A cryogenic low-noise JFET-CMOS preamplifier for the HPGe detectors of GERDA

Cryogenic low-noise charge sensitive preamplifiers have been realized and tested for the GERmanium Detector Array (GERDA). In the search of neutrino-less double-beta decay of ⁷⁶Ge at LNGS, GERDA will operate bare segmented germanium detectors immersed in liquid argon. The front-end electronics will operate in the cryogenic liquid too. An integrated JFET-CMOS preamplifier, which is fully functional at cryogenic temperatures, has been developed and realized. It has been tested in conjunction with an unsegmented p-type HPGe detector. Both the crystal and the preamplifier were operated inside a liquid nitrogen dewar at 77 K. The detector capacitance was ∼ 60 pF. An optimum resolution of 1.6 keV fwhm was obtained on the pulser line at 6 μs shaping time. The obtained resolution for the 1.332 MeV line from a ⁶⁰Co source was of 2.2 keV fwhm. No peak shifts or line broadenings were seen during long-term acquisitions, thanks also to the extremely high preamplifier loop gain which yields a very high closed-loop gain stability. A wide bandwidth (rise time of 16 ns) permits use of pulse-shape analysis techniques to localize the position of the photon interactions inside the detector. A low power consumption (23.4 mW) makes the preamplifier suitable for the foreseen multi-channel array of germanium detectors.