Fiber-coupled radioluminescence dosimetry with saturated Al2O3:C crystals: Characterization in 6 and 18 MV photon beams

Radioluminescence (RL) and optically stimulated luminescence (OSL) from carbon-doped aluminum oxide crystals can be used for medical dosimetry in external beam radiotherapy and remotely afterloaded brachytherapy. The RL/OSL signals are guided from the treatment room to the readout instrumentation using optical fiber cables, and in vivo dosimetry can be carried out in real time while the dosimeter probes are in the patient. The present study proposes a new improved readout protocol based solely on the RL signal from Al₂O₃:C. The key elements in the protocol are that Al₂O₃:C is pre-dosed with ∼20 Gy before each measurement session, and that the crystals are not perturbed by optical stimulation. Using 6 and 18 MV linear accelerator photon beams, the new RL protocol was found to have a linear dose-response from 7 mGy to 14 Gy, and dosimetry in this range could therefore be performed using a single calibration factor (∼6 × 10⁶ counts per Gy for a 2 mg crystal). The reproducibility of the RL dosimetry was 0.3% (one relative standard deviation) for doses larger than 0.1 Gy. The apparent RL sensitivity was found to change with accumulated dose ((−0.45 ± 0.03)% per 100 Gy), crystal temperature ((−0.21 ± 0.01)%/ °C), and dose-delivery rate ((−0.22 ± 0.01)% per 100 MU/min). A temporal gating technique was used for separation of RL and stem signals (i.e. Cerenkov light and fluorescence induced in the optical fiber cable during irradiation). The new readout protocol was a substantial improvement compared with the combined RL/OSL protocol, that required relatively long readout times and where the optical stimulation greatly affected the RL sensitivity. The only significant caveat was the apparent change in RL-response with accelerator dose-delivery rate.     

Characteristics of LiAlO2 – Radioluminescence and optically stimulated luminescence

Radioluminescence (RL) and optically stimulated luminescence (OSL) results of LiAlO₂ were compared with Al₂O₃:C. For blue (470 nm) optical stimulation, RL + OSL signal in LiAlO₂ exhibited a sharp initial increase followed by a decay within the first 20 s of continuous wave (CW)-OSL and a very slow increase thereafter. The RL + OSL signal was about 1.25 times of its RL signal in LiAlO₂ as compared to 2.5 times in Al₂O₃:C. With the continued beta irradiation, the RL signal exhibited a faster growth in LiAlO₂ than that in Al₂O₃:C. Emission spectrum of LiAlO₂ exhibited multiple emission peaks in the range from 320 to 380 nm as against 410 nm of Al₂O₃:C. RL and OSL emission spectra were similar in both LiAlO₂ and Al₂O₃:C. The intense RL in LiAlO₂ (about 300 times of that of its background signal) for a beta ray dose rate of 4.6 mGy/s appears attractive for radiation dosimetry including real time/online dosimetry and dose mapping.     

Medical dosimetry using a RL/OSL prototype

A portable and robust instrument has been developed for the routine assessment of patient exposure to ionizing radiation during radiotherapy treatments. The design principles of hardware and software are described, along with preliminary measurements that illustrate the operation of the system and its capabilities. In this study the authors used radioluminescence (RL) and Optically Stimulated Luminescence (OSL) from Al₂O₃:C detectors coupled to a PMMA optical fibre to acquire dose in medical dosimetry. The RL/OSL prototype can provide two independent dose estimates from the same in vivo treatment: one integrated dose estimate (OSL) and one real-time dose estimate (RL), which can be compared to one another. The authors first characterized the dose–response to a calibration source (¹³⁷Cs), analysing the OSL and the RL signal to doses from 0.5 to 3 Gy. Later the percentage dose depth from RL is presented for two gamma (6 and 15 MV) and two electron (6 and 12 MeV) medical beams.     

