Design and performance of a system for two - dimensional readout of gas electron multiplier detectors for proton range radiography

The proton range radiography (PRR) technique being developed is expected to provide significant improvements in precision delivery of the therapeutic dose in the hadron therapy. The technique requires measuring residual energies and trajectories of mono-energetic protons passing through an object to be imaged. Such an imaging system can be operated in-situ before and after the treatment allowing real time monitoring of the irradiated tissue position. A detector system suitable for such applications must be capable of measuring proton tracks with submillimeter spatial resolution over area of 30 x 30 cm2. In order to limit the exposure time and obtain real-time images the detector and data acquisition must provide capability of measuring high proton rates up to 106 s-1. In this paper we present novel concept of two-dimensional (2-D) readout of the gas electron multiplier (GEM) detectors used as the position sensitive detectors. The key component of the system is an application specific integrated circuit (ASIC) named GEMROC. The GEMROC comprises 32 independent channels, each one capable of measuring the amplitude and the time of incoming signals. The positions are then reconstructed by coincidences of signals from the X- and Y-readout strips. The design and performance of the GEMROC ASIC are discussed and the results obtained for a test bench based on a smaller 10 x 10 cm2 GEM chamber are presented. The test results demonstrate clearly that with respect to the two critical parameters, i.e. the noise and the count rate capability, the developed readout system meets the requirements for clinical applications of the PRR technique.