Peculiarities of optically stimulated luminescence in halite

Sodium chloride – NaCl is one of materials indicating strong OSL signal after exposure on ionizing radiation. Previous studies of the OSL response in pure sodium chloride showed coexistence of fading and regeneration of the signal using the newly developed variable delay optically stimulated luminescence technique (VD-OSL). This paper presents investigations of some peculiarities of long time scale OSL properties of rock salt (halite) including sensitization phenomena for various bleaching methods. Dose response characteristics were studied in the range from 200 mGy to 1 Gy for series of halite aliquots without signal bleaching and for single aliquot using zeroing by blue and green light.     

Experimental demonstration of the variable energy of stimulation optically stimulated luminescence (VES-OSL) method

A new method of OSL stimulation using the continuous increase of stimulation energy (VES-OSL) has been previously suggested. Earlier computer simulations showed that this type of stimulation results in a peak-shaped OSL curve that closely resembles the TL peak. VES-OSL measurements were performed using a stimulation source that was specially constructed for this purpose, and the equipment is described in detail. The first results of these measurements are presented for quartz samples and the behaviour of the VES-OSL curves obtained is consistent with earlier theoretical expectations.     

Estimating the parameters of traps in quartz by the variable energy of stimulation optically stimulated luminescence (VES-OSL) method

Optical-cross section that is a trap parameter estimated from the measurements of optically stimulated luminescence (OSL) is not a uniquely determined physical quantity. It depends not only on temperature and the energy of stimulation light but also, in the simplest case, on the optical depth of trap, the frequency of vibration mode and on the Huang-Rhys factor, i.e. the average number of phonons involved in the process of optical excitation of electrons from trap to conduction band. Conventional OSL measurement techniques do not enable to determine directly these parameters but they could be estimated by applying the variable energy of stimulation optically stimulated luminescence (VES-OSL) method. Recently it was put in to practice and the first VES-OSL curves were presented. In this study the outcomes of VES-OSL experiments are presented together with the first attempt of direct estimating the optical depth of traps active in OSL process in quartz.     

Anomalous regeneration of OSL in sodium chloride – Experiment and modeling

Recently, a new optical phenomenon was observed in sodium chloride (NaCl). Performing series of optically stimulated luminescence (OSL) readouts it was found that subsequent OSL decay, after some delay, starts from significantly higher intensity level than the final intensity of the previous readout. This unusual behavior was called the ‘regeneration effect’ (Biernacka and Mandowski, 2013). Regeneration occurs in spite of the significant fading of OSL – well known in this material. Nevertheless, these two contradictory processes take place in two different time scales. Previously, it was proved also, that the regeneration phenomenon cannot be explained on the basis of the classical simple trap model (STM). This paper analyses optimal measurement conditions leading to the highest regeneration effect. It was found that the increase of OSL intensity could be as high as 190%. A simple phenomenological model explaining the mechanism of regeneration is proposed. The model involves simultaneous localized and delocalized (i.e. band-like) transitions. Characteristic lifetimes of regeneration and fading processes are of the order of 560 s and 22000 s, respectively.     

Influence of thermal treatment on OSL regeneration in potassium chloride

Optically stimulated luminescence (OSL) of pure analytical potassium chloride (KCl) prepared in two different forms (crystals and pellets) was studied. The occurrence of regeneration effect (self-renewal of the OSL signal) in the material was examined. The experiments using the variable delay OSL (VD-OSL) method were carried out. Performed measurements allowed to determine time scale of the phenomenon, as well as quantitative changes of regeneration depending on thermal treatment before and after irradiation. Significant increase of the OSL regeneration was noticeable for pellets after the application of the annealing before irradiation, while for crystals a substantial decrease of regeneration was observed. Preheating applied after irradiation caused that self-renewal of OSL signal was drastically reduced or completely suppressed depending on the form of KCl samples.     

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.     

Modelling the thermal quenching mechanism in quartz based on time-resolved optically stimulated luminescence

This paper presents a new numerical model for thermal quenching in quartz, based on the previously suggested Mott-Seitz mechanism. In the model electrons from a dosimetric trap are raised by optical or thermal stimulation into the conduction band, followed by an electronic transition from the conduction band into an excited state of the recombination center. Subsequently electrons in this excited state undergo either a direct radiative transition into a recombination center, or a competing thermally assisted non-radiative process into the ground state of the recombination center. As the temperature of the sample is increased, more electrons are removed from the excited state via the non-radiative pathway. This reduction in the number of available electrons leads to both a decrease of the intensity of the luminescence signal and to a simultaneous decrease of the luminescence lifetime. Several simulations are carried out of time-resolved optically stimulated luminescence (TR-OSL) experiments, in which the temperature dependence of luminescence lifetimes in quartz is studied as a function of the stimulation temperature. Good quantitative agreement is found between the simulation results and new experimental data obtained using a single-aliquot procedure on a sedimentary quartz sample.     

