Characterising large area silicon drift detectors with MOS injectors

In the framework of the INFN DSI project, the first prototypes of a large-a rea Silicon Drift Detector (SDD) have been designed and produced on 5 " dia meter wafers of Neutron Transmutation Doped (NTD) silicon with a resistivit y of 3000 Omega . cm. The detector dimensions are 6.75 x 8 cm(2), with a se nsitive-to-total area ratio of 86%. The detector is a "butterfly" bi-direct ional structure with a drift length of 32 mm and the drifting charge is col lected 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 ha s a new type of MOS injector. We present results obtained by injecting char ge at the maximum drift distance (32 mm) from the anodes by means of the MO S injecting structure. As front-end electronics, we have used a 32-channels low-noise bipolar VLSI circuit (OLA, Omni-purpose Low-noise Amplifier) spe cifically 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.