Keeping track of the damage

Keeping track of the damageSilica (silicon dioxide)
is the most abundant mineral in the earth's crust and consequently is 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, by using x-ray beams from the U.S.
Department of Energy’s Advanced Photon Source at Argonne National Laboratory.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.The
world of subatomic particle interactions is 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 you 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 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 ChemMatCARS 15-ID
beamline at the U.S. Department of Energy’s 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."Contact: *Patrick.kluth@anu.edu.auSee: P. Kluth*, C. S. Schnohr, O.
H. Pakarinen, F. Djurabekova, D. J. Sprouster, R. Giulian, M. C. Ridgway, A. P.
Byrne, C. Trautmann, D. J. Cookson, K. Nordlund, and M. Toulemonde, “Fine
Structure in Swift Heavy Ion Tracks in Amorphous SiO[subscript 2],” Phys. Rev.
Lett. 101, 175503 (2008). DOI: 10.1103/PhysRevLett.101.175503.The authors
acknowledge the ARC and the ASRP for financial support. O. H. P., F. D., and K.
N. acknowledge support from the Academy of Finland as well as the CONADEP and
OPNA projects, and grants of from CSC. ChemMatCARS Sector 15 at the APS is
principally supported by the NSF/DOE under Grant No. CHE0087817, and by the
Illinois Board of Higher Education. Use of the Advanced Photon Source at Argonne
National Laboratory was supported by the U. S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under Contract No.
DE-AC02-06CH11357.Argonne National Laboratory seeks solutions to pressing
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