The role of simulations in the study of thermoluminescence (TL)

The traffic of charge carriers in a luminescent material during its excitation by irradiation and during readout either in the measurement of thermoluminescence (TL) or optically stimulated luminescence (OSL) is governed by sets of coupled nonlinear differential equations. The analytical solution of these sets is usually not possible, and one can resort to one of two options. Some researchers preferred to make simplifying assumptions and thus got approximate solutions whereas others performed simulations by solving the simultaneous equations numerically. Each of these routes has its pitfalls. The simplifying assumptions, mainly the quasi-equilibrium assertion or the assumption that certain relations between the relevant parameters and functions hold, may be valid in certain ranges of the TL or OSL curve, and may cease to be valid, say at the high-temperature range in TL. Performing simulations using the numerical solution of the relevant set of equations may yield results which are accurate, but cannot be considered as being general because they depend on the specific choice of the parameters. Repeating the simulations with several sets of the physically plausible parameters would add credibility to the conclusions drawn. The combination of the two approaches is highly recommended, i.e, if similar results are found by approximations and simulations, the validity of the conclusions is strengthened. Evidently, the comparison of these theoretical results to experimental effects is essential. In the present work we consider the occurrence of unusually high and unusually low values of the activation energy and the effective frequency factor. In particular, we can simulate a recently discovered behavior of TL in LiF:Mg, Cu, P at the ultra-high dose range and get qualitatively the main elements of the experimentally found results.     

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.     

Thermoluminescence and optically stimulated luminescence in various phases of doped Na2SO4

The dependence of optically stimulated luminescence (OSL) and thermoluminescence (TL) response due to crystal phase in Cu and Cu,Mg-doped Na₂SO₄ was studied. Study shows that the slowly cooled samples which crystallize in phase V show good OSL sensitivity whereas the quenched samples of Na₂SO₄ which crystallize in phase III irrespective of doping show no OSL sensitivity. However, during storage when phase III samples get converted to phase V, samples show OSL sensitivity comparable to freshly prepared samples in phase V. Hence, it is observed that TL–OSL properties of doped Na₂SO₄ are phase dependent .This study will be helpful in developing OSL phosphors in which phase plays an important role in deciding the desired properties.     

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.     

Ionization density dependence of the curve shape and ratio of blue to UV emissions of Al2O3:C optically stimulated luminescence detectors exposed to 6-MV photon and therapeutic proton beams

In this work we characterized the dose and linear energy transfer (LET) (ionization density) dependence of commercial Al₂O₃:C optically stimulated luminescence (OSL) detectors (OSLDs) exposed to clinical photon and proton beams. We characterized the dose-dependence of the OSL signal, OSL curve shape, and the relative intensities of the blue and ultraviolet (UV) OSL emission bands using different readout protocols and beam qualities. We irradiated OSLDs with absorbed doses ranging from 0.1 Gy to 100 Gy in a 6-MV photon beam and from 0.1 Gy to 50 Gy in 140- and 250-MeV proton beams. Readouts were done with both continuous-wave (CWOSL) and pulsed (POSL) stimulation. The linearity of the OSLD dose–response depended on readout protocol and radiation type. Improved linearity was found for OSLDs irradiated with beams of increased LET, and for OSL signals containing only the blue emission band of Al₂O₃:C (which remained linear for doses up to 10 Gy for 140-MeV proton beam irradiations). The OSL curve shape did not vary with dose in the low-dose region (below 5 Gy depending on readout protocol), but beyond this, curves decayed more rapidly with increasing dose. Similarly, the ratio of blue to UV emission band intensities in the OSL signal did not vary with dose up to 5 Gy (depending on readout protocol), beyond which the ratio decreased with increasing dose. Because both the OSL curve shape and the ratio of blue to UV emission intensities have been investigated as potential parameters for measurements of LET, the constancy of these two quantities at doses relevant to radiotherapy is encouraging for the potential development of novel OSL methods to measure LET. Our findings are expected to contribute to the development of (i) improved readout protocols for commercially available Al₂O₃:C OSLDs and (ii) methods to measure radiation quality and LET.     

