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The COBRA Idea

In order to perform a search for the 0νββ decay, a material containing an isotope which is expected to perform such a decay is needed. Additionally some kind of detector is necessary. One needs to have a way of noticing decays in the source material and characterizing them. CdZnTe serves both purposes at the same time. It contains nine isotopes capable of double beta decays, but can also be used to build a semiconductor detector.

CdZnTe contains five isotopes capable of β-β- decays and four isotopes which can transition with a β+β+ decay mode. An overview of those decays possible in the detector material is given in the table. Of special interest are the three following isotopes:
116Cd and 106Cd both have the advantage, that the Q-values of their decays lie higher than the energy of the most energetic natural background line. This background is caused by a photon with an energy of 2614 keV emitted by the nucleus after the β- decay of 208Tl and the following deexcitation into the ground state or the α decay of 212Po to 208Pb. Both processes are part of the natural 232Th decay chain. 106Cd is also interesting because it is one of the very few isotopes which are capable of all three β+β+ decay modes. The third very interesting isotope contained in CdZnTe is 130Te. Its Q-value does not lie above the 2614 keV γ-line, but between this line and its according Compton edge, thus it still lies in a region of a potentially very low background.
The other contained ββ emitters play a less important role due to their lower Q-values, which lead to a higher background as well as bigger half-lives.
The decay of 113Cd is well separated from the higher Q-value region and dominates the background at low energies, but is also of interest itself. 123Te only emits a low energetic X-ray during its transition, which lies below the energy threshold of the detector, which is different for every crystal and lies at values of about 100 keV, and thus does not cause any severe background.

CdZnTe is a semiconductor and can thus, equipped with two electrodes, be used as a semiconductor detector. Semiconductors in general offer good energy resolutions, which is vital for a double beta decay experiment like COBRA. Additionally CdZnTe detectors operate at room temperature, allowing an experiment which does not suffer the complication of cooling. For an experiment housed underground in a remote laboratory this characteristic is extremely valuable. CdZnTe detectors are produced commerially - usually for radiation detection in medicine, baggage scanning or bomb detection - and thus are easily available as well as under constant further development.

One layer of the COBRA detector array, containing 4x4 CdZnTe crystals.

The idea of the COBRA detector geometry is, to use an array of multiple such CdZnTe detector crystals. In this manner one is able to exploit to some degree the advantages of two basic detector concepts. On the one hand the detector crystals are both the source of decays as are they able to detect them; source and detector can be the same. This results in a high detector efficiency. On the other hand it is also possible for a decay to be detected in a different crystal than the one it originated from. In this case source and detector are separate objects. This makes it possible to gain information about single particles’ energies and the direction they are heading into, thus the kinematics of the decay. The COBRA detector hence is able to exploit to some degree the advantages of both "source equal detector" and "source unequal detector".

 

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