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

Abstract

X-and UV radiation induced thermostimulated luminescence (TSL), thermostimulated exoelectron emission (TSEE) and X-ray induced luminescence (XIL) studies in CaSO 4 : Eu and CaSO 4 : Eu, Na reveal that the X-ray induced TSL peaks exhibit Eu 2+ (blue) and Eu 3+ (red) emissions ± ± but their relative ratio varies with the peak. In contrast, UV induced TSL peaks exhibit only Eu 3+ emission. Even in the CaSO 4 : Eu sample, which contains both Eu 2+ as well as Eu 3+ , no Eu 2+ emission is seen in all the TSL peaks produced by UV irradiation. These results confirm that the X-ray induced TSL in these phosphors is best explained by the energy transfer model while UV induced TSL is best explained by the charge transfer model. In CaSO 4 : Eu, the X-ray induced TSEE peak coincides exactly with the 400 K TSL peak whereas the UV induced TSEE is negligible. This indicates an Auger-like decay of electrons following the self-absorption of 400 nm Eu 2+ emission due to the 400 K TSL peak. Intense XIL in CaSO 4 : Eu occurs mainly due to Eu 2+ emission which decreases with increasing irradiation temperature from 100 K perhaps due to thermal quenching. But the significant variation in XIL intensity in the temperature interval 350 to 450 K is attributed to Eu 2+ 3 Eu 3+ oxidation. XIL in CaSO 4 : Eu increases with X-ray intensity but is nearly independent of X-ray exposure time. These results suggest that XIL emission is probably caused by metastable Eu 2+ centers formed during X-irradiation.A.R. Lakshmanan and A. Tomita: TSL, Exoelectron Emission and Luminescence

Effect of Thermal Preannealing on Fluorescence, Thermostimulated Luminescence and Cathodoluminescence of CaSO4 Doped with Europium

“…In the``as-preparedº samples, we find only three of the five TSL peaks mentioned by Lakshmanan and Tomita [10] at about ±160 (I), 35 (II) and 117 C (III) after X-irradiation at ±196 C (Fig. 8).…”

“…), the transformation Eu 3+ 3 Eu 2+ takes place. Lakshmanan and Tomita [10] have obtained similar results in the X-ray luminescence of CaSO 4 : Eu and assumed a formation of metastable Eu 2+ centres during X-irradiation. In CL, we always observe after thermal annealing a weak decrease of Eu 3+ emission correlated with a small increase of Eu 2+ emission.…”

“…The major TSL emissions (Fig. 9) are linked to Eu 3+ ions; Eu 2+ emission is present only in peak III, as already noted by some researchers [7,10,14,15]. After annealing (>600±900 C), the important change is the strong increase of the Eu 2+ emission in TSL peak III only; the other TSL peaks show Eu 3+ emission only (Fig.…”

“…Very few differences in the emission intensities are observed with annealing temperature. Contrary to the optical fluorescence data, a dominant emission of Eu 2+ appears in as-prepared samples; recently, a similar observation has been made in X-ray induced luminescence, with CaSO 4 : Eu obtained by the Yamashita method but annealed at 400 C (instead of 700 C) [10]. The Eu 2+ /Eu 3+ emission ratio remains unchanged up to about 100 ppm, then varies between the as-prepared and annealed samples as shown in Fig.…”

Reduction from Eu3+ to Eu2+ in BaAl2O4:Eu phosphor prepared in an oxidizing atmosphere and luminescent properties of BaAl2O4:Eu

“…Sometimes, NH 3 and metal vapor are also used as reducing agents. Another way is to irradiate the Eu 3+ -containing sample with high-energy rays, like UV lights [11,12], or g-or X-rays [13,14], or ultrafast femtosecond laser [15,16]. Lastly, Eu 3+ -Eu 2+ can be realized even when the Eu-containing sample is prepared in an oxidizing atmosphere, e.g.…”