text: Nine different candidates of nuclei are being considered in experiments to confirm neutrinoless double beta-decay: . They all have arguments for and against their use in an experiment. Factors to be included and revised are natural abundance, reasonably priced enrichment, and a well understood and controlled experimental technique. The higher the -value, the better are the chances of a discovery, in principle. The phase-space factor , and thus the decay rate, grows with .
  Experimentally of interest and thus measured is the sum of the kinetic energies of the two emitted electrons. It should equal the -value of the respective nucleus for neutrinoless double beta emission.
  The table shows a summary of the currently best limits on the lifetime of 0νββ. From this, it can be deduced that neutrinoless double beta decay is an extremely rare process, if it occurs at all.