Front-End Electronics and Mechanical Developments for the N3G Project

N3G (Next Generation Germanium Gamma Detectors) is an Italian national experiment aimed at the development of techniques for the production, readout, conservation and maintenance of segmented high-purity germanium (HPGe) detectors. The germanium crystals are sourced in an un-doped form: the detector junction and contacts are realized with PLM (Pulsed Laser Melting) technique. The detector enclosure and contact system are designed to minimize microphonics and stray capacitances and ensure good electric contact especially at liquid nitrogen temperature. Pre-amplifiers are being designed both in integrated and hybrid format. The final goal, still to be tested, is the capability to operate with the full circuit at cryogenic temperatures. The design of such circuit is difficult due to the lack of cryogenic transistor models from the foundries. Both simulations and experimental results are shown. The proposed circuit is characterized with particular focus on energy resolution, linearity, bandwidth and power consumption. Specifically, a 1.08 keV resolution was measured on the test bench simulating the detector with a 15 pF ceramic capacitor and filtering the signals with a quasi-Gaussian shaping amplifier. Circuit linearity is compliant with gamma spectroscopy requirements. The preamplifier is also equipped with an innovative solution that allows for high-resolution spectroscopy in an energy range that exceeds the natural saturation limit of the circuit and, at the same time, reduces dramatically the dead-time in case of high-energy events inside the crystals due to heavy ions or cosmic rays. For events of energy greater than 15 MeV, a resolution, expressed as FWHM, better than 0.11% was measured.