Phosphor Grains Research Articles

New optically stimulated luminescence dosimetry film optimized for energy dependence guided by Monte Carlo simulations

Optically stimulated luminescence (OSL) film dosimeters, based on BaFBr:Eu2+ phosphor material, have major dosimetric advantages such as dose linearity, high spatial resolution, film re-usability, and immediate film readout. However, they exhibit an energy-dependent over-response at low photon energies because they are not made of tissue-equivalent materials. In this work, the OSL energy-dependent response was optimized by lowering the phosphor grain size and seeking an optimal choice of phosphor concentration and film thickness to achieve sufficient signal sensitivity. This optimization process combines measurement-based assessments of energy response in narrow x-ray beams with various energy response calculation methods applied to different film metrics. Theoretical approaches and MC dose simulations were used for homogeneous phosphor distributions and for isolated phosphor grains of different dimensions, where the dose in the phosphor grain was calculated. In total 8 OSL films were manufactured with different BaFBr:Eu2+ median particle diameters (D50): 3.2 μm, 1.5 μm and 230 nm and different phosphor concentrations (1.6%, 5.3% and 21.3 %) and thicknesses (from 5.2 to 49 μm). Films were irradiated in narrow x-ray spectra (N60, N80, N-150 and N-300) and the signal intensity relative to the nominal dose-to-water value was normalized to Co-60. Finally, we experimentally tested the response of several films in Varian 6MV TrueBeam STx linear accelerator using the following settings: 10 × 10 cm2 field, 0deggantry angle, 90 cm SSD, 10 cm depth. The x-ray irradiation experiment reported a reduced energy response for the smallest grain size with an inverse correlation between response and grain size. The N-60 irradiation showed a 43% reduction in the energy over-response when going from 3 μm to 230 nm grain size for the 5% phosphor concentration. Energy response calculation using a homogeneous dispersion of the phosphor underestimated the experimental response and was not able to obtain the experimental correlation between grain size and energy response. Isolated grain size modeling combined with MC dose simulations allowed to establish a good agreement with experimental data, and enabled steering the production of optimized OSL-films. The clinical 6 MV beam test confirmed a reduction in energy dependence, which is visible in small-grain films where a decrease in out-of-field over-response was observed.

Down and upconversion photoluminescence of ZrO2:Er3+ phosphor irradiated with 120 MeV gold ions

Down conversion and upconversion photoluminescence of pristine and gold ion irradiated ZrO2:Er3+ phosphor synthesized by solution combustion are reported. The crystallinity of the sample as analysed by x-ray diffraction shows a monoclinic phase having a crystallite size of about 57 nm calculated using Williamson—Hall formula. Field emission scanning electron microscopy shows that the shape and size of phosphor grains are non uniform. The down conversion photoluminescence of ZrO2:Er3+ has sharp emission bands in the red, green and blue regions of the spectrum. These emissions are corresponding to f–f transitions of Er3+ ions under excitation of 379 nm. In particular, the emission has maxima at 467, 492, 526, 548 and 660 nm correspond to 4F3/2 → 4I15/2, 4F7/2 → 4I15/2, 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions respectively. Interestingly, the PL emission intensity (excitation at 379 nm) is enhanced 1.6 times higher than pristine sample after 120 MeV Au ion irradiation for a fluence of 1 × 1010 ions cm−2. The blue to green emission ratio increases with the increase in ion fluence. Therefore, emission of the color shift towards bluish-white color with ion fluence. The lifetime of 4S3/2 level is found to be 16.9 and 71.5 μs for pristine and Au ion irradiated (1 × 1012 ions cm−2) ZrO2:Er3+ respectively. In near infrared (NIR) region, the PL emission band is observed at 1531 nm corresponding to 4I13/2 → 4I15/2 transition of Er3+ under excitation of 980 nm. The sample emits intense green emission and relatively weak red emission in up conversion PL under excitation of 980 nm. The ratio of intensities of red and green emission changes after Au ion irradiation is attributed to the corresponding change in the lattice symmetry in the host. Resulting in strong up conversion emission from two photon absorption process.

Light beam interactions and emission performance in powder phosphor materials: The role of the binder

Abstract The light emission performance of powder phosphors, employed in a variety of imaging applications, is affected by their structural (i.e., the particle size and packing) and optical properties (i.e., the wavelength of the light rays and the refractive index of the grains). In the chemical synthesis and the corresponding construction development of phosphor layers, the binder plays crucial role though its contribution to the way that the grains are glued together in a packed spatial distribution. In the present work, the influence of the binder on optical diffusion models was examined by using Mie theory and Monte Carlo simulation. To this aim, phosphor layers were investigated assuming different set-ups of refractive indexes (i.e., four cases of phosphor grains and binder combinations). Results showed that the choice of the binder affects the amount and distribution of light mainly through the: (i) domination of light absorption with respect to light scattering, and (ii) the direction of the light beam when light scattering takes place. It is important to note that by changing the refractive index of the binding material the light spread was not considerably varied through the number of light-grain interactions.

