Researchers beaming on positron anniversary

FRIDAY 14 MARCH 2008

On the eve of the 75th anniversary of the discovery of the first antimatter particle – the positron – researchers at The Australian National University are taking their first steps to investigate the interactions of antimatter with matter.

Researchers at the Australian Positron Beamline Facility at ANU have created a high-intensity positron beam with a very narrow energy distribution that can be used for highly detailed studies of atomic and molecular structure. The facility is part of the Australian Research Council’s Centre of Excellence for Antimatter-Matter studies.

Initial research work at the facility has involved investigating positron interactions with simple atoms such as helium. This work investigated the probability that a positron will interact with a helium atom and whether positrons can ‘bind’ to helium atoms to form a ‘positronic’ complex. The research revealed that recent experiments from Europe, that claimed to have found positronic complexes, are probably not correct.

Professor Stephen Buckman, Director at the ARC Centre for Antimatter-Matter studies, said that the research offers some exciting possibilities.

“While still at an early stage, the work offers potential for applied areas of science and medicine,” Professor Buckman said.

“Positrons are widely used in medical imaging in a technique known as Positron Emission Tomography (PET). PET scans are regularly used in our major hospitals as a diagnostic for cancers and brain function and rely on positrons released into the body annihilating with their antiparticle the electron. Positrons are also widely used in materials research to characterise the size and nature of defects and voids in materials at the nano-metre level.”

The experimental history of the positron began on 15 March 1933 when the US science journal Physical Review published a paper entitled The Positive Electron by Carl D Anderson of the California Institute of Technology. Anderson had examined more than 1300 photographs of cosmic ray tracks through a device known as a cloud chamber. In the cloud chamber, cosmic rays leave tracks and the shape and size of these tracks can be used to determine their charge and mass.

“Anderson found 15 tracks which could not be explained by the then current knowledge of elementary particles,” said Professor Buckman. “He postulated that these were in fact the ‘positive electrons’ that had been predicted theoretically by Paul Dirac a few years earlier and the rest is history.The discovery of the first antimatter particle – the electron’s antiparticle, the positron – had been experimentally verified.”

Martyn Pearce, ANU Media Office – (02) 6125 5575 / 0416 249 245