477 DEVELOPMENT OF ENRICHED CADMIUM TUNGSTATE CRYSTAL SCINTILLATORS TO SEARCH FOR DOUBLE BETA DECAY PROCESSES IN 106 Cd P. Belli 1 , R. Bernabei 1 , R. S. Boiko 2 , V. B. Brudanin 3 , R. Cerulli 4 , F. A. Danevich 2 , S. d’Angelo 1 , A. E. Dossovitskiy 5 , B. V. Grinyov 6 , A. Incicchitti 7 , V. V. Kobychev 2 , G. P. Kovtun 8 , A. L. Mikhlin 5 , V. M. Mokina 2 , L. L. Nagornaya 6 , S. S. Nagorny 2 , S. Nisi 4 , R. B. Podviyanyuk 2 , D. Prosperi 7 , D. A. Solopikhin 8 , V. I. Tretyak 2 , I. A. Tupitsyna 6 , A. P. Shcherban 8 , V. D. Virich 8 1 Dip. di Fisica, Universita’ di Roma “Tor Vergata” and INFN, Rome, Italy 2 Institute for Nuclear Research, National Academy of Sciences of Ukraine, Kyiv, Ukraine 3 Joint Institute for Nuclear Research, Dubna, Moscow region, Russia 4 INFN, Laboratori Nazionali del Gran Sasso, Assergi (AQ), Italy 5 Joint stock company NeoChem, Moscow, Russia 6 Institute for Scintillation Materials, National Academy of Sciences of Ukraine, Kharkiv, Ukraine 7 Dip. di Fisica, Universita’ di Roma “La Sapienza” and INFN, Rome, Italy 8 National Science Center “Kharkiv Institute of Physics and Technology”, Kharkiv, Ukraine R&D of cadmium tungstate (CdWO 4 ) crystal scintillators with cadmium enriched in 106 Cd is in progress with aim to realize an experiment to search for double beta processes in 106 Cd. Samples of cadmium (with natural isotopic composition and enriched in 106 Cd) were purified by vacuum distillation. Cadmium tungstate compounds (natural and enriched) to grow CdWO 4 crystals were synthesized from solutions. Contamination of the cadmium samples and synthesized compounds were measured by mass spectrometry. Concentration of iron was determined by atomic absorption spectroscopy. CdWO 4 crystal boule was grown by Czochralsky method from the natural cadmium tungstate compound. The total losses of cadmium on the stages of purification, raw material synthesis, crystal growth and scintillator production do not exceed 4 %. An excellent energy resolution FWHM = 8.1 % was measured by ∅22 × 29 mm CdWO 4 scintillator for 662 keV γ quanta of 137 Cs source. 1. Introduction Observations of neutrino oscillations manifest the non-zero neutrino mass and provide important motivation for high sensitivity experiments to search for neutrinoless double beta (0ν2β) decay. However, this process still remains unobserved, and only half-life limits for 0ν2β mode were obtained (see, e.g., reviews [1 - 9]). One of the most sensitive 2β experiments has been performed in the Solotvina Underground Laboratory with the help of enriched cadmium tungstate ( 116 CdWO 4 ) crystal scintillators [10, 11]. The half-life limit on 0ν2β decay of 116 Cd was set as T 1/2 ≥ 1.7 × × 10 23 yr at 90 % C.L., which corresponds to an upper bound on the effective Majorana neutrino mass 〈m ν 〉 ≤ 1.7 eV [11]. This result is among the strongest world-wide restrictions on 〈m ν 〉. The Solotvina experiment demonstrates that CdWO 4 crystals possess several important properties required for high sensitivity 2β decay experiments: low level of intrinsic radioactivity, good scintillation characteristics, pulse-shape discrimination ability, low cost, and stability of operation. We would like to mention that in [12] it was discussed the potential to develop CdWO 4 thermal bolometers with energy resolution ≈ 5 keV in a wide energy interval, and that in [13] the possibility to use CdWO 4 crystals as scintillating bolometers was demonstrated. Another application of CdWO 4 scintillating crystals is search for double β processes in 106 Cd. One of the highest for 2β + nuclides value of Q 2β = 2770(7) keV allows three modes of decay: 2β + (decay with emission of two positrons), εβ + (electron capture with emission of positron) and 2ε (double electron capture). In fact, 106 Cd is among the most investigated 2β + nuclides. Half-life limits on the level of 10 15 yr could be extracted from the old (1952) underground measurements of Cd sample with photographic emulsions [14]. Search for positrons emitted in 2β + decay was performed in 1955 with the Wilson cloud chamber in the magnetic field and Cd foil (30 g); this resulted in limit of 10 16 yr [15]. In measurements of 153 g Cd sample during 72 h with two NaI scintillators in coincidence, half-life limits on the level of ~10 17 yr were determined for 2β + , εβ + and 2ε processes in [16]. Data of the experiment with 15 cm 3 116 CdWO 4 crystal scintillator enriched in 116 Cd to 83%, performed in the Solotvina Underground Laboratory (1000 m w.e.), were used to set limits on 2β decay of 106 Cd on the level of 10 17 - 10 19 yr [17]. In the experiment [18], 331 g of Cd foil was measured in the Frejus Underground Laboratory (4800 m w.e.) with the 120 cm 3 HP Ge detector during 1137 h; γ quanta from annihilations of positrons and from de-excitations of the daughter 106 Pd nucleus were searched for. Half-life limits on the level of 10 18 - 10 19 yr were reached. In [19], a large (1.046 kg) CdWO 4 scintillator with natural Cd composition was measured in the Gran Sasso Underground Laboratory (3600 m w.e.) during 6701 h. Determined limits on half-life for 2β + and εβ + decays were on the level of ~10 19 yr for 0ν, and 10 17 yr for 2ν processes. Small (0.5 g) CdTe crystal was tested as bolometer in 1997 [20]; achieved sensitivity was ~10 16 y for 0ν2β + decay. An experiment using enriched in 106 Cd to 68% cadmium sample (154 g) was performed in 1999 [21]. Measurements in the Gran Sasso Underground Laboratory with two low background NaI(Tl) scintillators during 4321 h gave possibility to reach level of sensitivity of more than 10 20 yr for 2β + , εβ + and 2ε processes. The long-term (14183 hr) experiment in the Solotvina Underground Laboratory with 3 enriched 116 CdWO 4