Spectroscopic performances of the GERDA cryogenic charge sensitive amplifier based on JFET-CMOS ASIC, coupled to germanium detectors

In the GERDA (GERmanium Detector Array) double-beta-decay experiment, it is planned to operate in liquid Argon (LAr) germanium detectors, organized in three fold strings. In this application the use of cryogenic front-end (FE) electronics is mandatory. Two versions of Charge Sensitive Amplifier (CSA), namely a 1-channel (1-ch) and a 3-channel (3-ch), based on JFET-CMOS circuits, have been realized and tested. The 3-chs CSA are designed to serve the detector string. While in the reference test the 1-ch circuit and a custom encapsulated germanium (Ge) detector (SUB) were operated both submerged in liquid Nitrogen (LN₂), in the naked detector test both the 1-ch circuit and the naked unsegmented Ge detector were submerged in liquid Argon (LAr). A resolution of 3.2 keV FWHM at 1.332 MeV ⁶⁰Co has been obtained in the latter configuration to be compared to 2.2 keV obtained in the reference test. The 3-ch CSA, based on three JFETs connected to three channels of the CMOS ASIC, mounted on a Cuflon PCB, has been tested both coupled to the reference SUB and to a naked prototype detector. The obtained resolution for the 1.332 MeV line of ⁶⁰Co was 2.4 keV with the CSA coupled to the SUB, and 2.9 keV with the naked Ge detector. The spectroscopic performances have been measured connecting the CSA output to an acquisition system through 10 m long cryogenic cables to simulate the real FE connection in the GERDA environment.