  Zinc tungstate (ZnWO$_4$) crystal scintillators are promising detection
material for the experiments searching for double beta decay, dark matter, and
investigating rare alpha decays. An extended R&D was performed to develop
advanced quality ZnWO$_4$ crystal scintillators. The R&D programme included the
selection of the initial materials, the variation of the compound
stoichiometry, the application of single and double crystallization, and the
annealing of the crystal boules. The optical transmittance of the produced
boules was measured, and the luminescence under X-ray excitation in the
temperature region from 85 K to room temperature was studied (thermally
stimulated luminescence was measured till 350 K). The energy resolution and the
relative scintillation pulse amplitude were measured with gamma-sources
demonstrating high scintillation properties of the samples produced by single
crystallization from deeply purified zinc and tungsten oxides, with
stoichiometric composition, annealed in air atmosphere.