New luminescence measurement facilities in retrospective dosimetry

This paper gives a review of recent developments in luminescence measurement facilities on the Risø TL/OSL reader including radio-luminescence (RL), exo-electron and violet stimulation attachments, and a method for characterising and if necessary correcting for beta irradiation source non-uniformity.We first describe improvements to the existing RL option to allow near infra-red detection (NIR) during irradiation by the built-in ⁹⁰Sr/⁹⁰Y beta source. The RL optical signal is collected by a liquid light guide through an F34-901 interference filter and detection is based on a dedicated thermoelectrically cooled NIR sensitive PMT (detection window peak at 855 nm, FWHM 27 nm). Software and electronics have been modified to allow standard TL and OSL measurements in the same sequence as RL measurements. Together with a new bleaching source based on a high-power UV LED (395 nm; 700 mW/cm²), this facility has been used to measure natural doses in feldspar using the decaying NIR RL signal.Secondly, we present a method for mapping radiation field of the built-in ⁹⁰Sr/⁹⁰Y β-source and estimating grain-location specific dose-rates. This is important for the accuracy of single grain results, when radiation field is spatially non-uniform across the sample area. We document the effect of this correction method and further investigate on the effect of lifting the source to achieve a better dose-rate uniformity.Finally we summarise two recently-developed novel facilities to help investigate (i) the time scales involved in OSL processes (time-resolved exo-electron detection) and (ii) extending the age range (violet stimulated signals from deep quartz OSL traps).     

The central role of oxygen on H-irradiated Lu2SiO5 luminescence

The behavior of self-trapped defects (STDs) in ion-beam irradiated Lu₂SiO₅ (LSO) crystal has been investigated via temperature-dependent radioluminescence (RL) measurements. Production of oxygen vacancies is the major effect of H⁺ irradiation on luminescencent properties of this phosphor. Luminescence centers for self-trapped exciton (STE) and self-trapped hole emission are assigned to oxygen vacancies and oxygen ions, respectively. Ion-induced structural damage modifies the thermal stability of the STDs and creates perturbed STEs. A striking effect of ion irradiation is the approximate factor-of-two enhancement of STE RL intensity that results from implantation of only a thin (∼250 nm) surface layer of LSO. This enhancement is attributed to ion-beam modification of a surface dead layer.     

Luminescence of Strontianite (SrCO3) from Strontian (Scotland, UK)

An historic Strontianite-type specimen from Strontian, Scotland, UK, was characterized to broaden our knowledge on luminescence properties of common carbonates. These fibrous aggregates are Strontianite (Sr_xCa_1−xCO₃) with circa 6% of CaO, interfacial water, hydrosilicate anions and substitutional divalent cations, e.g., Ca²⁺, Mn²⁺, Fe²⁺ in structural Sr²⁺ positions. The specimen was analyzed by X-ray Fluorescence Spectrometry (XRF), Environmental Scanning Electron Microscopy coupled with an Energy Dispersive X-ray Spectroscopy (ESEM-EDS) probe, Spatially-resolved Cathodoluminescence under the Scanning Electron Microscope (SEM-CL), Differential-Thermal Analyses (DTA), Thermogravimetry (TG), Thermoluminescence (TL), Radioluminescence (RL) and High Resolution Spectra Thermoluminescence (3DTL), to gain an overview of the spectral emissions, the defect linkages were modified by heating from room temperature (RT) up to 500 °C. Substitutional transition elements are probably responsible for the spectral emission bands from 500 nm to 800 nm and hydrous molecules from 300 nm to 400 nm. DTA–TG analyses performed on little chips, to preserve the fiber interfaces coherence, exhibit minor endothermic peaks attributed to outflow of water groups in fiber interfaces. Both, CL and RL curves show common spectral positions but UV–blue and red emission intensities are counterbalanced since electron irradiation reduces the UV–blue emissions while X-irradiation increases them. The TL curves show a top thermal limit at 300 °C for the 300–400 nm TL emissions which become irreversibly destroyed, whereas the longer wavelength region emits at higher temperature. The non-reversible changes observed in the 320 nm and 360 nm bands during the spectra 3DTL emission could be linked with non-bridging oxygen defects, protons and hydroxyl groups and the red emissions to the ⁴G (⁴T_1g)–⁶S Mn²⁺ ion transition. Following assignations and similar spectral CL patterns of Russian Strontianite samples, the emission-defect assignments: Dy³⁺ 480 nm; Tb³⁺ 540 nm; Dy³⁺ 580 nm and Sm³⁺ 640 nm cannot be disregarded.     

