Positron Imaging Centre
Positron Emission Particle Tracking
The technique of Positron Emission Particle Tracking (PEPT), invented at Birmingham, enables a single radioactive tracer particle moving inside a piece of equipment to be tracked accurately at speeds up to 2ms-1.
Using the original positron camera, a slow moving tracer (speed less than about 5cm/s) could be located to better than 1 mm approximately 10 times per second, whereas at 1 m/s it could be located to within about 5 mm roughly 250 times per second. Using the Forte, even greater accuracy is possible: at 1 m/s a tracer can be located to within 0.5mm 250 times per second. These accuracies are reduced if the tracer is surrounded by a considerable thickness of dense material. Examining the change in the tracer's position with time gives an estimate of its instantaneous velocity which is accurate to 10%.
Tracking is possible over the volume lying between the two detectors of the positron camera. Tracking can be carried out inside almost any equipment provided this can be set up with the volume of interest confined between the two detectors. This gives a maximum field of view of 60x60x30cm3 (original camera) or 80x50x40cm3 (Forte) although accurate tracking is impossible near the edges of the field of view. The original camera is also mounted on rails with a motorised drive under computer control so as to follow a tracer in real time, and this extends the effective field of view to nearly 2m for horizontal axis systems in which the tracer moves reasonably slowly (eg. continuous flow systems in which the axial velocity is less than about 5cm/s); the Forte will similarly be mounted on rails during the year 2000. It may also be possible to tackle larger equipment by dismounting the two detectors of the original camera onto separate trolleys, and simplified transportable PEPT systems are also under consideration.
Early studies with PEPT used as the tracer a glass bead which was directly irradiated with the beam from a cyclotron to induce radioactivity within it. Using this approach suitably radioactive glass tracers down to 1mm in size can be produced, and low density tracers can be produced using foamed glass. More recently, 600mm tracer particles of polystyrene resin are routinely used, into which the activity is introduced by ion exchange, and still smaller tracer particles are being developed.
Many of the early PEPT studies have been on the behaviour of dry granular materials. Studies of gas-fluidised and vibrating beds of particles and of various designs of stirred mixer are continuing, and the work has recently been extended to the study of viscous fluids. As well as following the motion of the tracer particle directly on a computer screen, four principal types of information can be extracted from following the particle track over an extended period during which it circulates throughout the mixing vessel:
First, the fraction of time spent by the tracer particle at each point in the bed can be plotted. Over an extended run this should come to represent the density of particles at each point in the bed. By using different sizes or densities of tracer, segregation effects can be investigated.
Secondly, the average velocity at each point in the bed can be extracted, and the velocity field compared to models. Alternatively, for non-laminar flow the distribution of instantaneous velocities at a point may be of interest.
Thirdly, for mixing studies in particular it may be most useful to select an initial "tagged" volume and, by following the history of the tracer particle each time it emerges from this volume, to build up a picture of how material initially within this volume disperses with time. When presented in this way the data can be used to determine a mixing index as a function of mixing time.
Fourthly, for some systems it may be natural to divide a vessel into compartments and measure the residence time distribution within each compartment. It may also be instructive to observe whether transfer between compartments occurs preferentially at certain points.
How PEPT Works
A radioisotope which decays by a form of beta-decay involving emission of a positron is incorporated into the tracer particle. Once emitted from the nucleus, the positron annihilates with an electron, releasing energy in the form of two 511keV gamma-rays which are emitted back-to-back, 180° apart to within about 0.5°. The tracer particle is introduced into the system under study, which is mounted between the two detectors of the positron camera. Each detector is able to detect incident gamma-rays and determine their interaction coordinates to within a few mm. Only coincidence events in which gamma-rays are simultaneously detected in both detectors are recorded. From a small number of such detected events the tracer position can be determined by triangulation. The gamma-rays are quite penetrating (50% are transmitted through 11 mm steel) so that tracking is possible inside real process equipment. Some gamma-rays are scattered prior to detection, but the tracking algorithm used is able to discard these events. It should be noted that all measurements are made in three dimensions, and that the accuracy quoted is limited by the difficulty in determining the position along the axis normal to the detector faces.
See also the more detailed description of the physics of PEPT.