An electron spin resonance study of thermoluminescence mechanisms in CaSO₄:Dy

Abstract

Mechanisms of thermoluminescence (TL) in CaSO4:Dy are studied using electron spin resonance (ESR) in single-crystal samples. The ESR analysis indicates the presence of several variations of a distorted SO4- centre locally stabilised by a nearest neighbour Ca vacancy. This centre's structure, which has not been reported previously, relates to several centres observed in this system and could also explain results in related compounds. The presence of the Dy impurity, which increases the concentration of Ca vacancies due to charge neutrality requirements, enhances both the concentration of these centres and the intensity of the major TL peaks near 220 and 350 degrees C. The specific role of these and other previously reported centres in the TL of this material is followed using heat treatments. This enables one to identify centres that change in the range of the observed TL peaks as well as centres that seem to be related to one another by charge transfer or rearrangement. The behaviour of the observed centres and their relationship to the TL behaviour of the system reemphasises the complexity of this system.

Thermostimulated Luminescence, Exoelectron Emission and X-Ray Induced Luminescence Studies in CaSO4 : Eu and CaSO4: Eu, Na

“…Mere presence of Eu 3+ emission (in peaks III and V) does not support model I automatically. For example, an earlier study [6] luminescence when the irradiation temperature was increased from 350 to 450 K. But the above redox model predicts an increase in Eu 2+ luminescence during the readout of peak IV at 400 K. The energy transfer model [13] is, however, more consistent with these results. Table 1 shows that the X-ray induced TSL emission spectra of Da-II also shows more or less a similar behaviour to Y-700, the only difference being that the Eu 2+ emission could not be seen at all in peaks III and V of Da-II.…”

“…The generally held view is that during TSL readout of RE doped CaSO 4 phosphors, it is the holes and not electrons that are released which on recombination with the electrons trapped elsewhere give rise to TSL emission following X-or g-irradiation. In this model, denoted as model II first proposed by Huzimura et al [12] and later on expanded by Morgan and Stoebe [13], the electron traps have not been so far identified (see the recent review by Lakshmanan [14]). RE 3+ activators have been originally proposed as the electron traps, i.e.…”

“…2c, curve c) partly due to its occurrence at a higher temperature at which all the radiation induced Eu 2+ ions are oxidized to Eu 3+ ions. Furthermore, the sulphate radical related hole traps giving rise to this peak are stabilized near Ca 2+ vacancies [13,14] and the Eu 3+ ions near such vacancies are not reduced to Eu 2+ ions on X-irradiation. Thus during peak V readout, the electron±hole recombination/exciton annihilation energy is transferred to nearby Eu 3+ ions which results in Eu 3+ emission.…”

Positron lifetime studies of CaSO4: Dy thermoluminescence phosphors

“…− with the release of an electron upon irradiation and these ions are stabilised by a nearest neighbour Ca vacancy [3]. The decrease of τ 1 upon irradiation may be due to the release of these free electrons near to Ca vacancy sites.…”

“…Besides thermoluminescense (TL), other methods like TL emission spectra, Photoluminescence (PL), Optical absorption (OA), Electron Spin resonance (ESR) are commonly used for the identification of lattice defects. Though a variety of defect centres are reported in sulphate based phosphors, Morgan and Stoebe [3] reported that 4 …”