α-Al2O3:C晶体的热释光和光释光特性

以高纯α-Al₂O₃和石墨为原料,采用温梯法生长了α-Al₂O₃：C晶体,使用Ris TL/OSL-DA-15型热释光和光释光仪研究了其热释光和光释光特性.α-Al₂O₃：C晶体在462K附近有单一热释光峰,发射波长位于410nm.随着辐照剂量的增加,热释光强度逐渐增强,462K的热释光特征峰位置保持不变.α-Al₂O₃：C晶体的 关键词： 2O₃：C')" href="#">α-Al₂O₃：C 热释光 光释光     

Time-resolved OSL studies of YAlO3:Mn2+ crystals

Time-Resolved Optically Stimulated Luminescence (TR-OSL) from single crystalline YAlO₃:Mn²⁺ samples was investigated using a green light emitting diode (λ ∼ 525 nm) as stimulation light source. The TR-OSL decay curve of the material can be described with a single exponential decay function with a lifetime about 80 ms that does not depend on irradiation dose in the range from 50 mGy to 1 kGy. This OSL decay is superposed on a photoluminescence signal with a much shorter (3.5 ms) decay lifetime. The Mn²⁺ photoluminescence decay with a lifetime of 3.5 ms can be easily eliminated by corresponding time resolution using pulsed OSL readout. Dose response and thermal stability of the OSL signal are consistent with the previous thermoluminescent (TL) studies of the material.     

Real-time luminescence from Al2O3 fiber dosimeters

The real-time luminescence signal from Al₂O₃ single crystal fibers, monitored during simultaneous irradiation and optical stimulation, was investigated using computer simulations and experimental measurements. Both radioluminescence (RL) and optically stimulated luminescence (OSL) signals were studied. The simulations were performed initially using a simple one-trap/one-recombination-center energy band model, and then extended to include shallow and deep electron traps as well. Real-time luminescence experiments were performed for different radiation dose rates and optical stimulation powers using periodic laser stimulation of the samples through a fiber optic cable, and the experimental results were compared with the predictions from the computer simulations. The luminescence signal was observed, both theoretically and experimentally, to increase from its initial value to a steady-state level. The steady-state RL and OSL levels were found to be dependent on dose rate, the steady-state level of the real-time OSL being independent of laser power. It was also shown that the total integrated absorbed dose throughout the irradiation period can be determined by correcting the real-time OSL signal for depletion caused by each laser stimulation pulse. The effects of the shallow and deep traps on the time-dependence of the real-time luminescence signal were studied comparing the experimental data from several Al₂O₃ fibers known to have different trapping state concentrations. The additional traps were found to slow the response of the real-time luminescence such that the time to reach steady state was increased as the additional traps were added.     

Ionoluminescence studies of combustion synthesized Dy doped nano crystalline forsterite

Ionoluminescence (IL) of nano crystalline Mg₂SiO₄:Dy³⁺ pellet samples bombarded with 100 MeV Si⁺⁸ ions with fluences in the range (1.124–22.480) × 10¹² ions cm⁻² have been studied. Two prominent IL bands with peaks at ∼480 nm and ∼580 nm and a weak band with peak at ∼670 nm are recorded. The characteristic peaks are attributed to luminescence center activated by Dy³⁺ ions due to the transitions ⁴F_9/2→⁶H_15/2,⁶H_13/2 and ⁶H_11/2. It is found that IL intensity initially decreases rapidly and then continuous to decrease slowly with further increase in ion fluence. The reduction in the Ionoluminescence intensity with increase of ion fluence might be attributed to degradation of Si–O ( 2ν₃) bonds present on the surface of the sample and/or due to lattice disorder produced by dense electronic excitation under heavy ion irradiation.     