Optically stimulated luminescence in doped K3Na(SO4)2 phosphors

Optically stimulated luminescence (OSL) in Cu and Eu doped K₃Na(SO₄)₂ is reported for the first time. The Cu-doped sample shows OSL sensitivity which gets enhanced by co-doping with Mg²⁺ ions. The Cu-doped and quenched sample shows better sensitivity which is almost double than that of the slowly cooled sample, whereas the sensitivity of Mg co-doped sample remains nearly same irrespective of the thermal treatment. The Cu-doped sample shows TL peak around 200 °C and moderate OSL sensitivity. Doping of Mg shifts the TL peak to around 160 °C and is correlated with good OSL sensitivity. Eu-doped sample does not show OSL sensitivity. However, relatively good OSL sensitivity is observed in Aluminium co-doped and slowly cooled sample, which is about 15% of the commercial Al₂O₃:C(Landuer Inc.). A near fully optically sensitive TL peak around 155 °C is observed. The dose response is linear and practically no OSL fading is observed in first five days of storage in slowly cooled sample. This study on conventional sulphate-based TL phosphors will be useful in developing OSL phosphors for radiation dosimetry.     

Photon dosimetry methods outside the target volume in radiation therapy: Optically stimulated luminescence (OSL), thermoluminescence (TL) and radiophotoluminescence (RPL) dosimetry

Dosimetry methods outside the target volume are still not well established in radiotherapy. Luminescence detectors due to their small dimensions, very good sensitivity, well known dose and energy response are considered as an interesting approach in verification of doses outside the treated region. The physical processes of thermoluminescence (TL), radiophotoluminescence (RPL) and optically stimulated luminescence (OSL) are very similar and can be described in terms of the energy band model of electron-hole production following irradiation.This work is a review of the main dosimetric characteristics of luminescence detectors which were used in experiments performed by EURADOS Working Group 9 for in-phantom measurements of secondary radiation (scattered and leakage photons). TL LiF:Mg,Ti detectors type MTS-7 (IFJ PAN, Poland), types TLD-100 and TLD-700 (Harshaw), OSL Al₂O₃:C detectors type nanoDot™ (Landauer Inc.) and RPL rod glass elements type GD-352M (Asahi Techno Glass Coorporation) are described. The main characteristics are discussed, together with the readout and calibration procedures which lead to a determination of absorbed dose to water.All dosimeter types used show very good uniformity, batch reproducibility and homogeneity. For improved accuracy, individual sensitivity correction factors should be applied for TL and OSL dosimeters while for RPL dosimeters there is no need for individual sensitivity corrections.The dose response of all dosimeters is linear for a wide range of doses.The energy response of GD-352M type dosimeters (with Sn filter) used for out-of-field measurements is flat for medium and low energy X-rays.The energy dependence for TLDs is low across the range of photon energies used and the energy correction was neglected. A significant over response of Al₂O₃:C OSLDs irradiated in kilovoltage photon beams was taken into account. The energy correction factor f_en was calculated by using the 2006 PENELOPE Monte Carlo code.With suitable calibration, all dosimeter types are appropriate for out-of-field dose measurements as well as for the in-phantom measurements of radiotherapy MV X-rays beams.     

Investigation of defect centres responsible for TL/OSL in MgAl2O4:Tb

Magnesium aluminate doped with Tb³⁺ (MgAl₂O₄:Tb³⁺) was prepared by combustion synthesis. Three thermoluminsence (TL) peaks at 120, 220 and 340 °C were observed. PL and TL emission spectrum shows that Tb³⁺ acts as the luminescent centre. Optically stimulated luminescence (OSL) was observed when stimulated by 470 nm blue light.Electron spin resonance (ESR) studies were carried out to identify the defect centres responsible for the TL and OSL processes in MgAl₂O₄:Tb³⁺. Two defect centres were identified in irradiated MgAl₂O₄:Tb³⁺ phosphor by ESR measurements which was carried out at room temperature and these were assigned to V and F⁺ centres. V centre (hole centre) is correlated to 120 and 220 °C TL peaks and F⁺ centre (electron centre), which acts as a recombination centre is correlated to 120, 220 and 340 °C.     

