Increase In Light Output Research Articles

Improvement of BGO ceramic scintillators through hot‐pressing sintering methodology

AbstractIn the present work, the sintering of bismuth germanate through hot pressing and the improvement of scintillator performance were investigated. The linear shrinkage, crystalline structure, microstructure, transparency degree, and radioluminescence (RL) were studied as functions of the sintering pressure. X‐ray diffraction revealed that the samples were predominantly composed of the Bi4Ge3O12 phase, accompanied by small amounts of Bi12GeO20, with concentrations varying according to the sintering parameters. These concentrations were quantified through Rietveld refinement, which also indicated a tendency for the cell parameters to shrink as the sintering pressure increased. The microstructure of the ceramic pellets produced under varying hot‐pressing parameters was investigated using scanning electron microscopy (SEM), revealing that while the grain size was preserved, the porosity and grain boundary thickness were reduced by the hot pressing, forming a quasicontinuum in some areas of the sample. The RL of all the samples exhibited a green color, with a maximum at 540 nm, ascribed to the transitions from the 3P0,1,21P1 excited states to the fundamental 1S0 state of the Bi3+ ions. For samples sintered under a pressure of .18 MPa, the enhancement in the optical transmittance, accompanied by a 61.36% increase in light output at the maximum wavelength, was observed.

Efficient and Ultrafast Oxyorthosilicate Scintillator by Activator Valence State Manipulation.

There is an urgent need for faster, brighter, and more controllable scintillation materials in advanced nuclear medicine, high-energy physical experiments, and dark matter particle detection. Nevertheless, the trade-off between high emission efficiency and fast timing characteristics remains a common challenge in the entire optical field. To address this issue, we develop a composition engineering strategy that involves multisite selective doping. This strategy aims to transform nearly all Ce3+ into fast-emitting Ce4+ while synergistically suppressing the electron traps. Even at very low doping concentrations, the designed Ca2+, Al3+, and Ce3+ tridoped oxyorthosilicate exhibits a scintillation decay (τd) acceleration of 20%, accompanied by a 25% increase in light yield (LY). The ratio of emission efficiency and timing characteristics (LY/τd) can be enhanced by 56%, which realizes the perfect balance of high LY and fast τd. Our work provides a method for designing efficient, ultrafast, and controllable scintillators in multicomponent systems, thus paving the way for high-resolution radiation detection and imaging applications.

Absolute energy-dependent scintillating screen calibration for real-time detection of laser-accelerated proton bunches.

Laser-plasma accelerators (LPAs) can deliver pico- to nanosecond long proton bunches with ≳100 nC of charge dispersed over a broad energy spectrum. Increasing the repetition rates of today's LPAs is a necessity for their practical application. This, however, creates a need for real-time proton bunch diagnostics. Scintillating screens are one detector solution commonly applied in the field of electron LPAs for spatially resolved particle and radiation detection. Yet their establishment for LPA proton detection is only slowly taking off, also due to the lack of available calibrations. In this paper, we present an absolute proton number calibration for the scintillating screen type DRZ High (Mitsubishi Chemical Corporation, Düsseldorf, Germany), one of the most sensitive screens according to calibrations for relativistic electrons and x rays. The presented absolute light yield calibration shows an uncertainty of the proton number of 10% and can seamlessly be applied at other LPA facilities. For proton irradiation of the DRZ High screen, we find an increase in light yield of >60% compared to reference calibration data for relativistic electrons. Moreover, we investigate the scintillating screen light yield dependence on proton energy since many types of scintillators (e.g., plastic, liquid, and inorganic) show a reduced light yield for increased local energy deposition densities, an effect termed ionization quenching. The ionization quenching can reduce the light yield for low-energy protons by up to ∼20%. This work provides all necessary data for absolute spectral measurements of LPA protons with DRZ High scintillating screens, e.g., when used in the commonly applied Thomson parabola spectrometers.

