Dynamic behavior of the charge-to-voltage conversion in Si-drift detectors with integrated JFETs

We report on the transient behavior of the charge-to-voltage conversion of a silicon drift detector with an integrated JFET in a source follower configuration, suitable for X-ray spectroscopy and imaging applications. The experimental results are based on the measurements of the JFET´s source voltage in the time domain. The dependence on count rate up to 600 kcts/s and on photon energy up to 15.7 keV are presented at operating temperatures around 20°C. The charge collecting capacitance shunting the gate of the JFET is discharged by a continuous reset current, which is attributed to the I-V characteristic of the JFET´s gate-to-channel junction. Its influence on the signal amplitude is discussed with respect to the voltage dependence of the charge-collecting capacitance. A set of analytical equations and an equivalent circuit fully describes the dependence of the mean source voltage, of the discharging current, and of the signal amplitude as a function of count rate and photon energy. The simulated responses show a good quantitative agreement with the measured characteristics.