Semiconductor Drift Detector’s Anode Capacitance: Measurement in Operating Conditions and Applications in Charge Readout

We describe a low-cost and high-accuracy technique for measuring the anode capacitance of a semiconductor drift detector (SDD) fully biased in operating conditions and connected to the front-end electronics. Different contributions to the overall anode capacitance with respect to the front cathode and the adjacent drift cathodes have been measured under different bias condition. For the SDD sample employed in this work, a total anode capacitance between 60.0 fF and 76.6 fF, depending on the bias condition, has been experimentally determined. This result can be effectively used for the optimization of the front-end electronics for SDDs. Owing to the stability of the SDD electrical parameters, we show that it is possible to configure the detector capacitance at the anode electrode as a functional element of the charge sensitive amplifier (CSA) used to readout the detector signal, either as feedback or as test capacitance. Using the anode capacitance as feedback capacitance the removal of the external feedback capacitance can reduce the total capacitance at CSA input, so improving the system noise; the stability of the acquisition system gain over temperature and voltage variations have been characterized, using a ⁵⁵ Fe X-ray source, showing a 0.5 ppm/mV gain variation over bias voltage variations (±2V at the first cathode) and 0.2 ppm/°C over a temperature variation of ±10°C.