To summarise, in recent years the allowed
decay has been observed in several nuclei. QRPA and shell model calculations
reproduce fairly well the measured half lives. This gives some confidence
that the matrix elements needed to interpret the experimental limits on
decay can be calculated with reasonable reliability as well. Data from
the most sensitive experiments on that decay lead to the limit eV
for the effective neutrino mass. The Heidelberg-Moscow and IGEX experiments
on 76Ge are continuing. They may achieve sensitivities of order yr
in 76Ge, corresponding to eV.
The 20 crystal bolometer array being built by the Milano group will presumably
achieve comparable sensitivity, provided background problems can be solved.
The NEMO III detector, for which NEMO II was a prototype, is being built
and will be installed in the Fréjus lab. It is based on the same
principles, but has a much larger source foil, and has a magnetic field
for electron-positron identification. It is designed to study primarily
100Mo, of which it can accommodate 10 kg. Because of the high
sensitivity, but also the modest energy resolution (15 % FWHM) and the
half life in 100Mo, the major background source when looking
decay will be
decay! After a few years of data taking, a sensitivity of order yr
should be achieved corresponding to eV.
The MUNU detector, a CF4 gas TPC built for the study of neutrino-electron
scattering at the Bugey nuclear reactor, could also be used to search for
double beta decay. Installed underground, filled with 10 kg of enriched
136Xe, it would achieve a sensitivity comparable to that of
NEMO III. One can thus conclude that presently running or planned experiments
will explore effective neutrino masses down to, say, 0.3 eV. Independently
of what they find, it will be desirable to push further the sensitivity.
To achieve this it is necessary to go to yet larger source masses. Larger
detector sizes can be considered. Current double beta decay detectors,
even considering NEMO III, are small by many standards. For instance, solar
neutrino detectors are significantly larger. Also it does not seem impossible
to produce enriched sources with masses as large as 100 kg. To take full
advantage the background will need to be reduced in parallel. This seems
possible with the experience gained in present generation double beta decay
experiments and solar neutrino experiments. Also detector technology has
brought advances leading to better event signature. Therefore a next generation
experiment with a source of 100 kg of some enriched material appears feasible.
It would have a sensitivity to
of order 1026 yr, corresponding to 0.1
eV. Such a project can however only be carried out by a large collaboration.
Several of the groups presently involved in various smaller experiments
would have to join efforts. To go further, it seems impossible to envision
enriched sources. This means that much larger detector masses, and still
better event identification are necessary. One idea put forward is to use
a liquid TPC filled with natural Xe, in which not only the electrons are
detected, but also the positive ions. Much development needs however to
be done before the construction of a full scale detector can be considered.
of the data NuPECC WebForce,