1. Ivan Solskyy International Workshop on Double Beta Decay Searches Oct. 15, 2009 - Oct. 17, 2009 Seoul National University, Seoul, Korea

2. Main technologies: Growth of complex oxide single crystals by Czochralski method Mechanical and chemico-mechanical polishing of single crystals Technological developments are mainly focused on: Obtaining of large-sized single crystals with high structural perfection for solid state lasers, scintilators, acousto-electronics, acousto-optics, non- linear optics and different kinds of substrates Mechanical processing of single crystals (precision cutting, grinding, lapping, polishing, finishing and chemico-mechanical polishing) resulting in fabrication of working elements for different devices of modern electronics (substrates with epitaxial grade surfaces, acousto-optic and electro-optic devices, ultrasonic delay lines, frequency transducers, resonators, laser rods and slabs, microchip and disk lasers, solid state detectors of high energy radiation, etc.)

3. Application of the CaMoO 4 single crystal: Scintillation detectors to search for rare nuclear and subnuclear processes. Scintillation (cryogenic) detectors to search for double  decay and dark matter. Objects for investigation The CaMoO4 single crystals (60 mm in diameter, the cylindrical part of length 150 mm) grown by Czochralski technique in SRC “Carat” (Lviv, Ukraine) from platinum crucible

4. The laboratory of crystal growth includes 8 furnaces with RF heating for Czochralski growth of bulk single crystals of high temperature melting complex oxides with diameter up to 100 mm and length up to 50 cm Furnaces of the “Galaxie – III” type (Physitherm, France) modernized by “Carat”

5. The laboratory of row materials preparation Furnaces for charge synthesis and after growth annealing Equipment for raw materials milling and pressing

6. The need to use single crystals of calcium molybdate as active elements of ionizing radiation detectors emerged relatively recently in connection with the execution of projects studying dark matter. A characteristic feature of scintillators based on the calcium molybdate crystals is their rather high light output and high radiation and chemical stability. Transparency of the crystal in a wide range of wavelengths and high thermal conductivity have allowed to use it as a matrix for laser. Ability to obtain satisfactory separation of power, high performance log of the resistance to climatic and mechanical factors allows the use of Calcium molybdate single crystals where the use of hygroscopic and less mechanically stable alkali-halogen crystals is impossible.

7. We provided growing crystals of calcium molybdate by Czochralski method in a platinum crucible from pre-synthesized batch. Getting charge for growth is based on the solid phase synthesis reaction between molybdenum oxide and calcium carbonate. It should note especially that in the process of synthesis an evaporation of component Molybdenum oxide may take place. This causes a violation of stoichiometric composition of the grown crystal and is one of a reason of the defect formation. To achieve high Calcium molybdate scintillation parameters it is important to establish the relationship between optical, lighting characteristics and concentration as well as type of crystal lattice defects. To study this relationship we have investigated an optical absorption and light output of scintillation elements fabricated from Calcium molybdate crystals grown in batch with different content of molybdenum oxide, concentration of which was changed by addition of certain amount for every process of growth.

8. Optimal process in terms of achieving the greatest values of the optical transparency and light output is growing crystals of calcium molybdate from double crystallized raw with correction of charge towards 1.0 weight percent of Molybdenum oxide surplus. Also we should note that the presence of optically active centers associated with lattice defects significantly affect the scintillation parameters of crystals.

9. In crystals grown from the charge with lack of more volatile component of Molybdenum oxide alone length of the crystal boule many macrodefects with the size of 10-30 microns are observed. These crystals are not suitable for practical application, as the presence of inclusions leads to an increase in light scattering and deterioration of the detector energy resolution. Addition of Molybdenum oxide surplus (1-1.5 percent) to melt improves the optical quality of crystals, but does not lead to complete disappearance of macrodefects. At the same time making a strictly stoichiometric charge don’t allow to obtain single crystals of optical quality, because overheating of the melt near crucible walls causes violation of stoichiometry. In particular, high-quality crystals with a light output that is 30 percent or more relative Natrium Iodine Thallium, can be obtained only with the use of mixtures containing impurities less than 10 -4 (ten in minus four degree) weight percent.

