GERDA experiment
The GERDA experiment ( english The Germanium Detector Array ) is an experiment to search for the neutrino-less double beta decay of germanium -Isotops 76 Ge. Unlike the normal double beta decay , in which two neutrinos are emitted, this process can not be explained with the standard model of particle physics , so experimental evidence would be a sensation.
Look for neutrino-free beta decays
The nuclide 76 Ge is stable against single, but not against double beta decay. The half-life is about 1.84 · 10 21 years. The energy of 2039 keV released during the decay is distributed over two electrons and two neutrinos. The detectable total energy of the two electrons therefore has a broad distribution.
A neutrino-free double beta decay would be much rarer. It would be characterized by the fact that the sum of the energies of the emitted electrons is always equal to the decay energy. With a good energy resolution of the detector, both types of decay can be distinguished from one another, because a single “line” would have to be superimposed on the continuous distribution.
construction
In phase I of the experiment, eight detectors made of high-purity germanium are used, which also represent the radiation source. To increase the sensitivity of the experiment, the germanium was enriched to a 76 Ge content of 86% . These detectors come from the former Heidelberg-Moscow experiment and the IGEX experiment. For phase II, the detector system was expanded to include newly acquired germanium. The detectors are located in a tank with liquid argon, which serves as a coolant and as a shield against natural radiation sources. The cryostat for the liquid argon is in turn located in a water tank, which also serves as a shield, but also as a veto detector for cosmic muons.
To reduce the background effect , the experiment is carried out in the Laboratori Nazionali del Gran Sasso 1400 m underground.
In the future (as of 2019) GERDA is to continue to be operated as part of a larger, global collaboration called LEGEND, in which a total of 1000 kilograms of germanium-76 are to be used.
Results
From the data obtained between November 2011 and May 2013, a lower limit for the partial half-life of neutrino-free double beta decay of 2.1 • 10 25 years was obtained. This contradicts the results of the Heidelberg-Moscow experiment.
In phase II, the sensitivity was increased through improvements in equipment. The background radiation is therefore less than 10 −3 events / (keV kg year). This measurement phase can therefore be regarded as free of background up to an exposure of 100 kg * years. The latest (2018) minimum value of the partial half-life of the neutrino-free double beta decay of Ge-76 is 2 • 10 26 years (90% CL).
Web links
Individual evidence
- ↑ Journal of Physics G: Nuclear and Particle Physics, Vol 40 No. 3 (2013)
- ↑ J. Jochum and P. Grabmayr: Decay without neutrinos? , Physik Journal Vol. 18 (2019) Issue 4, pages 35–40
- ↑ GERDA collaboration, M.Agostini et al .: Results on neutrinoless double beta decay of 76Ge from GERDA Phase I . In: Phys. Rev. Lett. tape 111 , November 20, 2013, p. 122503 , doi : 10.1103 / PhysRevLett.111.122503 , arxiv : 1307.4720 .
- ↑ GERDA collaboration, M.Agostini et al .: Improved Limit on Neutrinoless Double-β Decay of 76Ge from GERDA Phase II . Phys. Rev. Letters 120 (2018) p. 132503