Design and Characterization of a Low-Noise and Low-Background Charge Sensitive Amplifier for the Readout of Germanium Detectors

Abstract-The Large Enriched Germanium Experiment for Neutrinoless double beta Decay (LEGEND) is a ton-scale experimental program to search for neutrinoless double beta (0νββ) decay in the isotope ⁷⁶ Ge by means of High-Purity Germanium (HPGe) detectors operated in Liquid Argon (LAr). The observation of 0νββ decay would have major implications in the understanding of the origin of the matter in the universe and establish neutrinos as Majorana particles, i.e. their own antiparticles. In this framework, the LUIGI (LEGEND Ultra-low background Integrated circuit for Germanium detectors Investigations) ASIC was designed. The Application Specific Integrated Circuit (ASIC) technology enables the implementation of the whole Charge Sensitive Amplifier (CSA) into a single low-mass chip. The LUIGI ASIC can play a key role in order to obtain a good energy resolution (at 2039 keV i.e. the Q _ββ value of the ⁷⁶ Ge ββ-decay, a value of 2.49±0.03 keV FWHM is obtained) and a high radiopurity that are the main requirements for the readout electronics in 0νββ decay experiments. It was designed featuring a low noise CSA and an on-chip Low-DropOut regulator (LDO) (at a shaping time of 6 μs, an energy resolution at the noise peak of 500 eV FWHM is measured). Two different versions of the CSA were implemented. The LUIGI-IR (Internal Reset) variant has a dedicated compensation network and implements an integrated large-value resistor through an ICON cell. Instead, the LUIGI-RF (Feedback Resistor) variant works with a large value external feedback resistor. The LDO makes it possible to power the chip without by-pass capacitors, which are not compliant with the radiopurity requirement. A dedicated Line Driver circuit drives the signal in a differential way over a distance of about 10 m .