Phototransferred Thermoluminescence of Dosimetric α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Crystals Irradiated by a Pulsed Electron Beam

The thermoluminescence (TL) of deep traps of anion-defective alumina monocrystals irradiated by a high-dose (more than 1 kGy) pulsed electron beam (130 keV) is studied. The deep traps in the studied material are classified according to the TL temperature range. It is demonstrated that the phototransferred thermoluminescence (PTTL) in the temperature range of the main TL peak is induced by optical charge migration from deep traps that are emptied at 400–470 and 470–600°C. An anomalous PTTL enhancement in crystals subjected to stepped annealing in the 350–400°C interval is observed. It is demonstrated that this effect may be caused by competing processes of charge transfer that involve deep traps corresponding to the TL peak at 390°C. The applicability of PTTL in the dosimetry of high-dose (1–50 kGy) pulsed electron beams is established.     

Thermally assisted OSL from deep traps in Al2O3:C

The present work suggests an alternative experimental method in order to not only measure the signal of the deep traps in Al₂O₃:C without heating the sample to temperatures greater than 500 °C, but also use this signal for high dose level dosimetry purposes as well. This method consists of photo transfer OSL measurements performed at elevated temperatures using the blue LEDs (470 nm, FWHM 20 nm) housed at commercial Risø TL/OSL systems, after the sample was previously heated up to 500 °C in order to empty its main TL dosimetric trap. The influence of this procedure on specific features such as glow curve shape and sensitivity of the main TL glow peak was also studied.     

Thermoluminescence of anion-deficient aluminum oxide single crystals after high-dose irradiation by nanosecond electron pulses

The effect of deep traps filled by a pulsed electron beam on the dosimetric thermoluminescence (TL) peak at 450 K in anion-deficient aluminum oxide single crystals has been investigated. After filling deep traps, the dosimetric TL peak becomes nonelementary and is characterized by a complex dependence of the TL intensity on the crystal annealing temperature with alternating fall and rise portions. The influence of the occupancy of deep centers of different nature and different energy depths on the structure of dosimetric TL peak is analyzed. The suggestion that basically electron traps are depleted in the temperature ranges of 600–750 and 900–1000 K while holes are depleted at T = 780–900 and above 1000 K is substantiated. The possibility of using TL deep traps for high-dose dosimetry of pulsed electron beams is demonstrated.     

Effect of deep traps on sensitivity of TLD-500 thermoluminescent detectors

It is studied experimentally how the dosimetric sensitivity in the dosimetric thermoluminescence (TL) peak at 450 К in TLD-500 detectors depends on the occupancy of deep traps by charge carriers of unlike signs. A kinetic model for the TL of F-centers taking into account the nonradiative capture of electrons in deep hole traps is proposed. The model explains the variation dynamics of the dosimetric sensitivity and dependences of the TL output on the heating rate during filling of deep centers.     

Optical depth of traps in AL2O3:C determined by the variable energy of stimulation OSL (VES-OSL) method

Standard methods of OSL measurements (CW-OSL or LM-OSL) do not allow for the direct determination of optical depth of traps. The variable energy of stimulation optically stimulated luminescence (VES-OSL) method gives such possibility. It consists in optical stimulation with the continuous increase of stimulation light energy and is analogous to the glow curve method in TL measurements. The VES-OSL curve shape and maximum position can be regulated by the stimulation photon flux, the rate of stimulation energy increase and by measurement temperature. This allows for detecting the OSL from very deep traps that give the TL signal overlapping with strong incandescence. The VES-OSL measurements carried out for Al₂O₃:C showed that traps having the optical depth between 2.0 and 2.8 eV are responsible for the OSL signal related to TL peak at about 200 °C. The OSL signal from the much deeper traps from the range 2.8–3.3 eV was also detected. The TL signal related do these traps cannot be detected below 500 °C.     

Gamma ray induced sensitization of 110°C TL peak in quartz separated from sand

We have investigated the gamma ray induced sensitization of the 110°C TL peak in quartz, separated from sand, in the dose range 30–750 Gy. A pre-dose of 100 Gy followed by annealing at 500°C for 1 h yielded an optimum sensitization factor of 14.2 for a test gamma dose of 0.5 Gy; this factor decreases slowly up to the studied pre-gamma dose of 750 Gy. From ESR studies carried out in the temperature range 25–550°C, the formation of E′₁-centres and their subsequent decay (at temperatures >400°C) have been observed. The released charge carriers (electrons) may lead to elimination of competitors (as a result of their filling up). Thus, the radiation-induced sensitization of the 110°C TL peak could be due to elimination of the competing deep traps.     

