it's not just geologists that have an interest in the interaction of energetic ions with solids. An improved knowledge of such interactions is also pivotal to emerging technologies such as nanofabrication, nuclear waste management, fusion power and long distance space travel. The problem to date has been that remarkably little is known about such ion track damage in solids. The traditional etching technique reveals the number of tracks but removes the tracks themselves, so tells you little about the underlying material science.
This lack of detailed information has created debates and arguments amongst scientists for more than 50 years. However, a research team from The Australian National University led by ARC Australian Research Fellow Dr Patrick Kluth has recently solved the mystery.
Dr Kluth explains, "The exact nature of ion track damage has been very difficult to determine because the tracks are only a few tens of atoms in diameter with often only subtle differences in structure to the surrounding material. A lot of times we are getting localized disorder in a material that is itself highly disordered."
To generate the ion tracks in a controlled manner, the researchers have used Australia's largest and most powerful accelerator, the 14UD at ANU where they bombarded amorphous silica targets with very energetic gold ions.
In the world of subatomic particles interactions are very different to our experience of collisions in everyday life. If you're throwing rocks at a tin can the likelihood of you scoring a hit depends on your aim and the size of the can. So long as your aim doesn't falter the likelihood of scoring a hit doesn't change with the speed of the rock. However in the microscopic domain, this common sense no longer holds. The velocity and thus energy of subatomic particles has a large bearing on the likelihood of them hitting each other. This counter intuitive situation arises because the particles aren't really colliding like two solid objects; rather it's their wave functions that are interacting. And wave functions are diffused through local space and time. To keep things convenient, scientists still express the likelihood of two particles colliding in terms of a collision cross section. Bigger cross-section, better chance. The only tricky thing is that this collision cross section changes as the particle energy changes. It's like your tin can getting smaller as the rocks get faster.
For this reason, ions of different energy interact with different components of the target material. Very energetic ions from either natural radioactive decay or the powerful accelerator are very unlikely to collide with the nuclei in the target, as the collision cross section for this interaction is essentially zero at these velocities. This means that the ion loses energy by interaction with the electrons of the host material, not the atoms. The result is a sudden and massive local heating along the ion's trajectory by several thousand degrees. This causes a violent expansion of the silicon dioxide reducing the density along the core of the track and compressing the material in the surrounding cylinder. The area is so localized that the subsequent cooling down is almost instantaneous, preventing the material from returning to its original structure. The net result is a tunnel shaped shock wave frozen in time.
The big breakthrough came with design of high-resolution x-ray scattering experiments to study the structure in the ion tracks. The tracks in the silicon dioxide are amorphous ¬ meaning the crystal lattice structure has no long-range order. However the target silicon dioxide also has an amorphous structure. "It's very hard to see tracks of new disorder in an already disordered material." Dr Kluth explains, "the new measurements, however, enable us to resolve the small density changes in the ion tracks which has not been possible by other means before. We are now confident that we can apply this method to resolve the structure of ion tracks in a wide variety of other materials as well."
A crucial aspect for the measurements is that the accelerator-irradiated material differs from naturally occurring silica in one very important way. All the ions from the accelerator were travelling in exactly the same direction when they created tracks. This means that all the damage tracks are parallel. This is vitally important because it makes x-ray analysis viable. To obtain a suitable bright monochromatic x-ray source, the scientists travelled to Chicago to use the Advanced Photon Source synchrotron at Argonne National Laboratory.
In a natural sample with tracks at random angles, a beam of x-rays is scattered in a different direction by each track resulting in a blurring of the scattering signal. However when the tracks are all parallel each one scatters x-rays in the same direction reinforcing the signal. "What we see in a case like this is a clean superimposition of the signals from each track."
"Apart from solving a long-standing mystery in materials science, these findings have significant potential impact for interplanetary science. In space, equipment is exposed to very high energy cosmic radiation and the response of materials to that is important in designing reliable electric components."