Study of Gd-induced shift of luminescence spectra of YGAG:Ce micropowders as a function of grain size

Four YGAG:Ce phosphors in the form of micropowders synthesized by solid-state reaction and the small-size fractions isolated thereof were thoroughly studied in order to establish the relationship between luminescent characteristics, size and the composition of the phosphor grains. Both photoluminescence (PL) and cathodoluminescence (CL) spectra were measured and analyzed. The PL spectra excited by the blue LED were taken from the powder samples differing in the content of key dopants (Ce, Gd) as controlled by inductively coupled plasma mass spectrometry (ICP-MS). The CL spectra excited by the electron beam of a scanning electron microscope (SEM) were taken from individual grains of various sizes (1–40µm). It was found that in most cases the smaller is the grain size, the lower is the intensity of PL and the greater is the blue shift of spectra. This effect is strongly correlated with a content of Ce and Gd in the grains. The mechanisms of Gd-induced red shift of luminescence were considered in terms of the fine structure of CL spectra.

Investigating the particle packing of powder phosphors for imaging instrumentation technology: an examination of Gd2O2S:Tb phosphor

The present paper summarizes the examination of packing density effect within a phosphor layer by using Mie scattering theory and Monte Carlo simulation techniques. Light propagation in the phosphor layer was modeled by considering the following cases: (i) phosphors of different layer thickness, 100 μm (thin layer) and 200 μm (thick layer), respectively, (ii) packing density from 10% up to 90%, (iii) three values of phosphor grain diameter 100 nm, 500 nm, and 1 μm, (iii) light wavelength and complex refractive index of the phosphor grains which correspond to Gd2O2S:Tb granular phosphor (light wavelength: 545 nm refractive index real part: 2.3, refractive index imaginary part: 10−6). Results showed the linear dependence of light extinction coefficient mext with packing density for all particle diameters considered, and, in particular, the highest variations occurred for particle size 500 nm (from 1.2 μm−1 up to 10.8 μm−1, considering packing density in the range 10% – 90%). Due to this variation, the highest spatial resolution was achieved for the same grain size (500 nm) and for the highest value of packing density (90 %). However, a very significant outcome of the present investigation was that improved spatial resolution can be achieved without significant decrease in light collection efficiency, if particles in the submicrometer scale (e.g., 500 nm) with relatively low packing density (e.g., 40%) are used.

Optical absorption characteristics in the assessment of powder phosphor-based x-ray detectors: from nano- to micro-scale

X-ray phosphor-based detectors have enormously improved the quality of medical imaging examinations through the optimization of optical diffusion. In recent years, with the development of science and technology in the field of materials, improved powder phosphors require structural and optical properties that contribute to better optical signal propagation. The purpose of this paper was to provide a quantitative and qualitative understanding of the optical absorption characteristics in the assessment of powder phosphor-based detectors (from nano- scale up to micro-scale). Variations on the optical absorption parameters (i.e. the light extinction coefficient and the percentage probability of light absorption p%) were evaluated based on Mie calculations examining a wide range of light wavelengths, particle refractive indices and sizes. To model and assess the effects of the aforementioned parameters on optical diffusion, Monte Carlo simulation techniques were employed considering: (i) phosphors of different layer thickness, 100 μm (thin layer) and 300 μm (thick layer), respectively, (ii) light extinction coefficient values, 1, 3 and 6 μm−1, and (iii) percentage probability of light absorption p% in the range 10−4–10−2. Results showed that the coefficient is high for phosphor grains in the submicron scale and for low light wavelengths. At higher wavelengths (above 650 nm), optical quanta follow approximately similar depths until interaction for grain diameter 500 nm and 1 μm. Regarding the variability of the refractive index, high variations of the coefficient occurred above 1.6. Furthermore, results derived from Monte Carlo modeling showed that high spatial resolution phosphors can be accomplished by increasing the parameter. More specifically, the FWHM was found to decrease (i.e. higher resolution): (i) 4.8% at 100 μm and (ii) 9.5%, at 300 μm layer thickness. This study attempted to examine the role of the optical absorption parameters on optical diffusion studies. A significant outcome of the present investigation was that the improvement of phosphor spatial resolution without decreasing the light collection efficiency too much can be better achieved by increasing the parameter rather than the parameter p%.

