GERmanium Detector Array (GERDA)
GERDA is a low background experiment located at the LNGS laboratory in Italy designed for rare event searches, using high-purity germanium detectors. Its main goal is to detect neutrinoless double-beta decay (0νββ), a non-standard model process, whose discovery would shed light on the nature of the neutrino and its mass. The 0νββ decay would appear as a peak in the energy spectrum above the 2νββ spectrum, as illustrated in Fig.1. Furthermore, GERDA will also be used for measurements of other rare processes, such as the standard double beta decay (2νββ), and dark matter searches.
Our group is responsible for the calibration of the germanium detectors and is also involved in background modelling, the dark matter search, detector simulation, and data analysis.
The GERDA setup is shown in Fig.2. At its heart, acting simultaneously as source and detector, are 7 strings of 3-8 enriched germanium diodes, These 41 detectors each weigh between 0.4 and 2.8 kg and add up to a total of 38 kg of active mass. Since 0νββ can only be potentially observed in 76Ge, which has only a 7.6% abundance in natural germanium, the detectors are isotopically enriched to about 87%. Applying a reverse bias voltage to the detectors, a current signal occurs if an ionizing interaction happens inside the detector, e.g., from the two electrons emitted by a 0νββ decay within the detector, or background events such as external gamma rays.
The main challenge for the measurement is then to avoid all undesired backgrounds from external sources of radioactivity. Therefore, several layers of active and passive shielding surround the germanium detectors. 1.4 km of rock above the experiment reduce background events due to cosmic rays from ~1 000 000 to 1 per hour square meter. Additional plastic scintillator panels above the experiment detect and exclude the remaining few cosmic ray particles. A 10 m diameter water tank equipped with photosensors surrounds the experiment, acting as shielding and a veto for background events. Inside the water tank is the cryostat, a cylindrical 2 m diameter tank, passively shielded on the inside with pure copper, and filled with liquid argon. The argon fulfils two tasks: It acts as a coolant for the detectors and, being a scintillating material, is equipped with photosensors inside the cryostat to detect and exclude events from particles crossing the cryostat. The remaining background originates from the holding structure and cables for the detectors and unavoidable impurities in the argon. A recorded spectrum with identified background sources is depicted in Fig.3.
The excellent energy resolution of the germanium detectors in addition to the stringent background reduction allows GERDA to operate in the so-called ‘background-free’ regime, where its sensitivity to the 0νββ is only limited by its exposure. In its final data release in June 2020, GERDA Phase II has acquired an exposure of 103.7 kg yr, reaching a background rate of of 5.2*10-4 events per kg yr keV, and a world-leading sensitivity to the 0νββ decay half-life of 1.8*1026 yr assuming no signal.
Event rates in the region of interest around 2039 keV, where the 0νbb signal is expected, are consistent with background expectations. Thus, no significant signal from 0νββ decay was observed. By combining the data sets from GERDA Phase I and II, a world-leading limit on the 0νββ half-life in germanium of 1.8*1026 yr was set, which coincides with the sensitivity. The search continues with the next generation experiment LEGEND.