XEUS wide-field imager: first experimental results with the X-ray active pixel sensor DEPFET

A new type of Active Pixel Sensor is proposed which will be capable to meet the requirements of the wide field imager of ESA's future X-ray mission XEUS: the simultaneous energy and position resolved detection of X-rays at high count rate on a large format sensor. The Active Pixel Sensor is based on the integrated detector-amplifier structure DEpleted P-channel Field Effect Transistor (DEPFET). The device operates on a fully depleted bulk and provides internal signal amplification at the position of the charge generation. A very low value of the overall output capacitance leads to extremely low read noise. In the matrix arrangement of an Active Pixel Sensor the single DEPFET pixels can be randomly accessed for readout, and various flexible readout modes are possible. In contrast to CCDs the DEPFET-based Active Pixel Sensor avoids the transfer of signal charges over long distances within the detector bulk, and related problems of transfer loss or out-of-time-events cannot occur. An interesting feature is the non-destructive nature of the DEPFET readout which can be used for the reduction of the low-frequency noise contribution by repetitive readings of the signal information. The device principle of the DEPFET based pixel sensor is explained. First results of single DEPFET measurements are presented.