Study of thermoluminescence response of purple to violet amethyst quartz from Balikesir, Turkey

Highlights

• We reported on dosimetric characterisation of natural amethyst quartz specimens from Turkey, using TL technique.

• The thermoluminescence characterisation tests were performed under the beta radiation exposure.

• The IT peaks ∼230 °C show superlinear dose response behavior (g(D) > 1) between 1 Gy and 5 kGy. The HT peaks ∼300 °C show linear behavior (g(D) = 1) at low dose levels (1 < D < 20 Gy) and superlinearity (g(D) > 1) between 20 Gy < D < 2 kGy.

• Deviations were determined for recycling measurements for various dose values of 0.1, 0.5, 0.8 and 1 kGy.

• Amethyst quartz has great potential to be investigated for dosimetry purpose.

Abstract

In thermoluminescence (TL) dosimetry, the phosphor amethyst quartz as a thermoluminescent, appears to be one of the materials arousing the highest interest. In this study the dosimetric characteristics of natural amethyst quartz crystals collected from Balikesir–Dursunbey (Turkey) were investigated for the purpose of determination of the general properties that phosphors should have in order to be useful for thermoluminescence dosimetry. The natural thermoluminescence was drained by annealing the powder samples at 450 °C for 1.5 h. The effects of high temperature annealing, dose response curves, glow curves after a postirradiation annealing, reusability of the samples and storage of trapped electrons in dark at room temperature were clarified through irradiating the samples with the desired exposures by ⁹⁰Sr/⁹⁰Y beta particles.

Isothermal annealing before and after irradiation was found to have a definite effect upon the TL glow curve of amethyst crystal powder. The same sample varied in sensitivity depending upon its previous thermal and radiation history. The peak heights of the glow peaks were examined with respect to dose response at dose levels between 1 Gy and 5 kGy. The intermediate temperature (IT) and high temperature (HT) peaks of 230 °C and 300 °C, respectively, exhibit dose–response curves as superlinear when dose is on the logarithmic scale except the dose response of 300 °C peak for the dose values of 1 < D < 20 Gy in which linear dose response was acquired. At the end of the storage time between exposure and readout which was about one month at room temperature, the emitted light reduction was 14% comparing to the initial state. Repeating the measurements of the same sample, exposed with 0.1, 0.5, 0.8 and 1 kGy beta exposures, resulted in between 4% and 11% increase in the TL sensitivity of the material.

Introduction

Quartz is the second most abundant mineral found in rocks and soils at and near the earth’s surface. It is an important constituent of many rocks of sedimentary, igneous and metamorphic origin and demonstrates excellent luminescence properties. Due to chemical differences of mineral forming fluids and physical conditions prevalent during the formation of quartz, all the types of the mineral behave differently regarding their thermoluminescence properties [42], [30]. All types of quartz contain mainly traces of iron, aluminum, lithium and slight amount of water. This natural mineral is potentially useful for a wide spectrum of investigations which depend on crystal lattice defect characteristics and concentrations, and as such it has been extensively studied [6]. Natural amethyst samples are purple varieties of α-quartz (SiO₂) [41]. Colorless samples of quartz that has become amethyst after irradiation have infrared spectra at room temperature with a broad band at 3441 cm⁻¹ and a sharp band at 3595 cm⁻¹. The color of amethyst is assigned to F centers formed by the exposure to ionizing radiation and to the influence of lattice distortions due to the content of iron as a substitute for silicon and a high content of trace elements of large ionic radius like potassium [10], [15].

Heat treatment of amethyst converts it to yellow, yellowish brown, green, or even makes it colorless. After heating the amethyst samples in the air at 300–560 °C for several hours the following general sequence of colors was noted: violet–colorless–green–yellow–brown, although not all samples displayed the entire sequence of changes [33]. Optical absorption spectroscopy of irradiation and thermal effects on samples of amethyst from Brazil has been reported by Dotto and Osotani [13]. The isothermal decay and irradiation growth of the studied three bands, 10,500 cm⁻¹ (κ), 18,300 cm⁻¹ (θ) and 28,000 cm⁻¹ (ξ), were considered to reveal a complex kinetics. Correlated Thermally Stimulated Depolarisation Currents (TSDC) and Electron Paramagnetic Resonance (EPR) studies were performed on natural samples of Brazilian amethyst [11].

Over the past few decades, many papers have been published that describe the thermoluminescence analysis as well as suitability of quartz and amethyst quartz for dosimeters [29], [30], [18], [43], [40], [9], [5], [44]. Attempts were made to examine amethyst for thermoluminescence characteristics and it was determined that both natural and artificial pressure and thermal effects may be of considerable importance in modifying the thermoluminescence of amethyst [36]. In the same study, it was introduced that static loading of the order of 1 kilobar may cause increased thermoluminescence in one peak and impact loading of the order of 100 kilobars may decrease the height of the peak and increase the height of another. The response of thermoluminescence to subsequent X-ray irradiation in natural amethyst samples subjected to shock pressures between 10 GPa and 50 GPa has been observed. This work determined the activation energies from the glow peaks of the glow curves and investigated them being not reliable because of the wide distribution of trap depths.

The ongoing discussions in the literature concerning thermoluminescence spectra of natural and synthetic amethyst and quartz samples show that conflicting identifications are characteristic for host lattice and impurity generated recombination sites. Zhang et al. [47] in his work indicates that quartz was dominated by emission bands at 250–800 nm. Emission bands for amethyst quartz were at the level of 740–750 nm. The possible explanation of the difference pointed out by Zhang was Fe ion impurity.

In the study of Rocha et al. [40] the main dosimetric characteristics of Brazilian amethyst were investigated in order to verify the possibility of its utilization for gamma-radiation detection using the thermoluminescence (TL) technique. The samples were tested in x- and gamma radiation beams and their TL glow curves, dependence of the dose response and energy response on the absorbed dose and reproducibility were examined. The study hints the feasibility of utilizing the luminescence properties of amethyst quartz for dosimetry purposes in the range 50 mGy to high doses.

This paper summarizes the dosimetric properties of natural amethyst samples collected from Balıkesir–Dursunbey (Turkey), determined with the use of thermoluminescence technique. The thermoluminescence characteristics of amethyst specimens were analyzed to confirm that consideration should be given to amethyst quartz to be utilized as a dosimeter. The characteristics of the natural amethyst quartz glow curve as a function of annealing time and temperature, both before and after irradiation, were studied. The effects of high-temperature annealing and high beta dose were clarified. The TL signal as a function of the absorbed dose was determined for both ∼230 °C peak at intermediate temperature (IT) region and ∼300 °C peaks at high temperature (HT) region of glow curve. The enhancement in sensitivity under high beta dose was examined by the dependence on both prior and subsequent (to beta irradiation) heat treatments. In order to have a better understanding of the luminescence characteristics, the reusability and fading of the material were also investigated.