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

The large enriched germanium experiment for neutrinoless double beta decay (LEGEND) is a ton-scale experimental program to search for neutrinoless double beta ( $0\nu \beta \beta $ ) decay in the isotope $^{76}\textrm {Ge}$ by means of high-purity germanium (HPGe) detectors operated in liquid argon (LAr). The observation of $0\nu \beta \beta $ 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 LEGEND ultralow background integrated circuit for germanium detectors investigation (LUIGI) application-specific integrated circuit (ASIC) was designed. The 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 to obtain good energy resolution (at 2039 keV, i.e., the $Q_{\beta \beta }$ value of the $^{76}\textrm {Ge} \; \beta \beta $ -decay, a value of $2.49 \; \pm \; 0.03\,\text {keV}$ full-width at half-maximum (FWHM) is obtained) and a high radiopurity which are the main requirements for the readout electronics in $0\nu \beta \beta $ decay experiments. It was designed featuring a low-noise CSA and an ON-chip low-dropout (LDO) regulator. (At a shaping time of $6\,\mu $ s, an energy resolution at the noise peak of 500 eV FWHM is measured.) Two different versions of the CSA were implemented. The LUIGI-internal reset (IR) variant has a dedicated compensation network and implements an integrated large-value resistor through an ICON cell. Instead, the LUIGI-feedback resistor (RF) variant works with a large value external RF. The LDO makes it possible to power the chip without bypass 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.