Normal and anomalous heating rate effects on thermoluminescence of Ce-doped ZnB₂O₄

Highlights

• The effect of heating rate on TL glow-peaks were studied.

• FWHM values were investigated to evaluate the increase on TL intensity.

• The reusability of the Ce-doped ZnB₂O₄ phosphors were investigated.

• Activation energy (E) and frequency factor (s) were calculated using VHR method.

Abstract

The effect of heating rate (HR) on thermoluminescence (TL) glow curves of 1%, 4%, and 10% Ce^{3 +} doped ZnB₂O₄ phosphors was investigated in detail. The glow peaks are examined and, activation energy (E) and frequency factor (s) are determined by using various heating rate (VHR) method. In the obtained glow curves with nine different HRs between 2 and 10 °C/s, it was observed that the TL intensities of the first peaks of all three samples and the second peak of 10% Ce^{3 +} doped sample decrease with increasing HR. The decrease in TL intensity was investigated whether it may be due to the presence of thermal quenching or not. On the other hand, the second glow peaks of 1, 4% Ce^{3 +} doped ZnB₂O₄ phosphors show an anomalous TL behavior, which the probability of the radiative processes increases due to recombination of free electrons, so the TL intensity increases with the HR. The graphs of full width at the half maximum (FWHM) versus HR were also plotted to evaluate the influence of HR on TL intensity. In this paper, we suggest that the non-localized Schön – Klasens model may give an explanation for the second peaks of 1, 4% Ce^{3 +} doped ZnB₂O₄ phosphors showing an unexpected increase with the increasing HR. In addition, the calculated E values of all doped phosphors were found similar in the range of 0.47–0.53 eV for peak 1 and 0.61–0.66 eV for peak 2. However, $s$ values of Ce^{3 +} doped ZnB₂O₄ phosphors were found slightly different according to the dopant amount and the equation used. Furthermore, different amount of Ce^{3 +} doped samples indicate the similar properties for the repeated cycles of 5 Gy in the same irradiation conditions.

Introduction

Borates, Sulphates, Fluorides, Oxides and Silicates are some of the important Thermoluminescence (TL) materials that have been developed and used over many decades. Especially, the borate based phosphors among them are preferred to be studied. Borate based phosphors are popular to study their TL characteristics due to neutron and gamma sensitivity, near tissue equivalence, basic structure of glow curve, synthesis feasibility in bulk quantities and simple thermal treatment procedure.

Several studies have been performed to determine the TL properties of borates using various preparation techniques, different modifiers and activators. Lately, ZnB₂O₄ has started to draw increasing research interest because it can be used as a host in phosphor applications (Li et al., 2007, Li et al., 2008, Zheng et al., 2009, Liu et al., 2010, Kucuk et al., 2013). Kucuk et al. (2015) reported the TL dosimetry properties of Lanthanum doped ZnB₂O₄ phosphor synthesized by nitric acid method. Yazici et al. (2006) reported that Ce doped BaB₄O₇ compounds could be used in dosimetric applications. Li et al., 2007, Juan et al., 2008) reported photoluminescence (PL) and TL dosimetry properties of Terbium and Dysprosium doped ZnB₂O₄ phosphors synthesized by using solid state technique. Dogan and Yazıcı (2009) reported fading and reusability properties in dose linearity region of Ce doped MgB₄O₇.

Heating rate (HR) is one of the important factors affecting the dosimetric characteristics of many TL materials. The various heating rate (VHR) method considers how the traps are emptied at different HRs while all other parameters are held constant (Topaksu et al., 2015). There are lots of studies in the literature which prove that the glow peak height decreases with an increase in HR as a normal behavior under the constant dose (Jiang et al., 2008) whereas there are also studies indicating the opposite interpretation (Akselrod et al., 1990). It is called as anomalous HR effect which the area under the normalized TL signal (TL/β) against temperature increases when the HR increases. This anomalous HR effect has been explained by Mandowski and Bos (Mandowski and Bos, 2011) using the semi-localized transition (SLT) model developed by Mandowski (Mandowski, 2005). According to SLT model, the localized transition from the excited state into the recombination center and the delocalized transition from the conduction band into the center are assumed to produce measurable TL (Chen et al., 2012). On the other hand, there are some studies having different aspects to this effect (Kitis et al., 2006, Bos et al., 2010, Chen and Pagonis, 2016). In two-stage model, it appears similar the above mentioned SLT model, but the concentration is given only on recombination of electrons going through the conduction band (Chen et al., 2012). Recently, the approach of non-localized Schön – Klasens model draws attention to both a decrease and increase on normalized TL intensity and the area under the glow peak to explain two presentations of TL in their simulation study (Chen and Pagonis, 2016). The model presents one electron trap and one recombination with one reservoir as two centers in the model to clarify the normal and anomalous HR effects.

The aim of the present study is to understand the HR effects on TL peaks of Ce^{3 +} doped ZnB₂O₄ phosphors. For this purpose, we present TL glow curves, normalized TL peak area, the change of the full width at the half maximum (FWHM) as a function of HR and the reusability results of 1%, 4%, and 10% Ce^{3 +} doped ZnB₂O₄ phosphors. In addition, E and s have been calculated with the VHR method for Ce^{3 +} doped ZnB₂O₄ phosphors.