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2018
The report describes the current status of the COBRA setup at the Laboratori Nazionali del Gran Sasso (LNGS) and analysis aspects of recent data.


COBRA - the Cadmium Zinc Telluride 0-Neutrino Double-Beta Research Apparatus

More than 60 years have passed since the experimental detection of the neutrino. And yet, some of its properties still remain unknown. The most pressing question probably is the one after the value of the particle's rest mass. And is the neutrino to be categorized as a Dirac or a Majorana particle?

An investigation of the theorized neutrinoless double beta decay (0νββ decay) has the potential of answering both those questions. Such an investigation, however, puts up an ambitious challenge. Due to the extremely long half-lives of at least about 1024 years predicted for such a decay, both a very high amount of the decaying isotope as well as a background basically reduced to zero are necessary, if one wants to have a chance of observing the decay.

Many different experiments, using different potentially decaying isotopes and different detector approaches, are currently searching for the decay. One detector approach proposed in 2001, is to use CdZnTe crystals. The material contains nine isotopes, which are capable of different double beta decay modes, and at the same time such a crystal, equipped with an anode and a cathode, can function as a room temperature semiconductor detector, making it possible to detect potential decays. If one of the bigger 0νββ experiments succeeds in its endeavors, it will be important to verify the results with other experiments using different isotopes and different detector approaches. CdZnTe crystals would be an excellent candidates for such a successive experiment.

Since 2013 the COBRA demonstrator setup has been in operation with the goal to investigate the experimental issues of operating CdZnTe detectors in low background mode and to explore this technique's prospects. It is built from 64 monolithic CdZnTe detector crystals with a size of 1x1x1 cm3 each, arranged in an array of 4x4x4 crystals. Based on the knowledge gained from the work with this demonstrator, the experiment has been upgraded to COBRA XDEM (short for eXtended DEMonstrator) in 2018. Nine additional, larger CdZnTe crystals with a volume of 6 cm3 each are now under operation, almost doubling the active volume of the experiment and expected to show significantly reduced background levels.

The setup is located at the Laboratori Nazionali del Gran Sasso (LNGS), an underground laboratory beneath the Apennine Mountains and hence covered by 1400 m of rock. This provides an effective shielding of the experiment from cosmic rays.

 

 
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