10. An important factor that determines the quality of oxide crystals grown by Czochralski method, is a temperature gradient on the crystal-melt interface. This gradient in conjunction with other parameters (crystal pulling rate, the ratio of the crucible diameter to crystal one) determines the form of interphase boundaries, the amplitude and frequency of temperature oscillations, processes microsegregation of impurities. It appears that radical means of avoiding defects and modularity is to prevent supercooling melt at the initial stage of growth. This is because the mechanism of formation of defects is mainly related with disparity of habitus of growing crystal and internal symmetry in the melt in supercooling state. Preventing of the melt supercooling clearly settles to avoid appearance of macroincludings, reduce blocks and also significantly reduce the content of the scattering centers.

11. While growing, due to high temperature, Mo 6+ (molybdenum six plus) is restoring and appears colouring of crystals. We can avoid this by creating in raw material small surplus of Molybdenum oxide over stoichiometry. Coloration can be reduced also with annealing crystals in the oxygen atmosphere. In Calcium tungsten scintillator anisotropy of the light output is revealed: in the direction of optical axis, its value turned out to be the lowest compared to other crystallographic directions. A few words about the technological methods that were used to obtain high-quality crystals of calcium molybdate.

12. During development of technological process of growing Calcium molybdate crystals of large size (diam. 60 mm and a length of cylindrical part 150 mm) we took into account the following factors: Content of impurities Fe (ferrum), Ni (nickel), Cr (chromium), Cu (cuprum), Co (cobalt) in the feedstock should not exceed 10 -4 (ten in minus four degree) weight percent. Chemical composition of raw material should take into account the evaporation of Molybdenum oxide in the growing crystal. Technological conditions of batch mixing and synthesis should provide a monophase compound Calcium molybdate. 

13. Axial temperature gradient should provide a smooth convex shape of interface phase (crystallization front) during the whole process of growing crystal. The design of the thermal unit should provide maximum thermal field symmetry and the minimum radial gradients of temperature and equilibrium annealing of crystal after growth. Crystal pulling rate is set in view of lowering the level of melt for the crystal growth rate to be constant. Rotation rate of crystal should vary with regard to changes in heat from the crystallization front during the growth process.

14. During our researches a thermal unit was designed and manufactured, which gave the opportunity to optimize a set of factors that ensure the stability of thermal conditions and crystallization front. Scheme of the thermal unit for growing Calcium molybdate crystals: 1.ceramic rod, 2.quartz cylinder, 3.Alumina Ceramics, 4.Platinum test tube, 5.Platinum screen, 6.Crystal, 7.Platinum crucible, 8.Melt, 9.Inductor, 10.Metal table.

15. Platinum crucible and active thermal screen with diameters 150 mm create the thermal unit basis. Crucible and screen are surrounded by three layers of insulation Alumina Ceramics, which allowed to establish a working volume of required axial and radial temperature gradients. Single crystals were grown on air on oriented seed priming. The direction of growth [001] lies in the plane of intense shear. Grown rate of Calcium molybdate single crystal growing during the whole process was kept till 3 mm/h (mm per hour) and the pulling rate was calculated by computer on basis of weighing the growing crystal, crystal density and the melt and automatically monitored the lowering of the level of the melt. We investigated the optical absorption, light output and decay kinetics at different orientations of incident radiation beam and crystal axes.

17. The figure represents the difference absorption spectrum of Calcium molybdate single crystal in the direction [001], which coincides with the direction of crystal growth (Z) and crystal annealed in oxygen atmosphere. Low-intensity band of absorption of about 0.8 cm -1 (zero point eight cm in minus one degree) in the maximum near 500 nm extends from 400 nm to 1100 nm. This is the band responsible for the gray-blue coloring of freshly grown Calcium molybdate crystals and may be associated with defects that arise during the growth. In the difference spectrum of the grown and annealed Calcium molybdate crystals for the direction of X (Y) the emergence of an additional maximum with a ~ 0.1 cm -1 (approximately zero point one cm in minus one degree) at a wavelength of 500 nm is observed.

18. Thus, we can conclude that annealing of Calcium molybdate crystals in the oxygen atmosphere for 100 - 150 hours at temperature more then 1200 C (one thousand and two hundred degree centigrade) leads to the disappearance of anisotropy of optical absorption. After annealing the crystals in flow oxygen atmosphere decay time increases to 19 ms and practically does not depend on orientation. Summarizing all above, we can conclude that annealing of Calcium molybdate crystals in the oxygen atmosphere leads to the disappearance of anisotropy of the scintillation characteristics.

22. Direction of  irradiation  /  ratio Average decay time [001] 0.201  0.00420.5  0.8  s [100] 0.197  0.00420.7  0.7  s [010] 0.194  0.00520.5  0.8  s

23. Thank you for attention!