Characterising large area silicon drift detectors with MOS injectors

In the framework of the INFN DSI project, the first prototypes of a large-area Silicon Drift Detector (SDD) have been designed and produced on 5 " diameter wafers of Neutron Transmutation Doped (NTD) silicon with a resistivity of 3000 Omega . cm. The detector dimensions are 6.75 x 8 cm(2), with a sensitive-to-total area ratio of 86%. The detector is a "butterfly" bi-directional structure with a drift length of 32 mm and the drifting charge is collected by two arrays of anodes (384 anodes for each half) having a pitch of 200 mu m. The high-voltage divider is integrated on-board and is realised with p(+) implantations. For test and calibration purposes, the detector has a new type of MOS injector. We present results obtained by injecting charge at the maximum drift distance (32 mm) from the anodes by means of the MOS injecting structure. As front-end electronics, we have used a 32-channels low-noise bipolar VLSI circuit (OLA, Omni-purpose Low-noise Amplifier) specifically designed for silicon drift detectors. The uniformity of the drift time in different regions of the sensitive area and its dependence on the ambient temperature are studied.