Vertex detection in a stack of Si-drift detectors for high resolution gamma-ray imaging

Usually the position resolution in imaging applications with gamma rays is limited due to the range of secondary reaction products to a few mm. Here a new approach will be presented using the vertex of the gamma interaction as the quantity for position measurement. Because the dominant interaction is the Compton effect the vertex detection method as used in particle physics needs a number of essential modifications. Therefore a detector system is investigated consisting of a stack of Si-drift detectors for the vertex detection of the first Compton interaction and a secondary absorption detector where the position of the scattered photon is detected. A number of effects and possible solutions will be discussed including depth of interaction measurement and track reconstruction of the Compton electron which yields useful information. Another very promising approach of measuring the track projection as in a TPC (Time Projection Chamber) will be presented including first successful measurements in a gas detector. Because the expected rate in the first and second detector is of the order of 1 MHz and 10 MHz respectively the concept of read out electronics and data processing based on VLSI custom chips for signal shaping and first buffering and digital pipe-line processors based on FPGAs is presented. Finally a full system for small animal imaging based on the principle of a Compton Camera will be discussed in terms of achievable resolution and sensitivity.