Assessing Spatial Resolution of Cathodo-Luminescence Imaging and Spectroscopy at the Nanoscale for Inorganic Phosphor Powders

It is now possible to easily collect CL images (using a photomultiplier tube using total or filtered light) and emission spectra from individual nanometer sized particles using the commercially available Gatan Vulcan attachment for a transmission electron microscope (TEM). In our case, at Brunel University London, we have fitted this to a JEOL 2100F field emission TEM. This work demonstrates the spatial resolution that has been achieved when imaging various phosphor nano materials and discusses the limitations of the technique. It also demonstrates the uniformity of excitation/emission from various phosphors, and hence the effectiveness of the synthetic method used to manufacture the phosphor can be seen. Conventionally, photoluminescence (PL) and cathodoluminescence (CL) spectra are collected from bulk phosphor samples and are the average of all of the particles in the sample. This is true of most samples made either in the laboratory for research or commercial materials prepared by industrial companies. Cathodoluminescence spectra have been collected from nanometer-sized crystals of quantum dots (spherical and rods) and various oxide and oxysulfide phosphors. The latter phosphors have been implemented into CL applications on a vast scale whereas the quantum dot materials are not designed for CL application and therefore great care is required when imaging these materials to minimize the effect of beam damage. When observing mixtures of phosphors (Gd2O2S:Tb3+ and Y2O3:Eu3+) it is apparent that when the electron beam is positioned onto one of these two materials and the spectrum observed, then the other phosphor emission spectrum can be observed if the particles are nearby. However when observing quantum dot materials the spatial resolution is only limited by the particle size. A tentative explanation for this effect is given. In this work we compare the spectra of nanometer sized phosphor crystals/particles in order to understand any variation in the emission spectra between the phosphor grains and to understand the effect of temperature on their luminescent properties. Our results will be presented that could only have been found by using CL spectra obtained directly from single nanoparticles. In conclusion: 1) The effect of different TEM machine parameters will be discussed 2) The CL spatial resolution will be demonstrated Figure 1. Left) HAADF STEM of nanometre-sized single crystals of Y2O3Eu3+ and Gd2O2S:Tb3+; The green line spectrum image shows where 10 emission spectra were recorded at equally spaced intervals. Right) Extracted spectra taken from opposite ends on the line scan show the characteristic CL emission spectrum for both the Eu3+ and Tb3+ are observed in both spectra. Figure 1

Fabrication of fine imaging devices using an external proton microbeam

We have successfully fabricated novel microscopic imaging devices made from UV/EB curable resin using an external scanning proton microbeam. The devices are micro-structured fluorescent plates that consist of an array of micro-pillars that align periodically. The base material used in the pillars is UV/EB curable resin and each pillar contains phosphor grains. The pattern exposures were performed using a proton beam writing technique. The height of the pillars depends on the range of the proton beam. Optical microscopy and scanning electron microscopy have been used to characterize the samples. The results show that the fabricated fluorescent plates are expected to be compatible with both spatial resolution and detection efficiency.

Understanding and arresting degradation in highly efficient blue emitting BaMgAl10O17:Eu2+ phosphor—A longstanding technological problem

Blue emitting BaMgAl10O17:Eu2+ (BAM) phosphor is indispensable for Plasma Display panel and lighting because of high luminescence efficiency. However, thermal degradation (annealing in air at 500–600°C) of BAM (upto ∼30%) remains an intriguing problem for display industry worldwide. In the present study, a systematic approach is pursued to develop highly efficient BAM phosphor that exhibits least degradation, understand the role of Eu2+ site occupancy in such BAM phosphor and encapsulate individual phosphor grains with a shell of silica nanoparticles. The approaches lead to highly efficient BAM:Eu2+ phosphor that showed no degradation against thermal baking (annealing at 500°C in air) for both UV and VUV radiation under UV and VUV excitation. An optimum solid state chemical route including precursor phases, dopant concentration, and thermal regimes has been evolved to develop BAM. Emission from Eu2+ occupying three different sites is identified with energetically stable anti Beevers Ross as the dominant contributor. Coating by nano sized amorphous silica sol with subsequent sintering lead to uniform silica shell. This nano silica layer also helps to enhance the luminescence from phosphor grains.

