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2.3 Experimental status 9
The 2νββ-decay to the first (2 + ) excited state has not been measured
for 116Cd yet. The experimental limit for this decay mode
is T 2ν
1/2 > 2.4 × 1021 years (90% C.L.) and expected half lives were reported
as T 2ν
1/2 = 2.0 × 1023 years [12]. The neutrino accompanied double
beta decay is of special interest for the testing of nuclear matrix
element calculations. Transitions to ground and excited states (0 + , 2 + )
can probe different aspects of matrix element calculations.
A variety of experiments is investigating different promising double
beta isotopes. Especially isotopes with already high natural abundance,
with expected low half lives, due to a high Q-value and a large nuclear
matrix element are the most promising isotopes for a first double
beta decay detection. Calorimetric experiments used so far 48 Ca, 76 Ge,
116 Cd and 130 Te. The spectroscopic detectors NEMO-3 and ELEGANT
investigated isotopes with high Q-value like 82 Se, 100 Mo and 116 Cd.
The best result for the neutrinoless double beta decay was so far
achieved by the Heidelberg-Moscow collaboration, presenting a halflive
limit of T 0ν
1/2 > 1.9 × 1025 years at 90 % C.L [13]. A subgroup of this
collaboration claimed the observation of the 0νββ decay of 76 Ge with
T 0ν
1/2 = (0.69 − 4.18) × 1025 years [14].
Currently, there are a lot of double beta decay experiments under
progress, some like GERDA already have started or will soon start data
taking to prove or disprove the Heidelberg-Moscow claim.
The two large scale Ge-experiments are GERDA and MAJORANA.
GERDA plans to use 18 kg of high-purity enriched 76 Ge in phase one
(already mentioned) and to add another 20 kg in the next phase.
MAJORANA will use 60 kg Ge detectors. Both experiments plan to
merge for a future ton-scale experiment by using the best developed
and tested techniques from the former experiments.
CUORE is an expansion of CUORICINO, currently under construction.
They will use nearly one ton of TeO crystals for investigating the 0νββ
decay of 130 Te.
The SNO+ collaboration (also starting soon) is planning to use 0.1 %Nd
in their one kton scintillation detector.
Other planned experiments are for instance CANDLES ( 48 Ca),
MOON( 82 Se), COBRA ( 116 Cd), EXO ( 136 Xe) and SuperNEMO ( 82 Se)
[15].