Australian Positron Beamline Facility
A moderated source of positrons
The positrons we use come from a radioactive sodium-22 source. They are emitted with a wide range of energies, so some conditioning has to be applied to get a monochromatic beam. The first thing we do is pass the positrons through a layer of frozen neon. A small fraction (about 1%) of the incident positrons are reemitted with a small energy spread, of about 2 eV, in a process called moderation. Many positron research groups use a beam formed in a similar manner to perform their experiments. The beam is confined in the radial direction by using magnetic fields. Electrostatic potentials allows us to direct it along the apparatus.
This element of our apparatus is available commercially and we have purchased the moderating stage from First Point Scientific Inc., a company based in southern California.