Role of Ti in the luminescence process of Al2O3:Si,Ti

Al₂O₃:Si,Ti, prepared under oxidizing condition at high temperature, gives PL emission around 430 nm when excited with 240 nm. The Al₂O₃:C, TL/OSL phosphor, also shows emission around 430 nm, which corresponds to characteristic emission of F-center. Thus, to identify the exact nature of luminescent center in Al₂O₃:Si,Ti, fluorescence lifetime measurement studies were carried out along with the PL,TL and OSL studies. The PL and TL in Al₂O₃:Si,Ti show emission around 430 nm and the time-resolved fluorescence studies show lifetime of about 43 μs for the 430 nm emission, which is much smaller than the reported lifetime of ∼35 ms for the 430 nm emission (F-center emission) in Al₂O₃:C phosphor. Therefore, the emission observed in Al₂O₃:Si,Ti phosphor was assigned to Ti⁴⁺ charge transfer transition. Fluorescence studies of Al₂O₃:Si,Ti do not show any traces of F and F⁺ centers. Also, Ti⁴⁺ does not show any change in the charge state after gamma-irradiation. On the basis of the above studies, a mechanism for TSL/OSL process in Al₂O₃:Si,Ti is proposed.     

Investigation of a fibre-coupled beryllium oxide (BeO) ceramic luminescence dosimetry system

The purpose of this study is to investigate the potential use of a beryllium oxide (BeO) ceramic as a radioluminescence (RL) and optically stimulated luminescence (OSL) probe material for fibre-coupled luminescence dosimetry. A portable dosimetry system, named RL/OSL BeO FOD was developed, consisting of a 1 mm diameter, 1 mm long BeO ceramic cylinder coupled to a silica/silica optical fibre. The reader measures the RL signal and also uses a 450 nm laser diode to stimulate the BeO ceramic. A second background optical fibre is used to remove the stem effect. The RL/OSL BeO FOD was characterised in a solid water phantom, using a 6 MV x-ray beam. The RL was found to be reproducible and have a linear response to doses ranging from 30 cGy–15 Gy and dose rates from 100 cGy/min – 600 cGy/min. The OSL response was linear to doses of 10 Gy, becoming supralinear at higher doses. Measured percentage depth curves using the RL/OSL BeO FOD agreed with those measured using an IC15 ion chamber to within 5%, beyond the build up region. It was also found that the RL from BeO ceramic is unaffected by the delivered dose to the probe and hence, it remains constant for a given dose-rate. The insensitivity of the RL to accumulated dose makes BeO ceramic potentially capable of accurate dose-rate measurements without any corrections for the accumulated dose. This study demonstrates the feasibility of BeO ceramic as a versatile fibre-coupled luminescence dosimeter probe.     

Growth of α-Al2O3:C crystal with highly sensitive optically stimulated luminescence

In this work, an α-Al₂O₃:C crystal was directly grown by the temperature gradient technique (TGT) using Al₂O₃ and graphite powders as the raw materials. The optical, optically stimulated luminescence (OSL) properties and dosimetric characteristics of as-grown crystal were investigated. As-grown α-Al₂O₃:C crystal shows strong absorption band at 205, 230 and 256 nm. Three-dimensional thermoluminescence (TL) emission spectrum of the crystal shows a single emission peak at ∼415 nm. The OSL decay curve can be fitted to two exponentials, the faster component and the slower component. The OSL response of the crystal shows a linear-sublinear-saturation characteristic. As-grown α-Al₂O₃:C crystal shows excellent linearity in the dose range from 5×10⁻⁶ to 50 Gy. For doses higher than the saturation dose (100 Gy), the OSL sensitivity decreases as the dose increases.     