An efficiently excited Eu3+ luminescent site formed in Eu,O-codoped GaN

For the development of III-nitride-semiconductor-based monolithic micro-light-emitting diode (LED) displays, Eu,O-codoped GaN (GaN:Eu,O) is a promising material candidate for the red LEDs. The luminescence efficiency of Eu-related emission strongly depends on the local atomic structure of Eu ions. Our previous research has revealed that post-growth thermal annealing is an effective method for reconfiguring luminescent sites, leading to a significant increase in light output. We observed the preferential formation of a site with a peak at ∼2.004 eV by the annealing process. In this study, we demonstrate that it is a previously unidentified independent site (OMVPE-X) using combined excitation–emission spectroscopy and time-resolved photoluminescence measurements. In addition, we perform excitation power-dependent photoluminescence measurements and show that this OMVPE-X site dominates the emission at a low excitation power region despite its small relative abundance, suggesting a high excitation efficiency. Most importantly, applying our annealing technique to an LED exhibits a reasonably increased electroluminescence intensity associated with OMVPE-X, confirming that this site has a high excitation efficiency also under current injection. These results demonstrate the importance of OMVPE-X as a notable luminescent site for brighter and more efficient GaN:Eu,O-based LEDs.

Improving the resolution and light yield in CsI(Tl) scintillators

In the search for Dark Matter, very low-energy sensitive detectors are needed that would be able to distinguish minute differences in closely residing energy peaks. To increase the ability of CsI(Tl) scintillator detectors to do so, a different wrapping material will be used on the innermost layer of the detector. This wrapping material is a 3MTM Enhanced Specular Reflector (ESR) material which is hypothesized to produce a higher light yield with better resolution than the PTFE wrapping of plumber’s tape due to its high reflectivity. Two different types of measurements were carried out to test this hypothesis. Each test was run on a different half of a CsI(Tl) rectangular crystal of volume 48 in3. First, a source of 60Co was used to test the difference between the wrappings. The test was run an hour for the PTFE and the ESR wrapping. We observe a 35% increase in the ADC count for each 60Co energy peak and 15% decrease in the value of the standard deviation around those peaks. Secondly, a source of 241Am was used to test the low-energy capabilities of the ESR material. Here also we see a significant increase in the ADC count for the two energy peaks of the 241Am source. Due to the high photon reflectivity capability of the ESR film, our detector is more capable of collecting photons created by low-energy interactions. As we know Dark Matter produces very low-energy recoils, while interacting with the detector target material, so the ability to contain a maximum number of photons within the detector volume will help us move one step closer to detecting the rare interaction. Because of this significant increase in light yield and general decrease in standard deviation, it is clear that the ESR material has a greater ability to gather higher resolution data than its predecessor. This could lead to a greater understanding of low-energy particles, which in turn could help us understand what Dark Matter really is.

Measurement of LYSO crystal light output and energy resolution improvement with acid etching

In this study, we investigated the impact of chemical etching on the light output and energy resolution of LYSO scintillators using simple, affordable laboratory equipment. We found that etching with phosphoric acid at temperatures between 180 °C and 190 °C improved the light output and energy resolution compared to mechanically polished crystals, even after minimal etching times. Our results show that with 7.5 min of chemical etching, the light output increase rate is 45.7%, and the relative energy resolution improvement is 12%.

TOPS fast timing plastic scintillators: Time and light output performances

Organic plastic scintillators are largely exploited for fast timing detectors thanks to their short scintillation time with respect to inorganic crystals. Plastic scintillators are cheap to produce, light and easy to manipulate (any standard mechanical workshop can handle the cutting, polishing, etc.). Nowadays, fastest plastic scintillators available on the market are EJ-232 (Eljen Technology) and BC-422 (Saint Gobain) with a rise time of 350ps, a decay time of 1.6ns and a pulse width of 1.3ns. Foreseen the performance improvement of timing detectors based on plastic scintillators, the development of faster scintillators can give a crucial contribution. TOPS is a project focused on the development of a new class of organic scintillators producing several liquid and solid samples. Comparing the light output and the timing properties of the samples exploiting minimum ionising particles, a selection of the highly performing TOPS scintillators has been investigated and characterised. The performance achieved with TOPS samples are extremely promising: a time resolution improvement from 10 up to 35% with respect to the EJ-232 has been demonstrated. In addition, an increase of light output has been obtained for all samples with a consequent potential improvement in energy resolution measurements of a factor up to 35%.

