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8 2 Neutrino physics
mi:
〈mνe〉 = | �
i
U 2 eimi| (2.9)
The decay rate of 0νββ-decays scales only with Q 5 in comparison to the
Q 11 dependence for 2νββ-decays [1].
2.3 Experimental status
The double beta decay studies were first made with geochemical methods,
searching for an excess of ββ-decay products in old ores. These
inclusive method is not able to distinguish 2νββ and 0νββ-decays.
Exclusive experiments on 0νββ can distinguish both decay modes and
are of two different concepts:
source = detector
source �= detector
Calorimeters follow the source = detector concept. They can achieve
good energy resolutions, detect the sum of both electron energies, have
a very high detection efficiency and can improve the sensitivity by increasing
the detector material.
Tracking detectors are representative for the source �= detector concept.
Their advantage is the possibility of identifying separately both
electrons, which improves the background level. The drawback is a
low detection efficiency and the circumstance that electrons loose already
a fraction of their energy in the source material, before leaving it.
Tracking detectors achieve also poor energy resolution in comparison
with calorimeters. The source material has often the form of thin foils
to improve the detection efficiency, but also complicate the increase of
the mass of the source material.
About a dozen 2νββ-decays have been measured yet, using both
detector concepts. Two of them, measured with the NEMO-3
detector (source �= detector) are the for COBRA relevant half
lives of 130 Te (T 2ν
1/2
= 7.0 ±1.0
0.8
(T 2ν
1/2 = 2.88 ± 0.04 ± 0.16 × 1019 years) [11].
(stat.) ±1.1
0.9 (sys.) × 1020 years) and 116 Cd