Optical photon transport in powdered‐phosphor scintillators. Part 1. Multiple‐scattering and validity of the Boltzmann transport equation

In Part 1 of this two-part work, predictions for light transport in powdered-phosphor screens are made, based on three distinct approaches. Predictions of geometrical optics-based ray tracing through an explicit microscopic model (EMM) for screen structure are compared to a Monte Carlo program based on the Boltzmann transport equation (BTE) and Swank's diffusion equation solution. The purpose is to: (I) highlight the additional assumptions of the BTE Monte Carlo method and Swank's model (both previously used in the literature) with respect to the EMM approach; (II) demonstrate the equivalences of the approaches under well-defined conditions and; (III) identify the onset and severity of any discrepancies between the models. A package of computer code (called phsphr) is supplied which can be used to reproduce the BTE Monte Carlo results presented in this work. The EMM geometrical optics ray-tracing model is implemented for hypothesized microstructures of phosphor grains in a binder. The BTE model is implemented as a Monte Carlo program with transport parameters, derived from geometrical optics, as inputs. The analytical solution of Swank to the diffusion equation is compared to the EMM and BTE predictions. Absorbed fractions and MTFs are calculated for a range of binder-to-phosphor relative refractive indices (n = 1.1-5.0), screen thicknesses (t = 50-200 μm), and packing fill factors (pf = 0.04-0.54). Disagreement between the BTE and EMM approaches increased with n and pf. For the largest relative refractive index (n = 5) and highest packing fill (pf = 0.5), the BTE model underestimated the absorbed fraction and MTF50, by up to 40% and 20%, respectively. However, for relative refractive indices typical of real phosphor screens (n ≤ 2), such as Gd2O2S:Tb, the BTE and EMM predictions agreed well at all simulated packing densities. In addition, Swank's model agreed closely with the BTE predictions when the screen was thick enough to be considered turbid. Although some assumptions of the BTE are violated in realistic powdered-phosphor screens, these appear to lead to negligible effects in the modeling of optical transport for typical phosphor and binder refractive indices. Therefore it is reasonable to use Monte Carlo codes based on the BTE to treat this problem. Furthermore, Swank's diffusion equation solution is an adequate approximation if a turbidity condition, presented here, is satisfied.

Research on Y 2O 3:Eu Phosphor Coated with In 2O 3

Y 2O 3:Eu red phosphor for FED application was prepared by high temperature solid-state reaction. The In 2O 3 coating by precipitation method to the phosphor was applied and the analyses of XRD, Zeta potential, SEM, EDS and low voltage cathodoluminescence (CL) were conducted for investigating the coating effect. The results showed that In 2O 3 coating promoted the low voltage CL of the phosphor efficiently. The promotion was possibly due to the enhancement of the surface conductivity of the phosphor grains.

Realization of Flexible Plasma Display Panels on PET Substrates

DC plasma display panels are fabricated on flexible polyethylene terephtalate (PET) substrates. Each pixel consists of laterally placed anode and cathode electrodes. All electrical elements are formed on a single PET substrate, whereas a second substrate is needed to encapsulate the panel. Silver is used as the metal for each electrode and standard photolithography is used to form each cell. A 150-/spl mu/m-thick layer of a UV-curable polymeric adhesive was used to form barrier ribs to both electrically isolate neighboring cells and to encapsulate the plasma. Conversion of vacuum UV into visible light is possible by blast-embedding of proper phosphor grains into the top substrate. The current-voltage and turn-on voltage versus pressure characteristics are examined. Effect of curvature on turn-on voltage is addressed.

A structural evaluation of ZnS-based electroluminescent phosphors and devices: a comparison with performance

A number of ZnS phosphor powders, both commercial and experimental, have been characterized by transmission electron microscopy (TEM). The microstructure of the phosphors has been compared with the light output (intensity and spectra) obtained when incorporated into an ac-driven electroluminescent test lamp. No evidence has been found in any samples examined by TEM of Cu2S precipitates. It was also found that the brightest emission is obtained when the phosphor particles have a very heavily multiply twinned structure together with a rough profile, and least output when they show the cubic phase. Examination of the electroluminescence output of individual phosphor grains indicates that emission is mostly from material close to the surface and suggests that field emission occurs from the surface which is aided by a marked surface roughness. Finally, the microstructure of a commercial ac powder phosphor sheet device was examined and also found to contain ZnS in the same heavily twinned form.

Monte Carlo calculation of CRT screen efficiency

Abstract— The influence of phosphor grain size on overall CRT screen efficiency has been studied by Monte Carlo calculation of the energy absorption and the photon propagation in the phosphor screen. To calculate the energy absorption, the phosphor layer is modeled as a randomly packed layer of spheres with defined size distribution and screen weight. The absorbed energy in the phosphor grains is obtained from cross sections of the physical processes involved. The optical efficiency is calculated taking into account the scattering and absorption probability of the photons within the phosphor screen. The optimal screen efficiency which can be obtained for CRT screens with constant phosphor powder efficacy and aluminum reflectivity increases with increasing median grain size of the phosphor powder. Depending on the actual anode voltage applied, the CRT screen efficiency increases for a 32‐kV anode voltage by 15%, going from 3‐ to 8‐μm median grain size of the phosphor powder. Larger grains at this anode voltage do not further increase the efficiency. Grain‐size distribution has no noticeable influence on screen efficiency.