Band Alignment Engineering in ns2 Electrons Doped Metal Halide Perovskites

AbstractLuminescent metal halide perovskites (MHPs) open new avenues for highly efficient radiation detection. To challenge the state‐of‐art technology, fundamental understanding of factors controlling radiation light yield of MHP scintillators is urgent. Herein, a design method is established by simultaneously considering charge‐transfer and recombination efficiencies via band alignment engineering in doped MHPs materials, and this strategy is corroborated experimentally and computationally by applying it to the luminescence of ns2 electron (Sb3+, Bi3+, and Te4+) doped vacancy‐ordered double perovskite Cs2ZrCl6. Alloying Te4+ into Cs2ZrCl6 is optimized and significantly improves the scintillation performance, including a twofold increase in light yield and a threefold increase in detection limit over pristine Cs2ZrCl6, and high‐resolution X‐ray imaging with 20 μm for 2D and 0.2 mm for 3D imaging. It is believed that doping engineering in MHPs enabling band alignment method holds great potential for the development of next‐generation MHP scintillators.

Enhanced characteristics of 3D-Printed plastic scintillators based on bisphenol fluorene diacrylates

Three types of acrylates with the fluorenyl group were selected for the purpose of enhancing 3D-printed plastic scintillators. The mechanical and optical characteristics of all the selected monomers were investigated, and optical absorbance and band gap energy were measured and compared to the standard monomers of styrene, PMMA, and vinyltolune. By measurement of light output for the 3D-printed plastic scintillators, an increase in light output correlating with an increase in the number of benzene rings per molecule weight was observed in the results of all the selected acrylates. The analysis of Raman spectroscopy revealed a linear dependence (R2 = 0.9599) of light output on the Raman intensities at 3070 cm−1. In the use of ethoxylated bisphenol fluorene diacrylate (OPPEA 40%), the best mechanical and optical performance of the 3D-printed plastic scintillator was found to 72% transmission, maximum emission wavelength 470 nm, hardness 84 Shore-D, and light output 5300 photons/MeV.

Charge calibration of DRZ scintillation phosphor screens

As a basic diagnostic tool, scintillation screens are employed in particle accelerators to detect charged particles. In extension to the recent revision on the calibration of scintillation screens commonly applied in the context of plasma acceleration [T. Kurz et al., Rev. Sci. Instrum. 89 (2018) 093303], here we present the charge calibration of three DRZ screens (Std, Plus, High), which promise to offer similar spatial resolution to other screen types whilst reaching higher conversion efficiencies. The calibration was performed at the Electron Linac for beams with high Brilliance and low Emittance (ELBE) at the Helmholtz-Zentrum Dresden-Rossendorf, which delivers picosecond-long beams of up to 40 MeV energy. Compared to the most sensitive screen, Kodak BioMAX MS, of the aforementioned recent investigation by Kurz et al., the sample with highest yield in this campaign, DRZ High, revealed a 30% increase in light yield. The detection threshold with these screens was found to be below 10 pC/mm2. For higher charge-densities (several nC/mm2) saturation effects were observed. In contrast to the recent reported work, the DRZ screens were more robust, demonstrating higher durability under the same high level of charge deposition.

