EPR and Thermoluminescence Measurements on CaWO₄ Crystals

Abstract

After irradiation with X-rays in the energy range 50 to 150 keV at the temperature of liquid air paramagnetic centres are created in single crystals of CaWO,. The centre with a spectroscopic splitting factor slightly higher than g = 2 originates from a hole stably trapped between two tungstate tetrahedrons. The signal of the corresponding electron is broad and rather difficult to det,ect. The concentrat,ion of these centres depends linearly on the dose of irradiation. The decrease of the EPR signal of the hole and electron centres with temperature is correlated with the thermoluminescence intensity.Nach Bestrnhlung mit Rontgenquanten im Energiebereich von 50 bis 150 keV bei der Temperatur der fliissigen Luft werden in CaW0,-Einkristallen paramagnetische Zentren nachgewiesen. Das Zentrum mit einem g-Faktor etwas iiber 2 wird cinem Defektelektron zugeschrieben, das stabil zwischen zwei Wolframatkomplexen eingefangen ist. Das Signal des zugehorigen Elektrons ist breit 17nd schwer nachweisbar. Die Zentrendichte im Kristall hangt linear von der Bestrahlnngsdosis ab. Die Abnahme des EPR-Signals dieser Zentren mit der Temperatur ist mit der vom Kristall emittierten Thermolumineszenz korreliert.

Identification of Three Trapping Centers in Calcium Tungstate

“…The A and B lines are due to W+5 and the C line to Nbt4. As we suggested in our previous note, our A line perhaps can be compared with the high g-value line reported by Zeldes-Livingston and Born et al [7]. Sindarly, our B line possibly compares with the low g-value line reported by these authors.…”

“…It is apparent from these suggestions of possible defect sites that no definite identification of trapping centers has been possible from TL measurements alone. Electron spin resonance (ESR) spectra of paramagnetic tungsten similar to those of (W+a)A and (W+b\B found in this investigation3) have been reported by Zeldes and Livingston [6] and Born et al [7]. These spectra have been attributed to two tungsten atoms with a hole and to one tungsten atom with an electron.…”

“…The equivalent (W+6), spectrum is attributed to a single tungsten ion with an extra electron. Both spectra have been reported by these authors to disappear a t the same rate and temperature, and their concentration is said [7] to depend linearly on the irradiation dose.…”

Thermally and optically stimulated processes in X-irradiated scheelite type crystals

“…The latter are more correct seen as [Pb3+(W04)j-] complexes since they show a pronounced superhyperfine interaction with four neighbouring tungstate tetrahedra [5]. The intrinsic hole centres [6,7,81 are to some extent similar to Vk centres in alkali halides. Since they exhibit in their EPR line width and hyperfine splitting marked motional effects already at low temperatures we assume these holes to move through the crystal during the first glow peak, most probably by a polaronic hopping process [9].…”

Ultra‐Violet Excited Paramagnetic Centres in Calcium Tungstate

“…Since the site symmetry of the A-lines is identical to that of the substitutional copper ions, the centre may be defined as a single W0;-complex which has lost an electron. This centre is considerably simpler than those proposed by Zeldes and Livingston [l] and Born et al [ 5 ] , who postulate a two tungsten complex having a hole stably trapped between two tungstate tetrahedra.…”