a design study for a cobra upgrade to - Institut für Kern- und ...

12 3 The COBRA experiment
CdZnTe (CZT) contains nine double beta decay isotopes, which are
listed in table 3.1 with their natural abundance, the possible decay
mode and the Q-value.
Isotope Q (keV) nat. ab. (%) decay mode
64 Zn 1096 48.6 EC/EC, EC/β +
70 Zn 1001 0.6 β − β −
106 Cd 2771 1.3 EC/EC, EC/β + , β + β +
108 Cd 231 0.9 EC/EC
114 Cd 534 28.7 β − β −
116 Cd 2805 7.5 β − β −
120 Te 1722 0.1 EC/EC, EC/β +
128 Te 868 31.7 β − β −
130 Te 2529 33.8 β − β −
Table 3.1: Double beta isotopes of relevance for the COBRA experiment,
with Q-value, natural abundance and possible decay mode given
according to Kiel [8].
The most promising isotopes for the COBRA experiment are 116 Cd and
130 Te. Their decays have a high Q-value, which is favourable for the
expected event rate as it varies with Q 5 for 0νββ. Additionally, 116 Cd
has, with 2805 keV, an endpoint energy well over the highest occurring
natural gamma line at 2614 keV from the 232 Th decay chain [16]. The
Q-value of 130 Te is between this line and its Compton edge. This leads
to a reduced background for their expected 0νβ − β − ground state transitions.
A natural high abundance of 130 Te provides also relatively high
source mass, already without using isotopically enriched material. CZT
contains also 106 Cd, which is one of only 6 known β + β + decay isotopes.
CdZnTe crystals are intrinsic II-VI semiconductor crystals with a zinc
blende structure. They are produced by the High Pressure Bridgman
Method with low intrinsic contamination, but also with a limited detector
size of a few cm 3 . This requires a modular design for reaching
high source mass, for which the collaboration investigates the advantages
of a coincidence analysis. Owing to a large band gap of 1.6 eV,
CZT detectors can be operated at room temperature, which avoids extensive
cooling.
Currently, different CZT detector designs are under investigation.
These are coplanar grid CZT detectors, which will be discussed further
on and two different pixelated designs, which show promising possibilities
of a very effective background reduction. For the recent activities