The Australian Positron Beamline Facility

The low energy beam from the moderator is fed into the trap where it cools through collisions with a buffer gas (made up of N2 and CF4). The positrons are confined using electric and magnetic fields and cool to a temperature of around 40 meV (room temperature).

CollaboratorsPeople from a wide range of backgrounds are involved in the project, with expertise in Atomic and Molecular Physics and Materials Science. Our collaborators for the original equipment grant are listed below.

ANU CSIRO Griffith UniversityProfessor Steve Buckman Dr. Anita Hill Professor Birgit Lohmann

Dr. James Sullivan Dr. Tim Bastow Professor Evan Gray

Professor Jim Williams Flinders University Charles Darwin University

Professor Bob McEachran Dr. Michael Brunger Dr. Jim Mitroy

Mr Graeme Cornish Professor Peter Teubner

This is the first positron beamline to be constructed in Australia, and was funded through the ARC LIEF program. A team of 4 Australian Universities and the CSIRO have united to construct and operate the facility. The beamline will be used for a wide range of experimental programs, ranging from atomic and molecular physics, to bio- and medical physics and materials science.

The beamline has been designed with four stages, a moderator stage, trap stage and two experimental stages.

Background

The moderator stagePositrons are obtained from a radioactive, 22Na source. The positrons are emitted from the source with a wide range of very high energies, unsuitable for beam formation. The first stage of the apparatus freezes a layer of solid neon over the 22Na, which then acts as a moderator for incident positrons.

About 1% of the fast positrons hitting the frozen neon lose energy inside the neon crystal and are reemitted at low energy. We can selectively take this portion of the positrons and form them into a low energy beam, with a resolution of about 2 eV.

Magnetic fields

Experimental stages

The positron trap

Confining electric fields are provided by a cylindrical electrode structure. The arrangement allows the formation of a two step potential well to confine the positrons. The different diameters of the electrodes allow for regions of differing pressures of cooling gas.

A segmented electrode in also included to allow for compression of the trapped positron cloud using the “rotating wall” technique.

Custom built magnetic field coils confine the positrons along the length of the experiment, preventing them from hitting the walls of the vacuum chambers and annihilating. They also allow for the positrons to be switched from one experimental station to the other. The magnetic field have strengths up to 500 gauss.

Two experimental stages are being constructed for the beamline. One will be for experiments in atomic and molecular physics. The other will be for materials and bio science.

The stages will be located after the trap, side-by-side. A magnetic switch will allow the positron beam to be directed to the desired experiment. The energy scales of the two experiments are quite different. For atomic and molecular experiments, the positron beam from the trap will be up to 100 eV in energy and with a resolution of as low as 10 meV. The pulses will be approximately 1 s in length. For the materials experiments, the energy of the positrons will be up to 10 keV with energy resolution of approximately 30 eV. The pulses will be <1 ns wide.

This is the first time that the same apparatus will be able to be used for both types of experiments.

Making a positron beam

To make a beam from the trapped positron cloud, the confining potential of the electrostatic well is raised, spilling the positrons out of the trap. The positrons form a pulsed beam, and the trap can be refilled. It is anticipated that the filling cycle will take less than 1o ms to complete, giving a pulse of around 5,000 positrons each time. 100 trapping cycles per second will give us a positron beam of approximately 500,00 per second.