A short drift GEM tracking detector for measuring escaped positrons in PET

A short drift GEM detector has been studied for use as a tracking device for positrons emitted from isotopes used in PET. The detector utilizes a triple GEM detector with a short drift region above the GEM stack to collect the ionization from positrons that traverse the gap. An XV strip readout is used to measure two transverse coordinates and the drift time is used to measure the third coordinate in the drift direction. The three dimensional coordinates are used to construct a vector for the track which can be used to determine the direction of the incident positron. A full Monte Carlo simulation of the detector response was implemented and produced lines of response that were used for image reconstruction and compared with actual data from the detector, as well as to estimate the position and angular resolution for tracks incident on the detector at different angles. In addition, a separate Geant4 Monte Carlo program was used to evaluate the effect of multiple scattering of low momentum positron tracks. The motivation and possible applications for imaging thin structures in biological systems, such as plant leaves, is discussed and preliminary phantom results of positron imaging are presented.