Mechanism of thermoluminescence in CaF₂:Eu and CaSO₄:Eu phosphors

Abstract

CaF2:n~ and CXO~:RE are important phosphors. The phosphors corresponding to RE E Dy and Tm find applications h thermoluminescence dosimetry using ionizing radiations. Although these phosphors have been in use for several yeus. the mechanism of thermoluminescence in these phosphors is not well understood. The role of RE ions in emission has been clearly indicated. Two types of model have been proposed. In model I it is assumed that a hole recombines with a RE'' ion to convert it into RE3+. Model Z proposes that the energy of electron-hole recombination is mansferred to the RE ion. Experimental results on c1sOd:Eu and CaF2:Eu are presented which show that model 2 is closer lo reality.

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

“…Dy and Tm) are those which emit intense TSL (model II also known as energy transfer model). Both Eu 2+ and Eu 3+ ions in CaSO 4 can be excited by the above lattice emission though not to their optimal efficiencies [5,6]. Our recent studies [16] emissions.…”

“…For example, Danby [2] on the basis of photoluminescence (PL) studies states that in the CaSO 4 3+ /RE 2+ redox model for the 473 K peak, one possible explanation suggested by them was the Eu 2+ /Eu + redox process for the 383 K peak. In contrast, Upadeo et al [5] and Dhopte et al [6] is negligibly small and claim that this explains the absence of ESR signal due to Eu 2+ in g-irradiated CaSO 4 : Eu. In general, no correlation between TSL and Eu 3+ 3 Eu 2+ conversion was seen by them.…”

“…3 and Table 1 show that the Eu 2+ emission is dominant in the 400 K peak in all the samples. Upadeo et al [5] have also concluded that the g-ray induced TSL process in CaSO 4 : Eu process can be explained more realistically on the basis of model II. .…”

“…The enhancement in XIL intensity observed in this temperature range may be attributed to the high luminescence efficiency of Eu 2+ ions (allowed f±d transitions) over that of Eu 3+ ions (forbidden f±f transitions) and to the better matching of Eu 2+ spectra with the spectral response of the photomultipliers used. In CaSO 4 : Eu (which contains Eu exclusively in 3+ state before irradiation), post-gray irradiation (dose = 63.7 Ckg ± ±1 ) annealing studies by Upadeo et al [5] have shown that there is a sharp decline in Eu 2+ PL intensity in the temperature region 410 to 500 K though no TSL was observed by them in this region. Post-irradiation annealing is known to oxidize Eu 2+ to Eu 3+ ions.…”

“…Obviously, the Eu 3+ emission which consists of only red photons has insufficient energy to cause Auger-like decay of electrons; (iii) the electrons giving rise to the TSEE peak at 400 K in Fig. 6 thermal conversion takes place only above 410 K [5] and hence optical (400 nm) bleaching during the readout of peak IV (T max = 400 K) can easily release these loosely bound electrons thereby causing the TSEE; (iv) in X-rayed sample (Fig. 6), no TSEE occurs corresponding to the 320 K peak because it is dominated by Eu 3+ emission similar to the emissions observed in UV irrradiated sample.…”