Exploring the Potential of the Australian Positron Beam Line Facility
Most of the fundamental particles of physics have an associated antiparticle with the same mass but opposite electric charge. In the case of electrons, their antiparticle is the positron, which carries a positive charge. A group of scientists at the ARC Centre for Antimatter-Matter Studies (a collaboration of Australian universities and research institutions) have recently completed a dual beamline facility at ANU for the study of positrons. The scientist leading the development of the Australian Positron Beamline Facility is Dr James Sullivan.
“This facility will provide us with a new tool to study the basic processes underlying positron physics. How they scatter off, and interact with, atoms and molecules of matter and how they ultimately annihilate by combining with their antiparticle.” Dr Sullivan explains. “Essentially, we are interested in understanding the detail of the quantum mechanical processes that underpin such interactions.”
There is a great potential to expand our knowledge of science using positrons but they also have a number of very practical applications. One is Positron Emission Tomography or PET - a medical imaging technique in which a small quantity of positron emitting radioactive isotope is bound to a biological molecule, such as glucose, and injected into the patient. When the emitted positrons annihilate a pair of gamma rays are emitted. By monitoring the position and direction of the gamma rays, it’s possible to determine the location of the isotope/molecule complex within the body. PET scans are particularly useful in detecting local changes in biochemistry which often occur before the structural tissue changes that appear on other scans.
“Despite the usefulness of PET scans, the basic physics of the positron electron annihilation process is not well understood,” Dr Sullivan says, “perhaps if we can get a better handle on the physics, there may be opportunities to improve the resolution of scans and or reduce the necessary dose of radioactive marker.”
Science often works like this. A thorough understanding of what’s going on at the fundamental level often enables technological breakthroughs that can’t be achieved by simply refining the engineering.
The second beamline of the facility is devoted to materials science. When positrons enter matter they commonly join with an electron to form a short-lived exotic atom called positronium. In vacuum, orthopositronium has a lifetime of 140ns but in solids this can be reduced to a few hundred picoseconds. However, orthopositronium is attracted to voids in solid material and because these act like tiny vacuums its lifetime is extended where voids are present. In effect this means that by injecting a very short pulse of positrons into a material and measuring the timing of the gamma ray annihilation signatures, scientists can determine the size and density of imperfections in the material. This is especially useful for example, in high grade silicon used in microelectronics and in detecting early signs of fatigue in metals.
But given that antimatter and matter annihilate on contact, how do you create a positron beamline in the first place?
The positrons in the Australian Positron Beamline Facility are generated as a radioactive decay product of an isotope of sodium 22Na. The positrons emitted by the sodium source emerge at every angle and with a range of energies up to about 500keV. This energy spread creates a problem because the resolution of most measurements that can be made with positrons depends on the energy variation within the initial beam. Scientists get around this using what’s known as a moderator.
A common choice of moderator is tungsten. When the positrons enter the tungsten they very rapidly thermalise, that is dissipate any excess energy and slow down to the same speed as the electrons in the material. However, the work function of materials like Tungsten is negative for positrons so they tend to be very quickly ejected again. Of course quite a large number recombine and annihilate within the moderator, but those that don’t, emerge with a much narrower range of energies than when the beam entered.
In the Australian Positron Beamline Facility, the chosen moderator is frozen neon gas rather than tungsten because the rate of annihilation is far lower in neon than in tungsten. To capture as many of the positrons as possible the neon is frozen directly to the radioactive source casing. The moderated low energy positrons that emerge from the neon are channelled into a positron trap using electric and magnetic fields. The trap further reduces the energy of the positrons and concentrates them in space. The scientists then use these trapped positrons as a reservoir from which to make a positron beam.
For experiments on the first beamline where low energy spread is required the positrons are accelerated from the trap using electric fields and directed through a gas of the target atom or molecule. In the second beamline devoted to lifetime experiments, the positrons are compressed into a series of very short (<1 ns) and compact pulses and implanted into the material to be studied.