Characterisation of a real-time fibre-coupled beryllium oxide (BeO) luminescence dosimeter in X-ray beams

For the first time the feasibility of using beryllium oxide (BeO) ceramics as a fibre-coupled radioluminescent dosimeter is investigated. BeO ceramic exhibits both radioluminescence (RL) and optically stimulated luminescence (OSL), and has the potential to be a near tissue equivalent alternative to Al₂O₃:C. A BeO fibre-coupled radioluminescence dosimeter is demonstrated and characterised for 6 MV X-rays and superficial X-ray energies, 150 kVp and 120 kVp. Based on the results, we demonstrate the capability of the RL BeO FOD for accurate and reproducible dose measurements with a linear dose rate and dose response. It has also been found that the percentage depth dose curves for 6 MV agreed with ion chamber measurements to within 2%, except in the build up region. For the 150 kVp and 120 kVp photon beams, the depth dose measurements agreed with ion chamber measurements to within 2.5% and 4%, respectively.     

Photoluminescence and radioluminescence study of Eu and Mn doped RbMgF3 nanoparticles

Photoluminescence (PL) and radioluminescence (RL) measurements were made on RbMgF₃ nanoparticles doped with Mn or Eu. We find that the Mn doped samples contain only Mn²⁺ and the Eu doped samples contained Eu³⁺ and Eu²⁺. The Mn²⁺ PL lifetimes are nearly independent of Mn²⁺ concentration but the RL spectra increases at high doses for 1% Mn²⁺ and decreases slightly at high doses for 5% Mn²⁺. The 5% Mn²⁺ is more radiation hard and the integrated RL intensity only starts to significantly decrease above 1 kGy. The Eu doped sample displays a PL lifetime that is lower for high Eu concentrations and this can be accounted for by a model where there is energy transfer between Eu³⁺ and more nonradiative decay sites at the surface. The RL is independent of dose between 10 mGy and ∼200 Gy, where the 1% Eu sample is more radiation hard and the Eu³⁺ RL intensity has decreased by only 3.4% at 6.7 kGy.     

Thermoluminescence and optically stimulated luminescence studies on LiMgPO4 crystallized by micro pulling down technique

LiMgPO₄ (LMP) crystals were grown by micro pulling down technique. Samples were irradiated with different β-particle doses of the ⁹⁰Sr/⁹⁰Y source. Thermally and optically stimulated luminescence spectra were measured with the automatic Risø TL/OSL-DA20 reader under the different modes of stimulation. The dose–response dependence, reproducibility, the lowest measurable dose and short-time fading were investigated. TL and OSL dose–response of LiMgPO₄ crystals was found to be linear up to around 1 kGy, what makes this material suitable for high dose measurements. Discrepancies between successive measurements did not exceed 10%, regardless of the applied growth parameters. The lowest measurable dose, defined as three standard deviations of the signal of unexposed detector, was determined around 0.5 mGy. About 73% of the initial OSL signal value was measured 24 h after the irradiation. For longer periods of time the level of signal stabilizes so that there was no further loss of signal observed. In case of TL, the level of signal does not stabilize and decreases to 69% within 2 weeks after the irradiation. The obtained results tend to suggest that LiMgPO₄ crystals may be considered as promising dosimeters for both personal and high dose dosimetry.     

