Australian Positron Beamline Facility
Trapping and cooling
The second conditioning element is a buffer gas trap. This technique was developed by Cliff Surko at UCSD and provides a method to get a high resolution beam. Positrons enter the trap where they collide with gas molecules and lose energy. In the case of a nitrogen buffer gas, the energy is lost to electronic excitation of the molecule at about 10 eV. After losing this energy, the positrons are confined in an electrostatic potential well, where they make successive collisions with the buffer gas and cool to the gas temperature (ie: room temperature). Our trap is a two stage trap, which differs from the original San Diego design which incorporated three stages. This allows us to sacrifice the lifetime of the positrons in the trap for a more compact source.
