A charge sensitive preamplifier for high peak stability in spectroscopic measurements at high counting rates

In this paper, a charge preamplifier designed to achieve high stability in spectroscopic measurements at different counting rates is presented. The preamplifier operates with the input JFET and the feedback capacitor directly integrated on the silicon detector, in the specific case a Silicon Drift Detector (SDD). The motivation of the work is to overcome the peak shift, typically in the order of few percents and then comparable with the energy resolution achievable at 6 keV with a SDD, which can be measured when the on-chip JFET is operated in the conventionally used source follower configuration. In the proposed circuit, the SDD anode voltage is stabilized by means of a low-frequency feedback loop which operates according to the "drain feedback" technique. Moreover, the implemented design, differently from previous designs also based on a continuous reset of the leakage + signal currents, allows to obtain a sufficiently fixed decay time constant of the preamplifier with respect to event rate variations. This feature allows to maintain an optimal pole-zero compensation by the shaping amplifier when the event rate changes relevantly during the measurement. In an experimental characterization of the preamplifier with a SDD, a Mn-Kalpha peak-shift within ±0.03% (i.e., ±2 eV at 6 keV) has been measured changing the rate from few kcounts/s up to 400 kcounts/s.