The Current Setup

In the original proposal of COBRA a detector array of 1600 CdZnTe cubes with a size of 1 cm3 each, as a first stage of the experiment, was suggested. Such a detector would have a detector mass of about 10 kg and a size of only 12x12x12 cm3. Additionally this setup would be highly scalable, more such arrays could be added, towards a bigger experiment.
To investigate the prospects of such an experiment, to gain knowledge about how to work with the detectors, what problems might occur and how to overcome them, it was started to build a smaller version of the proposed experiment in 2011, the COBRA demonstrator. Completed in 2013 this demonstrator has been under constant improvement and development while taking
data, since.

The demonstrator is an array of 64 cubical CdZnTe detector crystals with coplanar grids and a size of 1x1x1 cm3 each. The crystals are arranged in four layers, each layer hosting 4x4 of the cubes. The different layers were installed after each other, in the time span from 2011 to 2013. Each crystal has a mass of roughly 6 g, resulting in a total detector mass of
nearly 400 g.

The originally proposed crystal size of about 1 cm3 was based on the extent of the largest CdZnTe crystals one was able to produce at that time. The crystal grow techniques have been being improved since, however, and the production of larger crystals is possible by now. The larger such a detector crystal is, the higher the according intrinsic detection efficiency gets. For a large crystal it is more unlikely that the particles created in a decay leave the detector without depositing energy. Additionally, one discovered that the main background of the COBRA demonstrator originates from contaminations on the crystals’ surfaces. Thus a larger detector crystal, having a smaller surface compared to its volume, might show a better signal to background ratio.

That is why in 2018 a set of larger crystals was added to the experiment, the extended demonstrator, short XDEM . Each of those XDEM crystals has a size of 2x2x1.5 cm3. This addition of the XDEM nearly doubled the experiment’s detector mass.

 

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