Optically stimulated luminescence from CaSO4:Eu – Preliminary results

A new OSL phosphor CaSO₄:Eu was developed. The phosphor shows good OSL sensitivity which is about 55% of commercially available Al₂O₃:C. The phosphor also shows good TL sensitivity and the dosimetric peak, which appears around 186 °C, has sensitivity nearly 50% of Al₂O₃:C. After OSL readout of the irradiated sample, the TL peak around 250 °C depletes completely, with partial depletion of peak around 186 °C. Since the traps responsible for the high temperature peak are involved for the observed OSL, the sample shows low post-irradiation fading. The OSL decay is similar to Al₂O₃:C. Thus this phosphor due to its good OSL sensitivity, linear dose response, low fading and simple preparation technique could be useful for radiation dosimetry applications.     

The contribution of hole traps to thermoluminescence of the dosimetric peak in anion-defect α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> single crystals

The thermoluminescent properties of anion-defect alumina single crystals with different FWHMs of the main (dosimetric) peak at 400–500 K are studied. New experimental evidence in favor of the hole nature of traps associated with the high-temperature part of this peak are presented. The introduction of hole trap centers into analysis provided theoretical justification for the experimentally observed dependences of the thermoluminescence (TL) intensity, the temperature position of the main peak, and its FWHM on the occupancy of deep traps. The hole nature of traps of the high-temperature part of the main TL peak is confirmed by the results of examination of specific TL features of shallow trap centers, which govern TL at 350 K, and the temperature variation of the main TL peak spectrum.     

Photoluminescence,thermoluminescence and electron spin resonance studies on Eu<Superscript>3+</Superscript> doped Ca<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>6</Subscript> red phosphors

Tricalcium aluminate doped with Eu³⁺ was prepared at furnace temperatures as low as 500°C by using the convenient combustion route and examined using powder X-ray diffraction, scanning electron microscope and photoluminescence techniques. A room-temperature photoluminescence study showed that the phosphors can be efficiently excited by UV/Visible region, emitting a red light with a peak wavelength of 616 nm corresponding to the ⁵D₀–⁷F₂ transition of Eu³⁺ ions. The phosphor exhibits three thermoluminescence (TL) peaks at 195°C, 325°C and 390°C. Electron Spin Resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the defect centres responsible for the TL process. Room-temperature ESR spectrum of irradiated phosphor appears to be a superposition of three distinct centres. One of the centres (centre I) with principal g-value 2.0130 is identified as O⁻ ion while centre II with an axially symmetric principal values g _∥=2.0030 and g _⊥=2.0072 is assigned to an F⁺ centre (singly ionized oxygen vacancy). O⁻ ion (hole centre) correlates with the TL peak at 195°C and the F⁺ centre (electron centre), which acts as a recombination centre, is also correlated to the 195°C TL peak. F⁺ centre further appears to be related to the high temperature peak at 390°C. Centre III is also assigned to an F⁺ centre and seems to be the recombination centre for the TL peak at 325°C.     

Post-IR IRSL production in perthitic feldspar

A museum sample of perthitic feldspar was used to study the production of post-IR IRSL signals. It was found that traps responsible for low temperature (∼230 °C) TL peaks play an unexpectedly important role in post-IR IRSL production. During the production of the IRSL signal during low temperature IR stimulation (100 °C), electrons are optically transferred from IRSL traps into these TL traps which have been emptied by the preceding preheat at 320 °C. Subsequent heating to 300 °C causes thermal transfer of these electrons from these traps back into previously emptied IRSL traps which are related to the high temperature TL peaks. IR stimulation of these electrons results in post-IR IRSL. Thus the initial source of the post-IR IRSL signal is the same as the IRSL signal, with a role being played by intermediate traps that give rise to TL signals between 200 and 250 °C, and the final source is similar to that of the IRSL signal. Therefore the post-IR IRSL signal is a by-product of the production of the IRSL signal. It was also found that post-IR IRSL production with high post-IR IR stimulation temperatures (e.g. >230 °C) additionally includes a small contribution from the post-IR isothermal decay of high temperature TL peaks that are not sensitive to IR stimulation at low stimulation temperatures.     

