The GERDA experiment was designed for two phases. The first phase used detectors that the collaboration had “inherited” from earlier experiments.
A further type of detector is used in the second phase of the experiment. It was developed and manufactured especially for the GERDA experiment. Moreover, the set-up was modified by installing further light sensors in the immediate vicinity of the germanium detectors. These enable researchers to recognize background events for what they are.
Phase I of the GERDA experiment ran from November 2011 to May 2013. Measurements were taken with a total of eight germanium detectors enriched with the germanium-76 isotope.
The energy spectrum acquired corresponded to the objectives set. The background index in the energy range of interest was 0.02 events/(keV kg year). The composition of the energy spectrum was developed mainly as part of a doctoral thesis at the MPP [Ref=Link].
At the energy where the signal was expected, it was not possible to measure an excess of events. Since this decay has not been observed, it is possible to set a lower limit for the half-life of neutrino-less double-beta decay of 2.1 x 1025 years. This corresponds to a confidence limit of 90%.
In order to be able to install the additional detectors and light sensors, the set-up had to be significantly modified. One very important aspect was the construction of a new lock system. This was again the responsibility of the MPP.
GERDA Phase II has been acquiring data with a total of 40 germanium detectors and the additional light detector system since 2015.
Since there was no measurable decay in the current experimental phase, it was possible to specify a new lower limit for the half-life of germanium-76. It is 5 x 1025 years, which can be converted into an upper limit for the Majorana neutrino mass of 150-250 millielectronvolts. This means that a neutrino that is identical to its antiparticle (Majorana character) would be at least 250,000 times lighter than an electron.
Furthermore, it was possible to show that the disturbing signals caused by natural radioactivity can be reduced tenfold with a novel screening concept - an important prerequisite to be able to start ambitious projects with even higher sensitivity.