Institute for Scintillation Materials Growth conditions of radiopure ZnWO4 scintillation crystals (Short presentation) L.Nagornaya Institute for Scintillation Materials ISMA RPSCINT for EURECA 2009 , Kyiv, 22-23 September, 2009

ISMA oxide scintillators to search for rare processes CdWO4 2 106,108,114,116Cd, 180W,186W ZnWO4 2 64,70Zn, 180W,186W; DM PbWO4 active shield & light guides for 116CdWO4 PbMoO4 2 100Mo CaMoO4 2 100Mo; DM ZnMoO4 2 100Mo, 64,70Zn MgWO4 DM ISMA RPSCINT for EURECA 2009 , Kyiv, 22-23 September, 2009

Requirements to scintillators a high light output at milli-Kelvin temperature an extremely low level of radioactive contamination presence of specific nuclei (2b decay) (for example 64,70Zn, 180,186W, 92,98,100Mo) a variety of elements in scintillation targets (Dark matter) ISMA RPSCINT for EURECA 2009 , Kyiv, 22-23 September, 2009

ZnWO4 for 2 and DM Large volume ZnWO4 crystal scintillators up to 50 100 mm 130 mm ZnWO4 is one of the best candidate as a detector for cryogenic 2 decay and Dark Matter experiments ! ISMA RPSCINT for EURECA 2009 , Kyiv, 22-23 September, 2009

L. L. Nagornaya et al, “Growth of ZnWO4 crystal scintillators for high sensitivity 2 experiments ” IEEE Trans. Nucl. Sci. 55 (2008) 1469 L.Nagornaya et al, “Large volume ZnWO4 crystal scintillators with excellent energy resolution and low background” IEEE Trans. Nucl. Sci. 56 (2009) 994 L.Nagornaya et al, “Tungstate and Molybdate Scintillators to Search for Dark Matter and Double Beta Decay ” IEEE Trans. Nucl. Sci. 56 (2009) 2513 P. Belli, et al “Search for 2b decay of Zinc and Tungsten with the help of low-background ZnWO4 crystal scintillators” Nuclear Physics A826 (2009) 256 Our joint publications with Oxford University (Great Britain), Gran Sasso National Laboratory (Italy) and the Institute for Nuclear Studies (Kiev, Ukraine) permit to make a conclusion that ZnWO4 single crystals are one of the most promising material for studies of rare events ISMA RPSCINT for EURECA 2009 , Kyiv, 22-23 September, 2009

Excellent spectrometric quality The energy resolution of ZnWO4 scintillation elements was 8.5 % with 1 cm3 sample and 10.7 % with a hexagonal scintillation element 40  40 mm for 662 keV  line of 137Cs L.Nagornaya et al, “Large volume ZnWO4 crystal scintillators with excellent energy resolution and low background” IEEE Trans. Nucl. Sci. 56 (2009) 994 ISMA RPSCINT for EURECA 2009 , Kyiv, 22-23 September, 2009

ZnWO4 temperature dependence of light output and decay time The light output of ZnWO4 demonstrates about three-fold increases and decay time becomes about three times longer with decrease of temperature from 300 K to 7 K. ISMA The relative light output of ZnWO4 at 10 K is 110-115 % that of CaWO4 EURECA 2009 , Kyiv, 22-23 September, 2009

ZnWO4 - an excellent radiopure ZnWO4(1) CaWO4(2) 232Th 0.0015 0.7 235U  0.003 1.6 238U 0.002 300 Total  0.2 400 40K  0.4 65Zn 0.5 90Sr-90Y  0.4 1)P.Belli et al., Nuclear Physics A826 (2009) 256 2)NIMA 538 (2004) 657; APP 23 (2005) 249 ZnWO4 is one of the best candidate as a detector for cryogenic 2 decay and Dark Matter experiments ! ISMA EURECA 2009 , Kyiv, 22-23 September, 2009

Radiopurity of this crystal is rather low but it exceeds by twice the requirements (0,01mBq/kg). In this connection a decision was taken to study the possibility to low the radiopurity by recrystallization of the crystal after the first growth. ISMA EURECA 2009 , Kyiv, 22-23 September, 2009

Activity in ZnWO4 (mBq/kg) after recrystallization Chain Nuclide Activity in ZnWO4 (mBq/kg) first growth (699 g) after recrystallization (142 g) 232Th 228Th  0.1 0.002(1)  0.04 0.002(2) 235U 227Ac  0.003  0.01 238U 230Th 226Ra 210Po  0.08  0.07  0.06  0.2  0.14 0.022(15) Total ‑activity 0.18(3) 0.61(4) Data of radioactive contamination of these samples are presented in the table. Contamination produced by recrystallization increased one order of magnitude for 226Ra and ≈ 3 times for the total α activity of U/Th chains. Increasing of 228Th activity was not observed. Dr. Poda will report the obtained results while his presentation ISMA EURECA 2009 , Kyiv, 22-23 September, 2009

For the analysis of possible sources or reasons of the crystal’s radioactive contamination after recrystallization at the instance of the organizers of the Workshop I will briefly describe different stages of technological process of the growth of ZWO4 crystals, the synthesis of charge, the processing and the manufacturing of scintillators. ISMA RPSCINT for EURECA 2009 , Kyiv, 22-23 September, 2009

Crystals are grown by Czochralski method in oxygen-containing medium furnace for growth is presented in the figure 1 5 1-ceramic (corundum) discs, screens, etc.; 2-platinum crucible, 3- platinum screens, 4-high-temperature induction heating (copper inductor) 5-platinum and leucosapphire seeding holder. 3 2 4 ISMA EURECA 2009 , Kyiv, 22-23 September, 2009

Thermal annealing is carried out in furnaces. ZnWO4 charge is produced by high-temperature solid phase synthesis from corresponding metals oxides Synthesis in air or oxygen is effected in platinum container in KC-520 or KU-14 furnaces. Thermal annealing is carried out in furnaces. Materials: ceramic tube, coiling – nichrome, kantal, Pt container ISMA EURECA 2009 , Kyiv, 22-23 September, 2009

Processing of the crystal turning lathe while cutting there is some transformer oil diamond discs for cutting grinding and polishing of crystals diamond powders 28/2 and 5/3 the powders are mixed with water. ISMA EURECA 2009 , Kyiv, 22-23 September, 2009

Any data on radioactive contamination of used materials are not available. While receiving the charge special protective measures from Radon were not taken. In the process of growth the contamination of the crystal with Pt is possible. ISMA EURECA 2009 , Kyiv, 22-23 September, 2009

Conclusion We would appreciate if specialists studying radioactive contamination give their suppositions as for possibility and ways of contamination of the crystal while recrystallization. Control of contamination and special clearance of the charge or source material is needed. ISMA EURECA 2009 , Kyiv, 22-23 September, 2009

The measurements of ZnMoO4 at milli-Kelvin temperature ZnMoO4 crystal mounted on copper cryostat. Scatter plot of the light signal versus heat signal for background exposition with ZnMoO4 crystal. Contamination by 210Po (suppose in equilibrium with 210Pb) is 360 mBq/kg, activity of 226Ra was determined as 56 mBq/kg by analysis of  events in the data. (Stefano Pirro, INFN - Sezione di Milano Bicocca I-20126 Milano - Italy)