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ScienceWise - Spring 2013

Science magnet

Article Illustration
Miro Peric machines one of the iron sections that will encase the completed solenoid

How a unique facility is attracting scientists to Australia

A series of massive slabs of iron are lined up on the floor of the mechanical workshop at the ANU Research School of Physics and Engineering. They’re waiting to be machined into a housing for a powerful solenoid funded by the Federal Government Superscience Initiative. Together, these will form part of a unique physics instrument, located at the ANU Heavy Ion Accelerator Facility. 

Watching the gantry crane heave a ton of solid iron from the floor onto one of the biggest lathes you’re likely to see in Australia, it’s hard to imagine that the vast majority of the space within that colossally heavy metal ingot is actually filled by lightweight electrons. 

Most of the 1000kg mass comes from the nuclei of the iron atoms, but those nuclei occupy only the smallest imaginable fraction of the total volume. If you could get rid of the electrons and pack the nuclei together you’d have something smaller than a particle of dust, but that still weighed almost 1000kg! 

The tiny size of nuclei creates a problem for scientists like Professors David Hinde and Mahananda Dasgupta. They study the processes of nuclear collision and fusion in order to understand how the heavy elements found on Earth were formed in ancient supernova explosions.

“We study fusion by bombarding a target with a beam of very energetic nuclei, but because the nuclei are so tiny, the chances of a direct hit on a target nucleus by a beam nucleus are very small indeed. We may only get a few fusion events for billions of ions hitting the target.” Professor Dasgupta says.

When a direct hit does happen the impact is enormous. So large in fact that not only do the two nuclei fuse, but the newly formed heavier nucleus is blown right out of the back of the target. The scientists need to isolate these newly formed nuclei so that they can be identified and characterised. However the problem is that for every fusion product leaving the back of the target, there are literally billions of beam particles that have passed through.  Somehow, the scientists have to separate the two and fortunately nature has provided a way.

Immediately after fusion, the new heavy nucleus is incredibly hot and emits energetic particles such as neutrons. These emissions perturb its trajectory, just like little rocket thrusters firing to the side, so the jet of fused nuclei leaving the back of the target isn’t all along the beam direction. Instead it forms a cone more like the spray from a shower head.

Using an enormously powerful magnetic field it’s possible to re-focus this cone of fused particles and collect them all in one place with excellent efficiency. The difficulty is that the field required to do this is truly enormous – 250,000 stronger than the Earth’s magnetic field. 

To generate a field of that strength over a large volume requires a superconducting electromagnet. When cooled close to absolute zero, the coils of special wire within the solenoid have no resistance, so once a large current is set up, it will circulate forever, or until the coil is allowed to warm up at the end of the experiment. 

The massive iron housing being manufactured in the workshop encloses the superconducting coil and serves a double purpose. Firstly it improves the uniformity of the magnetic field within the instrument. But it also prevents that field spilling out into the surrounding lab. “When you’re dealing with strong magnetic fields, 8 Tesla in this case, you really have to look at the safety aspects too,” Professor Hinde explains, “In a lab full of nuts, bolts and spanners, turning on an uncontained field of this magnitude would trigger a hail of metal!”

“There are very few workshops in the country that could handle the fabrication of such massive and complex parts. The Research School’s mechanical workshop has always been one if its great strengths and the expertise of our technical staff is what makes so much of the science we do here possible.” Professor Hinde says.

The ANU accelerator is one of the highest Voltage Van de Graff accelerators in the world attaining over 15 million Volts.  Scientists are attracted from around the world to make use of the laboratory’s unique capabilities. The new solenoid system will enhance both the Accelerator Facility and Australia’s global scientific reputation.

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