Development of Tracer Particles for Positron Emission Particle Tracking

Positron Emission Particle Tracking (PEPT) is a radioactive tracer-based approach to studying dynamic physical processes and multiphase flows. Short-lived positron-emitting isotopes are loaded onto suitable substrates used as tracer particle flow followers in physical and engineering-relevant systems. Coincident photons from electron-positron annihilation are detected using large arrays of pixelated scintillators, with the reconstructed photon trajectories collectively used to determine tracer particle dynamics. We have developed indirect radiochemical, and direct physical activation, techniques for producing tracer particles for such studies, and we report on the current state of the art with focus on the direct approach with high-energy alpha-particle beams. The O-16(alpha,x)F-18 reactions have been explored as viable candidates in producing the pure positron emitter F-18 from natural O-16-bearing targets. Silicon dioxide (SiO2) glass spheres of diameters of 5 to 10 mm were irradiated in a 100-MeV alpha-particle beam of around 800-nA current for approximately 2 h. Radioisotope activation yields were characterized by half-life measurements and gamma-ray spectroscopy, with the highest yield being F-18 (<2.5 mCi). Contaminants from other reaction channels were observed and characterized, including the positron emitter Sc-43 and negative beta emitter Na-24, produced from alpha and neutron activation of contaminant species in the target material, respectively. The activation technique is shown to be a reasonable candidate to complement and enhance existing tracer particle production techniques for PEPT and other radiotracer-based studies.