Lu2O3:Tb,Hf storage phosphor

Lu₂O₃:Tb,Hf ceramics containing 0.1% of Tb and 0–1.5% of Hf were prepared in reducing atmosphere at 1700 °C and their thermoluminescence properties were systematically studied. For comparison Tb,Ca co-doped specimen was also fabricated and investigated. The Tb,Hf ceramics shows basically a single TL band located around 180 °C as found with heating rate of 15 °C/min. Ceramics singly doped with Tb show complex TL glow curves indicating the presence of traps of very different depths. On the other hand Tb,Ca co-doping is beneficial for the development of shallow traps with the main TL band around 70 °C. Hence, the aliovalent impurities, Ca²⁺ and Hf⁴⁺, strongly influenced the traps structure in Lu₂O₃:Tb ceramics, each of them in its own specific way. Isothermal decay of Lu₂O₃:Tb,Hf at 185 °C was recorded and its shape suggest that multiple hole trapping occurs in the Lu₂O₃:Tb,Hf ceramics. Due to the different traps depths the Lu₂O₃:Tb,Hf ceramics possess properties typical for storage phosphors, while Lu₂O₃:Tb,Ca is a persistent luminescent material rather.     

Effect of Eu and Pb doping on the dosimetric properties of LiCAF

LiCaAlF₆ (LiCAF) crystals doped with two different ions (europium and lead) have been investigated as potential new dosimetric materials. The stability of thermally stimulated luminescence (TSL) glow peaks in LiCAF:Eu was evaluated by means of the initial rise technique. The decay times at room temperature of the traps related to the dosimetric glow peaks were found to range between 40 and 2 × 10⁴ years confirming the good dosimetric characteristics of this crystal. The glow curve of LiCAF:Pb is dominated by a peak at approximately 300 °C emitting in the UV region (³P_0,1–¹S₀ transition of Pb²⁺) superimposed to a very broad structure at lower temperature (20–200 °C) featuring recombination at an intrinsic defect centre. The anomalous behavior of the low temperature structure during thermal cleaning procedures prevented any reliable numerical analysis of the TSL glow peak at 300 °C.     

On the role of hole trapping centers in the interactive mechanism of the trap interaction in anion-defect alumina single crystals

The effect of deep hole traps on the intensity and shape of the dosimetric peak of thermoluminescence (TL) has been studied at 450 K in anion-defect alumina single crystals. It has been shown that filling of deep hole centers leads to a decrease in the sensitivity to radiation of crystals with a small half-width of the TL peak and has no effect on the sensitivity of crystals with a broadened peak. It has been assumed that traps responsible for the TL dosimetric peak broadening are of hole nature, which can be caused by the presence of Ti3+ ions in the corundum lattice. The results obtained have been interpreted within the modified model of the interactive system of traps.     

Further study on the influence of thermal treatments on the glow curve structure in LiF:Mg,Cu,P (GR-200A)

The influence of various annealing treatments with heating temperatures (T_A) from 240 to 700 °C, with re-annealing at 240 °C, and with a combined re-annealing procedure of 20 min at 270 °C followed by 10 min at 240 °C on LiF:Mg,Cu,P (GR-200A) was investigated. As the T_A increased, the intensity decreased rapidly to almost no signal at 340–380 °C then increased clearly and achieved a maximum at 540 °C. The position of the maximum intensity of the glow curve shifted basically in the direction of higher temperatures with an increase at T_A and achieved a maximum of 279 °C when annealed at 460 °C. The re-annealing influenced both the intensity and the glow curve structure at certain degree. The effect of re-annealing on the glow curve depended markedly on the T_A. With re-annealing at 240 °C, the intensity decreased as T_A increased up to 360 °C then increased and achieved a maximum at 540 °C. The intensity could be restored fully when annealed at above 500 °C, however, the glow curve couldn't be restored fully. With a combined annealing, the shape of glow curve of a sample annealed at above 540 °C or below 320 °C was similar to that of the standard glow curve of LiF:Mg,Cu,P and the intensity and glow curve could be restored completely when annealed in the range 620–660 °C. It seems that the main roles of the re-annealing at 240 °C are to restore partially the intensity of peak 4 and peak shape for LiF:Mg,Cu,P when annealed at above 260 °C, and restore fully the total TL intensity of LiF:Mg,Cu,P when annealed at above 500 °C and the main roles of the combined re-annealing are to reduce the intensity of peak 5 and the total TL intensity, increase the intensity of peak 4 and restore the glow curve shape.     

