Even though more than 50 years have passed since the experimental detection of the neutrino, many important properties of this outstanding lepton are still unknown.
It is the last elementary particle whose rest mast could not be measured yet, and because it is neutral it is also the only fundamental particle that may be its own anti-particle.
Observation of neutrinoless double beta decay can clarify both important questions at once. But the expected half-life of more than 1025 years for this decay is an ambitious challenge to experimental physics. Currently first experiments are under commissioning that will be able to probe this magnitude of half-lifes. If they succeed, it will be important to verify the results with other experiments using different isotopes and different approaches. If they do not succeed, again new approaches will be required.
COBRA provides the necessary properties to be an excellent candidate for such a successive experiment. It uses CdZnTe room-temperature semiconductor detectors that contain several double beta decay candidate isotopes, among them also beta+ emitters and two of the most promising isotopes, 130Te and 116Cd. 116Cd has a decay energy that lies even well above the naturally occurring gamma background. The comparatively new semiconductor material CdZnTe has received more and more interest in recent years. Consequently, there has been major progress with these detectors and an end of the boost of this technology is not conceivable.
The experiment is located in Italy, 150km North-East of Rome in Gran Sasso National Laboratory, LNGS. The laboratory is built approximately halfway through a 10.4km tunnel that passes underneath the Gran Sasso mountain, which is part of the Apennine mountain range. The highest mountain peak, the Corno Grande, is 2912m above sea level and sits just to the side of the laboratory. The quantity of rock above the laboratory is about 1400 m, which corresponds to 3600 m.w.e. of shielding against most cosmic rays.