C. Macolino: Search for Zero-v Double-Beta Decay with Gerda
Gerda (Fig.1) is a Germanium detector array, an ultra-low-background experiment looking for 0v2b decay of 76Ge, which occurs at a Q-value of 2039 keV. To observe that monochromatic line the key is of course to reduce backgrounds as much as possible.
The Gerda collaboration is an European group mainly from Italy, Germany and Russia. The detector has an outer water tank shielding from neutron contamination and providing a cherenkov muon veto, which also benefits from plastic scintillators on top of the cryostat. Inside the tank the germanium detectors are immersed in liquid argon.
To maximize the sensitivity to the half-life, one sees that the measurement depends inversely on the square root of background level and energy resolution, and proportionally to the square root of active target mass and measuring time.
Seven events are observed in the signal region in their data taking (see Fig.2), consistent with background-only hypothesis. They extract a lower limit on the half-life of 0v2b decay of Germanium at 2.1×10^25 years at 90%CL.
In phase-II Gerda will be upgraded to increase the total mass to 20 kg, reducing backgrounds by a factor of 10.