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GERDA research group at the University of Zurich


GERmanium Detector Array (GERDA)

GERDA is a low background experiment for rare event searches, which is based on Germanium detectors. Its main goal is to detect the neutrinoless double beta decay (0νββ), a non-standard model process, whose discovery would shed light on the nature of the neutrino and its mass. Furthermore, GERDA will also be used for measurements of other very rare events, like 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 modeling and data analysis.

The Detector
The detector setup of GERDA is depicted in Fig.1. Its heart, source and detector at once, are 7 strings of 3-8 enriched Germanium diodes, each weighting between 0.4 and 2.8 kg and adding up to a total of 38 kg of active mass. Since 0νββ can only be potentially observed in 76Ge, with only 7.6% natural abundance, the diodes are enriched to about 87%. Applying a reverse bias voltage to the diodes, a current signal occurs if an ionizing interaction happens inside the diode, e.g., from the two electrons emitted by a 0νββ decay.


Fig.1: Illustration of GERDA. The Germanium diodes are protected by several layers of avtive an passive shieldings from external radiation.


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 are put around the Germanium diodes. 1.4 km of rock above the LNGS reduce background from cosmic rays from ~10 000 000 to 1 per hour and square meter. Additional plastic scintillator panels above the experiment detect and exclude the ramining few cosmic ray particles. A 10 m diameter water tank is put around the experiment, equipped with photo multiplier tubes inside, as a shielding and veto. Inside the water tank resides the cryostat, a cylindric 2 m diameter tank, passively shielded on the inside with pure copper, and filled with liquid Argon. The Argon fulfills two tasks: It is a coolant liquid for the diodes and, being a scintillating material, photo multipliers inside the cryostat are used to detect and exclude events from particles crossing the cryostat. The remaining background originates from the holding structure and cables for the diodes and unavoidable impurities in the Argon. A recorded spectrum with identified background sources is depicted in Fig.2.


Fig.2: Energy spectrum of GERDA PhaseI after 1 year of operation. Identified sources of background are indicated.


Performance

As of the data release in June 2016, GERDA has acquired an exposure of 34.4 kg yr, reaching an unprecedented background rate of 10-3 events per kg yr keV in its second Phase. The 0νββ decay should appear as a peak in the energy spectrum slightly after the 2νββ spectrum, as illustrated in Fig.3. So far, 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 and new, world leading limits on the 0νββ half-life in germanium of 4*1025 yr were set. Details can be found in Ref. [1]. The quest for discovering 0νββ decay continues!
Fig.3: Signature of 0νββ decay in the energy spectrum. The real relative height of the 0νββ peak is unknown.

References

[1] GERDA collaboration, Background free search for neutrinoless double beta decay with GERDA Phase II, Nature 544 (2017) 47, DOI: 10.1038/nature21717 .