Modeling the optical properties of fluorescent powders: Y1.91Eu0.09O3

A numerical model is developed to study the optical properties of phosphor powder coatings. The method derived uses a combination of ray tracing and Monte Carlo modeling which allows us to calculate the absorption, reflection, and transmission for the exciting ultraviolet radiation as well as for the emitted visible light. Size, shape, morphology, density of packing of the phosphor grains, and the roughness of the coating can be studied in detail with our model. Calculations for Y1.91Eu0.09O3 red phosphor powder layers used in commercial fluorescent lamps are presented. Good agreement with experiment was found.

Cathodoluminescence, reflectivity changes, and accumulation of graphitic carbon during electron beam aging of phosphors

We demonstrate that extended e-beam exposure produces a contaminating overlayer on phosphors whose opacity increases roughly linearly with time. Raman scattering data and optical analysis indicate that this layer is graphitic in nature, arising from the electron-beam-stimulated conversion of hydrocarbons adsorbed from the vacuum ambient. The presence of this contamination optically attenuates emitted cathodoluminescence, prevents many low energy electrons from ever reaching the phosphor grains, and exacerbates surface charging which reduces the arrival energy of electrons above 1.5–2 keV. All of these effects are shown to impact cathodoluminescent output in an important way, but an accurate accounting of their total impact will be required to assess the importance of other degradation mechanisms like enhanced nonradiative electron-hole recombination at surfaces, both carbon and noncarbon related.

α-Fe 2O 3 Coatings on Y 2O 2S:Eu

Two methods are presented for applying α-Fe 2O 3 coatings to Y 2O 2S:Eu phosphor grains. In the first method α-FeO(OH) is precipitated homogeneously on the phosphor, followed by thermal decomposition into α-Fe 2O 3. The second method consists of precipitation of FeSO 4 on the phosphor, followed by thermal decomposition of FeSO 4 into α-Fe 2O 3. The material obtained can be applied as red primary in color television sets.

Scanning luminescence X‐ray microscopy: Imaging fluorescence dyes at suboptical resolution

SummaryScanning luminescence X‐ray microscopy is based on the use of the very small focused probe of a scanning X‐ray microscope to stimulate visible light emission from phosphors and dyes. Using an undulator X‐ray source and a Fresnel zone plate to produce a focused X‐ray probe, images of P31 phosphor grains with a resolution of 50–75 nm have been obtained, and luminescence from polystyrene spheres loaded with 50–100 μmol/g of fluorescent dye has been imaged. The resolution was not limited by the focused X‐ray probe (the microscope has imaged features at 36‐nm spacing in transmission mode) but by dark noise and the low net efficiency of the luminescence detection system used for this investigation. This technique may make it possible to image dye‐tagged sites of biochemical activity at the resolution of the X‐ray microscope in wet, unsectioned, and unfixed cells, especially with soft X‐ray optimized dyes. Because the image is formed from the detection of signal against a dark background, calculations suggest that the radiation dose for luminescence imaging of dye‐tagged features should be 2–22 times lower than it is in transmission X‐ray microscopy. A possible extension of the technique for three‐dimensional imaging at the transverse resolution of the X‐ray microscope is described, where visible light collection optics might be used to obtain submicrometre axial resolution.

Temperature measurements with micrometer spatial resolution

A novel technique has been developed to measure temperatures with a spatial resolution less than 10 μm. This method uses the temperature-sensitive time decay fluorescence of a phosphor as a surface sensor. The spatial resolution is obtained by using an electron beam to excite individual phosphor grains deposited on the surface of interest. The phosphor selected for its dynamic range and chemical stability is the inorganic compound magnesium fluorogermanate activated with manganese. The time decay constant of its fluorescence decreases monotonically with temperature between −200 °C to 450 °C. This method was verified on a cross-sectioned Zener diode. A description of the technique and a temperature map of the sectioned area of an operating diode are presented.

A theoretical model for the x-ray luminescence of granular phosphor screens

A theoretical model for the x-ray luminescence of a phosphor screen is presented. The model takes into account the granular structure of the screen and the random deposition of the phosphor grains on the screen substrate. Results based on this model are in very good agreement with the experimental data already known for the x-ray luminescence of the various phosphor screens.