  Results of several double beta decay experiments, performed with the help of
low background crystal scintillators, are presented. In particular, the
half-life value of the two-neutrino double beta decay of 116-Cd has been
measured as 2.9 10^{19} yr, and the new half-life limit on the neutrinoless
double beta decay of 116-Cd has been established as >1.7 10^{23} yr at 90%,
which corresponds to a restriction on the neutrino mass <1.7 eV. New half-life
bounds on the level of 10^{17}-10^{21} yr were set for double beta processes in
64-Zn, 70-Zn, 106-Cd, 108-Cd, 114-Cd, 136-Ce, 138-Ce, 142-Ce, 160-Gd, 180-W,
and 186-W by using low-background CdWO4, GSO, and ZnWO4 crystal scintillators.
The claim of discovery of the neutrinoless double beta decay of 76-Ge [Mod.
Phys. Lett. A 16 (2001) 2409] was analyzed. The demands of the future high
sensitivity double beta decay experiments, aiming to observe the neutrinoless
double beta decay or to advance restrictions on the neutrino mass to < 0.01 eV,
were considered. Requirements for their sensitivity and discovery potential
were formulated. Two projects of double beta experiments with a sensitivity on
the level of 10^{26}-10^{27} yr (CAMEO and CARVEL projects) were discussed.
Scintillation properties and radioactive contamination of CaWO4, ZnWO4, CdWO4,
PbWO4, GSO(Ce), CeF3, yttrium-aluminum garnet doped with neodymium (YAG:Nd)
crystal scintillators were studied. Applicability of these scintillators to
search for double beta decay was discussed.
