D.G. Desai

Effect of non-stoichiometry on some properties of lead tungstate single crystals

The effect of stoichiometric deviations on the optical absorption, gamma rays induced absorption and thermoluminescence (TL) from PbWO4 (PWO) single crystals has been investigated. The stoichiometric deviations are found to develop yellowish coloration in this crystal. This problem has been overcome by purifying the starting powder for minute stoichiometric deviations through repeated crystallisations. In the case of non-stoichiometric crystals a broad absorption over the wavelength range 400–500 nm is observed. This absorption gets further enhanced when the crystal is subjected to γ-ray irradiation. Whereas, the stoichiometric crystals, which are colourless, are found not to develop coloration for gamma exposures as high as 10 Mrad. The presence of weak TL in both types of crystals is observed for the radiation exposures of 10 Mrad. For colourless crystals, only one TL glow peak at 170°C is recorded over the temperature range 30–400°C. A shift in the glow peak temperature towards the lower temperature side is observed in the case of non-stoichiometric crystals. This result can be used to detect minute stoichiometric deviations in PWO.

Growth and characterization of lithium yttrium borate single crystals

Single crystals of 0.1% Ce doped Li6Y(BO3)3 have been grown using the Czochralski technique. The photoluminescence study of these crystals shows a broad emission at ∼ 420 nm corresponding to Ce3+ emission from 5d→4f energy levels. The decay profile of this emission shows a fast response of ∼ 28 ns which is highly desirable for detector applications.

Growth and luminescence properties of Ce doped Li6Gd(BO3)3 single crystals

Large size single crystals of undoped and Ce doped Li6Gd(BO3)3 oriented in direction have been grown using the Czochralski technique. The Transparency of these crystals was >85%. The Low temperature photoluminescence study of these crystals revealed that the temperature quenching of Ce3+ emission is compensated, to some extent, by an increased energy transfer between Gd3+ →Ce3+ with temperature.

Preparation and characterisation of radiation hard PbWO4 crystal scintillator

The selective loss of one of the crystal constituents is found to be responsible for the yellowish coloration of PbWO4 crystals. However, using the already pulled crystals as the starting charge for the subsequent growth, colorless crystals can be grown. The crystals exhibiting excellent transmission characteristics have been grown employing a low temperature gradient, a moderate rotation rate of 15 rpm and a pull speed of 1 mm/h. The colored crystals show some radiation damage on gamma irradiation, while the colorless ones remain unaffected even for irradiation doses as high as 10 Mrad. Both the types of crystals show the presence of weak thermoluminescence (TL) emission when high irradiation doses (∼10 Mrad) are given. Only one TL glow peak is obtained in both the cases but the peak temperatures are different. The emission centers responsible for the TL emission are found to be the ones which give rise to the scintillation emission in the crystal.

Scintillation yield uniformity studies on single crystals of Tl doped CsI

Single crystals of 0.1% Tl doped CsI (50 mm φ and 50mm L) have been grown using the Bridgman melt growth technique. Scintillation yield uniformity of radiation detector made from the as-grown crystals was checked using an 243Am α (5.25 and 5.5 MeV) source. It was found that yield variation along the radial direction was within 2% over 0-15 mm and was less than 5% for the whole crystal. A correlation between radial temperature profile of furnace and yield uniformity was observed.

Czochralski Growth of NaY(WO4)2 Single Crystals

Single crystal growth of NaY(WO4)3 was carried out by the Czochralski technique using a starting charge rich in Y2(WO4)3 composition. This off‐stoichiometric starting composition used for the growth has been found to yield reasonable size single crystals. The XRD measurements carried out on the powder from different positions of the grown crystal confirmed the presence of desired single phase NaY(WO4)2. Grown crystals were found to have good transparency.

Investigations On Stoichiometry And Melting Behavior Of NaY(WO4)2

Differential thermal analysis (DTA) and X‐ray diffraction (XRD) studies were carried out to understand the melting behavior of the NaY(WO4)2, an important functional material used for the laser production. It has been observed that the stoichiometric NaY(WO4)2 composition forms a solution with another phase of the Na2WO4‐Y2(WO4)3 pseudo‐binary system. This is found to be detrimental for the growth of single crystals of the material. Therefore, molar fraction in the starting charge was suitably altered to successfully restrict the formation of the undesired phase in the melt. A composition is suggested for the favorable crystal growth of this material.