The Germanium Detector Array (or GERDA) experiment was searching for neutrinoless double beta decay (0νββ) in Ge-76 at the underground Laboratori Nazionali del Gran Sasso (LNGS).
The collaboration predicted less than one event each year per kilogram of material, appearing as a narrow spike around the 0νββ Q-value (Qββ = 2039 keV) in the observed energy spectrum.
The LEGEND collaboration, continuing GERDA's work, was aiming at increasing the sensitivity to the half-life of 0νββ decay up to
[1] The experiment used high purity enriched Ge crystal diodes (HPGe) as a beta decay source and particle detector.
The detector array was suspended in a liquid argon cryostat lined with copper and surrounded by an ultra-pure water tank.
[1] Phase 2 increased the active mass to 38 kg using 30 new broad energy germanium (BEGe) detectors.
This limit could be combined with previous results, increasing it to 3·1025 yr, disfavoring the Heidelberg-Moscow detection claim.
Phase II had additional enriched Ge detectors and reduced background, raising the sensitivity about one order of magnitude.
[4] The background index for BEGe detectors was 0.7·10−3 counts/(keV·kg·yr), which translated to less than one count in the signal region after an exposure of 100 kg·yr.
The reported final lower limit agreed with the expected value for the sensitivity of the experiment, and was the most stringent value for the decay of any 0νββ isotope ever measured.
In its final phase GERDA deployed 41 germanium detectors with a total mass of 44.2 kg, with very high germanium-76 enrichment percent.