A FAST LOW-NOISE SILICON DETECTOR FOR ELECTRON-SPECTROSCOPY UP TO 1 MEV

A 1 mm thick silicon pad detector has been developed for the Atlas Positron Experiment (APEX) [1,2]. This experiment requires efficient detection of positrons and electrons with energies up to 1 MeV with an energy resolution of better than 10 keV and time resolution of less than 2 ns. To satisfy these requirements, the detectors should be 1 mm thick with low leakage current and be capable of operation at voltages greater than needed for full depletion. To enhance the charge collection time, the detectors should also withstand cooling to liquid nitrogen temperature. These requirements have been satisfied using diodes manufactured on high resistivity silicon in combination with a segmented guard ring structure (multiguard) and a process optimized to reduce the bulk generated leakage current. A room temperature leakage current of 1 nA/cm2 was achieved and an energy resolution of 3.6 keV FWHM was measured with 30 pF input capacitance on a charge sensitive preamplifier. At -130-degrees-C with 90 pF input capacitance an energy resolution of 3.4 keV FWHM and a time resolution of 1.2 ns was measured for 500 keV electrons in coincidence with gamma rays detected in a BaF2 Scintillator detector. Results from the evaluation of different designs and process approaches are discussed.