An ultra-low noise cryogenic CMOS charge sensitive preamplifier for large volume point-contact HPGe detectors

The paper presents the development of an ultra-low noise cryogenic CMOS charge sensitive preamplifier dedicated for the point-contact HPGe detectors for the CDEX dark matter search experiments. The noise of the preamplifier was optimized for 2 pF input capacitance, which was simulated including the detector capacitance and the parasitic capacitance of the interconnection. The prototype chip was implemented in a 0.35 μm CMOS process and the chip was electrically characterized. The rise time of the preamplifier output was increased to 31.7 ns when directly driving 11.5 meter long cable. The ENC was measured to be 10 and 13.3 electrons for 0 and 2 pF input capacitances at 12 μs shaping time at 77 K. A low mass front-end board was also developed and was mounted with a 0.5 kg point-contact HPGe detector. The energy spectrum of the 133Ba and 57Co radiation sources were measured. The energy resolutions of 530 eV FWHM and 640 eV FWHM have been achieved at 81 keV and 122 keV peaks respectively. The resolution for pulser signals was also monitored to be 186 eV, corresponding to 26 electrons (r.m.s.) ENC noise.