Characterization of deep energy level defects in α-Al2O3:C using thermally assisted OSL

Commercially available α-Al₂O₃:C powder was studied for deep energy level defects by a newly suggested method using thermally assisted optically stimulated luminescence (TA-OSL) phenomenon. The method involves simultaneous application of continuous wave optically stimulated luminescence (CW-OSL) as well as thermal stimulation up to 400 °C, using a linear heating rate of 4 K/s. By using this method, two well-defined peaks at 121 °C and 232 °C were observed. These TA-OSL peaks have been correlated to two different types of deeper defects which can be bleached at 650 °C and 900 °C respectively on thermal treatment. These deeper defects, having larger thermal trap depth and relatively lower photoionization cross-section at room temperature for stimulation with blue LED (470 nm), are stable up to 500 °C, so they can store absorbed dose information even if the sample is inadvertently exposed to light or temperature. As only a fraction of signal is bleached during TA-OSL readout, multiple readouts could be performed on an exposed sample using this technique. The dose vs TA-OSL response from deep traps of α-Al₂O₃:C was found to be linear up to 10 kGy, thus extending its application for high dose dosimetry. The value of thermally assisted energy (E_A) associated with these traps in α-Al₂O₃:C has been determined to be 0.268 eV and 0.485 eV respectively and the corresponding values of photoionization cross-section at room temperature (25 °C), for optical stimulation with blue light (470 nm), are 5.82 × 10^?20 and 3.70 × 10^?22 cm², respectively. The process of thermally assisted OSL has been formulated analytically as well as theoretically for describing the temperature dependence of optical cross-section and evaluation of thermally assisted energy associated with deep traps.     

Thermoluminescence and defect study of MgSiO3 ceramics

Enstatite (MgSiO₃) ceramic powders were synthesised by a low-temperature initiated self-propagating, gas-producing solution combustion process. The prepared powders were characterised by powder X-ray diffraction, scanning electron microscopy and Brunauer–Emmer–Teller specific surface area measurements. Defect centres induced by radiation were studied using the techniques of thermoluminescence (TL) and electron spin resonance (ESR). A well-resolved glow with peak at 178°C and a shouldered peak at 120°C were observed. Two defect centres were identified by ESR measurements, which were carried out at room temperature, and these were assigned to an O^? ion and F⁺ centre. The O^? ion (hole centre) appears to correlate with the main TL peak at 178°C.     

Features of thermoluminescence in anion-defective alumina single crystals after highdose irradiation

The effect of high-dose irradiation by electron beam with nanosecond duration and by gamma-rays on thermoluminescence (TL) yield of anion-defective dosimetric Al₂O₃:С crystals is studied. It is shown that in a wide dose range up to 10 kGy no significant changes in the TL curve shape and the temperature position of the main dosimetric peak (T = 460 K) are observed. The TL yield of this peak is in saturation in the high-dose range 5–80 kGy. Then anomalous increase in TL yield is registered at the dose growth up to 800 kGy. With that an intensive band appears in the green spectrum region in the photoluminescence spectrum. The role of aggregate defects forming F₂-type centers with the increase of TL yield in Al₂O₃:С crystals under high-dose irradiation is discussed.     

Thermoluminescence characterization of the Cu(Tb,Tm)-12CaO-7AL2O3 compounds obtained by combustion synthesis

The thermoluminescence (TL) properties of dode-calcium hepta-aluminate (Ca₁₂Al₁₄O₃₃) enabled by 12CaO-7Al₂O₃ doped with rare earth and transitions metals ions have been studied and their suitability for radiation dosimetry applications is discussed. It was observed that this calcium aluminate phase doped with Tm-Cu at concentration of 0.1 mol% is a good candidate for dosimetric applications since it presents well-defined single peak observed at 240 °C and 320 °C and a linear response to gamma radiation dose from 5 × 10⁻³ Gy up to 100 Gy.     

Effects of annealing and high radiation dose on the thermoluminescence characteristics of natural fluorite

In this study, the various dosimetric properties of the thermoluminescent (TL) phosphor of CaF₂:natural including response characteristics, sensitivity to thermal annealing and decay of stored energy were investigated in detail. The analysis of the peak temperature values and the half widths of the resolved components in the glow curves, by using the Computerized Glow Curve Deconvolution (CGCD) Method, resulted in the activation energy (E) of 0.85–1.83 eV and frequency factor (s) of 6.17E+9–2.90E+15 s^?1. The effects of pre-irradiation thermal annealing on the radiation-induced sensitization of the thermoluminescent response of CaF₂:natural were presented. An enhancement of sensitivity after being subjected to a high beta particle exposure was clarified. The effect was complicated by the influence of heat treatments before the exposure to radiation. Replicate runs on CaF₂:natural after irradiation with 1 Gy absorbed dose indicated good reproducibility of peak temperatures and intensities; they can be re-used for repeated measurements. It appeared that the main dosimetric peak at 280 °C exhibits negligible fading over 4 weeks and may be used for dosimetry. This study has demonstrated that the potential exists for the use of CaF₂:natural for TL radiation dosimetry.