Size-induced effect on cathode luminescence spectra of CsI(Na) and CsI(Tl) crystals

We investigated the cathode luminescence characteristics of CsI(Na)and CsI(Tl)crystals by the spectrum and structure properties at room temperature.We fabricated three different sizes of CsI(Na)and CsI(Tl)crystals and measured their luminescence spectra under cathode rays.We found that CsI(Na)cathode luminescence peaks appear at 420 and 305 nm,and CsI(Tl)cathode luminescence peaks are 540 and 410 nm,the grain size affects CsI(Na)luminescence significantly,and the Na-related420 nm luminescence intensified relatively when the average grain size reaches~20μm,which becomes weak when the grain size is down to nano-scale.But the cathode luminescence spectra of CsI(Tl)crystals with different size have no obvious changes.Our explanations for these phenomena are that the different impurities in the same host material CsI lead to different luminescence mechanisms.These cathode luminescence characteristics indicate the suitability of CsI(Na)and CsI(Tl)crystals to match photomultiplier tube for large area crystal detector development.     

Linear energy transfer dependence of Al2O3:C optically stimulated luminescence detectors exposed to therapeutic proton beams

Currently, there are no radiation detectors that can be used for routine measurements of linear energy transfer (LET) in particle therapy clinics. In this work, we characterized the LET dependence of Al₂O₃:C optically stimulated luminescence (OSL) detectors (OSLDs) exposed to therapeutic proton beams in order to evaluate their potential for clinical LET measurements. We evaluated OSLDs that were irradiated with an absorbed dose to water of 0.2 Gy in therapeutic proton beams with average energies ranging between approximately 25 MeV and 200 MeV, resulting in LET in water values between 0.45 and 2.29 keV/μm. We examined two properties of the OSL emission signal in terms of LET dependence: the signal intensities of the blue and ultraviolet (UV) emission bands, and the shapes of the OSL curves. We found that the signal intensity of the UV emission band increased consistently with LET within the range investigated, whereas the intensity of the blue emission band remained constant. Our results also demonstrated that the OSL curve shapes were more LET dependent for signals containing both the blue and UV emission bands than for signals containing only one of the bands. Both metrics we examined in this study – the relative UV/blue emission signal intensities and OSL curve shapes – show potential for LET detection in proton therapy.     

Theoretical analysis of pattern effect suppression in semiconductor optical amplifier utilizing optical delay interferometer

The ability of an optical delay interferometer (ODI) to suppress the pattern effect that is inherently present in a straightforward, solitary semiconductor optical amplifier (SOA) whose dynamic response is slower than the period of its driving high-speed return-to-zero (RZ) data signal is theoretically investigated. For this purpose an existing comprehensive model that simulates and links the operation of these two elements is methodically applied to their concatenated configuration. In this manner an extensive set of curves is numerically obtained, which allow to analyze and assess the impact of the input pulse energy and width as well as of the SOA carrier lifetime, linewidth enhancement factor and small signal gain on the amplitude modulation of the transmitted sequence at the output of each one of these block units. Their thorough study and interpretation reveals that the employment of the ODI can significantly reduce the value of this quality metric resulting from a single SOA only. The main offered benefit, however, is that any technical restrictions regarding the involved critical parameters can be considerably relaxed while at the same time their useful operational range can be extended. These important findings highlight the necessity of placing this passive device after the SOA and exploiting it in order to effectively alleviate the detrimental pattern-dependent degradation. This fact in conjunction with its overall practicality renders it a promising candidate for enhancing, within the frame of the proposed scheme, the performance of SOAs that are employed as pure amplification elements in fiber-optic communication systems and networking applications.     

Investigation of laterally single-diffused metal oxide semiconductor (LSMOS) field effect transistor

We propose a distinct approach to implement a laterally single diffused metal-oxide-semiconductor (LSMOS) FET with only one impurity doped p-n junction. In the LSMOS, a single p-n junction is first created using lateral dopant diffusion. The channel is formed in the p region of the p-n junction and the n region acts as the drift region. Two distinct metals of different work function are used to form the “n⁺” source/drain regions and “p⁺” body contact using the charge plasma concept. We demonstrate that the LSMOS is similar in performance to a laterally double diffused metal-oxide-semiconductor (LDMOS) although it has only one impurity doped p-n junction. The LSMOS exhibits a breakdown voltage of ∼50.0 V, an average ON-resistance of 48.7 mΩ-mm² and a peak transconductance of 53.6 μS/μm similar to that of a comparable LDMOS.