I.A. Tupitsyna

Growth of

Large volume zinc tungstate crystal scintillators (up to 50 mm in diameter and 100 mm length) with improved energy resolution were developed as a result of advances in crystal growth. The energy resolution of one-centimeter cube ZnWO4 crystals showed energy resolution in the range 8-10% for 662 keV gamma line of 137Cs. A value of 13.7% has been obtained with a large sample Oslash44 times 55 mm. The radioactive contamination of a ZnWO4 26 times 24 times 24 mm detector was tested in the Solotvina Underground Laboratory. The obtained results demonstrate ability to use ZnWO4 in order to search for 2beta-decay of zinc and tungsten.

{\rm ZnWO}_{4}

This paper reports the results of measurements of optical transmittance, thermoluminescence, and radiation hardness of lead and cadmium tungstate (PWO and CWO, respectively) doped crystals. It was found that three-valent impurities increase the optical transmittance of PWO crystals but decrease the transparency of CWO crystals. The radiation hardness for both types of crystals sufficiently increases after doping. Different kinds of defects that provide the observed effects are discussed.

Crystal Scintillators for High Sensitivity

Tungstate and double tungstate crystals of high scintillation efficiency and detectors based on them are applied widely in the medical imaging and radiation monitoring because of their high sensitivity to the ionizing radiation, small radiation length, high radiation hardness, low afterglow level. In this work a possibility to broaden the application field of CWO crystals have been investigated by improvement of their spectrometric quality and decreasing of their afterglow level. CWO crystals with improved characteristics have been obtained (resolution for 137 Cs o8%, afterglow o0.02% after 20 ms). A possibility is considered to use these crystals for spectrometry of thermal and resonance neutrons, which is possible due to the presence of nuclei with large cross-section for these neutrons in the crystal lattice. Compounds of a new type based on Cd, La-containing double tungstates doped with rare earth elements have been synthesized, and their luminescent characteristics have been studied. Calculation and measurement results are presented for these crystals under thermal and resonance neutron activation. The results obtained show that such crystals are very promising for dosimetry and spectrometry of neutrons at thermal energies. r 2002 Elsevier Science B.V. All rights reserved.

2\beta

Luminescent properties of magnesium tungstate monocrystals grown by two different methods are studied. Only the exciton luminescence of these crystals themselves is observed. Temperature dependence of the low-energy range in the luminescence excitation spectra is described by the Urbach rule. Slope coefficient σ0 = 0.74 obtained from this dependence implies autolocalization of the excitons in MgWO4. The processes of electronic excitations relaxation are considered depending on the structure of valence band in MgWO4 and in other wolframites, ZnWO4 and CdWO4. In contrast to ZnWO4 and CdWO4, the d-states of the cation do not participate in formation of the MgWO4 valence band. Using the excitation spectra measured in the range of the fundamental absorption (4–20 eV), it is shown that this difference manifests itself in relaxation of electronic excitations and may be the cause of the relatively low light yield of MgWO4.

Experiments

Results are reported on the effects of gamma-radiation upon scintillation characteristics of CdWO 4 (CWO) single crystals with different defects. The defect structure before irradiation determines the radiation-induced changes of the CWO crystals. Radiation effects in CWO crystals were shown to be related to the formation of F-centers. In some CWO samples, the dose of 103 Gy caused destruction of defects responsible for induced absorption. Doping by Li leads to reduction of defects responsible for color and trapping centers in CWO crystals. The annealing in a reductive medium (H 2 ) or doping with Me3+ significantly increases radiation stability of CWO.

Influence of doping on radiation stability of scintillators based on lead tungstate and cadmium tungstate single crystals

Results are presented of our latest research, aimed at the development and study of oxide scintillation crystals with high scintillation yield and low intrinsic radioactivity. We report on the improvement of these properties for conventional scintillators, as well as on new promising crystals based on metal tungstates and molybdates. The results are discussed in view of applying these materials in cryogenic experiments searching for dark matter or neutrinoless double beta decay.