Optically stimulated luminescence (OSL) and thermally assisted OSL in Eu2+ – Doped BaSO4 phosphor

BaSO₄:Eu²⁺ phosphor has been investigated for its photoluminescence (PL), thermoluminescence (TL), TL kinetics, optically stimulated luminescence (OSL) and thermally assisted OSL (TA-OSL) response. PL spectra showed the characteristic emission of Eu²⁺ ion at 375 nm when excited by 320 nm. The luminescence lifetime has been measured as 40 and 628 μs of fast and slow components respectively. The TL parameters such as trap depth (E), frequency factor (s) and the order of kinetics (b) are determined. The phosphor is found to be 6 and 4 times more sensitive than CaSO₄:Dy and α-Al₂O₃:C, respectively, in TL mode. However, its OSL sensitivity is 75% of α-Al₂O₃:C. It is found to possess three OSL components having photoionization cross-sections of 1.4 × 10⁻¹⁷, 1.2 × 10⁻¹⁸ and 5.2 × 10⁻¹⁹ cm² respectively. The temperature dependence of OSL studies showed that integrated TA-OSL signal increases with stimulation temperature between 50 and 250 °C, while between 260 and 450 °C the signal intensity decreases. This behavior is interpreted to arise from competing effects of thermal assistance (activation energy E_A = 0.063 ± 0.0012 eV) and depletion of trapped charges. This increase of OSL at elevated temperature can be employed for enhancing the sensitivity of phosphor for radiation dosimetry.     

Photoluminescence and radioluminescence study of NaMgF3:Eu nanoparticles

Photoluminescence (PL) and radioluminescence (RL) measurements were made on small (∼25 nm) NaMgF₃ nanoparticles doped with Eu concentrations ranging from 0.1% to 5%. We find that they contained Eu³⁺, Eu²⁺, and an additional unidentified defect with a broad PL emission ∼470 nm. Similar to previous measurements on larger (57 nm–77 nm) NaMgF₃:Eu nanoparticles with 1% Eu and 5% Eu, we find that the PL lifetime decreases with increasing Eu concentration that can be attributed to Eu energy transfer to non-radiative recombination sites. However, there is no change in the fraction of Eu³⁺ distorted sites. The ∼470 nm PL defect peak was also reported for larger nanoparticles, which suggests that this peak arises from similar unidentified point defects. However, the activated non-radiative decay for the small nanoparticles has a significantly lower activation energy. The Eu³⁺ RL decreases by only 2.3% at 10 kGy for low Eu concentrations.     

Optically stimulated luminescence in Ag doped Li2B4O7 single crystal and its sensitivity to neutron detection and dosimetry in OSL mode

Optically stimulated luminescence (OSL) measurements have been carried out on single crystals of Ag doped Li₂B₄O₇ (LTB:Ag) after exposure to various nuclear radiations. The time integrated OSL intensity is found to be linear in the range from 0.1 Gy to 500 Gy. Fading of the OSL signal was found to be around 36% in 48 h. The presence of ⁶Li and ¹⁰B has been gainfully utilized to measure doses of thermal neutrons. Further, the large difference between the wavelength of the stimulation source (∼460 nm) and emission from the LTB:Ag at 270 nm has enhanced the signal-to-noise ratio in a simple OSL set-up with suitable filters. The high sensitivity of the LTB:Ag to thermal neutrons will be useful in variety of applications including personal dosimetry in mixed-fields and imaging devices for neutron radiography.     

Investigation of the electrical properties of metal-oxide-metal structures formed from RF magnetron sputtering deposited MgTiO3 films

Thin films of MgTiO₃ high-k dielectric have been prepared by RF magnetron sputtering deposition at various substrate temperatures. X-ray diffraction, atomic force microscopy were used to characterize the deposited films. Experimental results show that substrate temperature has little effect on the stoichinometry. The electrical properties of MgTiO₃ metal-insulator-metal (MIM) capacitors were investigated at various deposition temperatures, Pt/MgTiO₃/Pt/SiO₂/n-Si, were studied. It is shown that the MgTiO₃ (210 nm) MIM capacitor fabricated at 200 °C shows an overall high performance, such as a high capacitance density of ∼1.2 nF/um², a low leakage current of 1.51 × 10⁻⁹ A/cm² at 5 V, low-voltage coefficients of capacitance, and good frequency dispersion properties. All of these indicate that the MgTiO₃ MIM capacitors are very suitable for use in Si analog circuit application or dynamic random access memory (DRAM) cell.