Atomic and Molecular Positron Beam
The atomic and molecular physics positron beamline uses a Surko trap to produce a pulsed positron beam with an enery resolution of 65 meV. The beam is confined in a strong magnetic field of approximately 500 gauss, more details can be found under the "beamline operation" heading on the menu at the right. Using the proerties of this magnetised beam, we can measure low energy scattering cross sections for a range of processes and target species.
After the trap section, the beam is directed through a retarding potential analyser (RPA) and a gas cell, kept in the same magnitude magnetic field as the trap. After passing through the gas cell, the beam travels to another RPA (in a separately adjustable magnetic field) and positrons passing through are detected using a micro-channel plate pair with a phosphor screen anode. Pulses can be imaged using the anode to provide information about beam alignment and density distribution, or they can be picked off the anode directly and amplified before being fed into a computer controlled data acquisition system.
Because the positrons are in a strong magnetic field, the energy of can be divided into two components, parallel and perpendicular to the field. Initially, the positron beam has almost all of its energy in the parallel direction. If the positron scatters elastically (ie: no energy transfer to the target), then energy is transfered into the perpendicular direction according to the scattering angle. As the RPA measures only the parallel energy of th beam, the energy distribution can be related to the angular differential scattering cross section. By adjusting the magnetic field, it is also possible to separate elastic and inelastic scattering events and thus measure inelastic scattering cross sections.
The first experiments using this beamline are underway, examining positron scattering from helium. An extensive research program is planned, from fundamental to applied studies of low energy positron scattering.
