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Riccardo Brugnera: Status of Gerda

March 14, 2013

Gerda is another player in the zero-neutrino double beta decay race. It uses Germanium as active isotope. Using Germanium as an active material has up and down sides. The sensitivity depends on the detector mass and time of running, background, energy resolution, and abundance of the isotope producing the decay. Germanium has high efficiency since the source coincides with the detector, and has small intrinsic background, excellent energy resolution. But the abundance of the isotope is not very high, and there are limited resources of crystal detector manufacturers.

Gerda uses Ge array in liquid Argon, with a high-purity liquid argon shield and water. In phase 1 the mass is of 18 kg, in phase 2 they will add 20 g of new enriched detectors.

The physics goal from phase 1 is to reach a sensitivity on the half-life of 2*10^25 years at 90%CL, or a limit on the effective majorana mass of .23-.39 eV. This will allow to check the claim of a signal. Then in Phase II they will reach a sensitivity of 1.35E26 year, testing majorana masses above 0.09 eV.

Gerda is situated in the Gran Sasso laboratory. Backgrounds are from photons from the thorium-uranium chain, neutrons, muons from cosmic rays. The strategy to reduce backgrounds are a graded shielding against ambient radiation, and a rigorous material selection, and avoidance of exposure above ground for the detectors.

The signal from neutrinoless double-beta decay is point-like (a single-site), with energy deposited inside one diode. Background has energy deposited in multiple sites. So one can use anticoincidence between detectors, and perform a pulse shape analysis with the detectors envisioned in phase II.

Data taking started in November 2011. Soon after the start they lost two of the germanium-enriched detectors due to high leakage current. In July 2012 they inserted BeGe detectors, restoring the nominal mass. In 373 days of exposure they have 16.7 kg*y effective exposure, and this phase I will last up to the collection of 20 kg*y.

Riccardo showed calibration spectra performed with a 228Th source. The energy scale stability is monitored over time.

The energy spectra show the main components from 39Ar beta radiation, and the two-neutrino beta decay. They blinded the region where they hope to see a signal.

Various background lines are observed. They compare what they find in the data with the results of HdM, seeing lower background levels. The spectrum can be decomposed in the sum of the contribution from various radionuclides and alpha-induced events.The main contribution around the Q-value of the signal are from potassium, bismuth and thorium. In the signal region the background is a factor of 10 lower than HdM and IGEX.

In phase I they already measured the half-life of the two-neutrino double beta decay. They fit the energy range where the signal over background rate is 4:1. They get a half-life of 1.84+0.09-0.08 (stat.)+0.11-0.06 (syst) *10^21 years. This result is the highest one when compared to all other determinations. The half life seems to have “grown with time”, as often happens to measurements that somehow correlate with previous determinations due to experimental bias.

For Phase II, they have produced and tested detectors. The first broad-energy germanium detector are already in the data chain of the phase I, so some experience has already beeh acquired with these devices.

Backgrounds need to be reduced by a factor of 10 with respect to phase I, and this may be achieved with new signal and HV cables with lower background budget, new front-end cards, and the pulse-shape discrimination with the new kinds of detectors. Also, they plan to add veto instrumentation. They also plan to change the lock system for the insertion of the detectors.

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