Materials Positron Beam
The materials positron beamline is currentl nearing the end of the construction phase. Like the atomic and molecular physics beamline, it is based on a Surko trap which produces a pulsed positron beam. In this case, however, the focus is not on energy resolution, but rather time resolution, so the microsecond scale pulses from the trap must be compressed to sub-nanosecond pulses before being injected into the materials to be studied at energies up to 10 kV.
The first stage of the materials beam is much like that for the atomic and molecular beam. However, the process of forming the beam is quite different. As the positrons are coming out of the trap, their energy is increased, with the later positrons having a greater energy than those first ejected. This means that the later positrons travel faster and catch up with the earlier ones - compressing the pulse in both time and space. Simulations have showed that it is possible to form a sub-nanosecond pulse in this manner, and this technique will soon be tested on the new beam. The end stage of the beamline incorporates a load-lock stage for introducing samples, along with a carosel that will allow up to 8 samples to be loaded at a time. The samples can be positioned one-by-one at the focal point of the positron beam, and annihilation gamma rays will be detected using fast detectors, allowing the time profile of the annihilation within the sample to be measured. This time signature can then be related to the structure of the material, in a technique known as PALS. This is explained in some more detail on our PALS page.
