Probing Processes Using Positrons

In positron emission particle tracking (PEPT), a single positron-emitting tracer particle is placed within the bulk of particles in the device of interest. The tracer will typically be of the same material as the bulk or compatible with it in size and density. A positron emitted from the tracer annihilates with an electron, leading to the production of two collinear 'back-to-back' gamma rays. These are detected by two large-area position-sensitive detectors, enabling the line on which the positron emitter lies to be determined. Successive such detections enable the tracer to be located in three dimensions simply by triangulation. Typically, a 100 μm particle moving at 1 m s−1 can be located to within 0.5 mm once every 2 ms. The tracer particle position data can then be processed to yield information such as the components of particle velocity, the residence time distributions within specified zones and the frequency of entering these zones. A pseudo-real time visualisation of the tracer movement enables the technique to be used for rapid diagnostic and development studies.