Improving light output and coincidence time resolution of scintillating crystals using nanoimprinted photonic crystal slabs

Scintillating crystals are used in numerous applications of ionizing radiation detectors. In time of flight positron emission tomography (TOF-PET) for example, both energy and coincidence time resolution (CTR) are important characteristics that could significantly benefit if more light from scintillators, otherwise trapped, could be collected by the photodetector. A novel and promising method to extract more efficiently the light produced in crystal scintillators with high index of refraction is to introduce a thin nanopatterned photonic layer on the readout surface. In this paper, we describe the patterning process of a photonic crystal layer made of TiO2 with 390 nm diameter ”pillars” in a square lattice with a periodicity of 580 nm and a structure thickness of 300 nm on one side of a 10x10x10 mm 3 LYSO cube. The production process used was nanoimprint lithography. A substantial increase in light yield of ≥ 50% has been measured in good agreement with our simulations. An interesting result from these measurements is that the improvement in light output is independent of whether the crystal is read out from its photonically patterned side or from the one opposite to it. For all cases studied, the energy resolution improved by a factor of 1.1. On the other hand, the CTR, being very threshold dependent, is unlike the light yield not subject to a constant improvement. It turns out that, at low thresholds, the gain (improvement) in CTR is limited to 1.2, and then rapidly increases to a value of more than 2 at higher thresholds. This is mainly explained by an additionally induced light transfer time spread of the photonic pattern. Several configurations with and without Teflon wrapping were investigated.

Electronic band modification for faster and brighter Ce,Mg:Lu3-xYxAl5O12 ceramic scintillators

Ce,Mg:LuYAG transparent ceramics were successfully fabricated by the solid state reaction method. The results of x-ray diffraction, photoluminescence, light yield, scintillation decay and thermoluminescence are analyzed to provide better understanding of the influence of the Y3+ admixture in LuAG on the lattice structure, electronic band structure, energy-level structure of Ce3+ centers and scintillation properties. The lattice parameter of Ce,Mg:LuYAG ceramics shows a linear growth with the increasing Y content, which is in agreement with Vegard's law. Reducing the Lu-to-Y ratio in the LuYAG matrix, the Ce3+ 5d1 energy level shifts to a lower energy, the forbidden gap becomes narrower and the electron traps become shallower. Furthermore, the increase of light yield and accelerated slow scintillation component are found with increasing Y admixture. After optimization, the light yield of Ce,Mg:Lu0.5Y2.5Al5O12 ceramics is achieved to 24,500 ph/MeV with 1 μs shaping time, which is 40% higher than the value in the Y-free Ce,Mg:LuAG ceramic sample.

Performance of InGaN/GaN Light Emitting Diodes with n-GaN Layer Embedded with SiO2 Nano-Particles

We demonstrate high-performance InGaN/GaN blue light emitting diodes (LEDs) embedded with an air-void layer produced by a dry-etch of nano-pillars in an n-GaN layer grown on patterned sapphire substrate (PSS), filling the space between nano-pillars with SiO2 nano-particles (NPs) and subsequent epitaxial overgrowth. The structure exhibits enhanced output power compared to similarly grown reference conventional LED without the air-void layer. This change in growth procedure contributes to the increase of internal quantum efficiency (IQE) and light extraction efficiency (LEE) resulting in a 13.5% increase of light output. LEE is 2 times more affected than IQE in the modified structure. Simulation demonstrates that the main effect causing the LEE changes is due to the emitted light being confined within the upper space above the air-void layer and thus enhancing the light scattering by the SiO2 NPs and preferential light via front surface.

Applications of fluorene moiety containing polymers for improved scintillation light yield

A terfluorene compound, designed and synthesized for its photophysical and polymerizable properties, was employed as a host material in polymer scintillators to achieve a 31% increase in light yield versus a commercial standard viewed with a Silicon Photomultilier. Monomers of the compound were mixed with a solubility promoting vinyl toluene and either a commercial or custom designed fluor containing fluorene moiety structures. Fluors were chosen with overlapping energy levels to promote resonance energy transfer from the host material and improve light emission. The mixture was cured via bulk polymerization into cylindrical polymer monoliths which were coupled to either a photomultiplier tube or silicon photomultiplier to measure the scintillation light yield upon exposure to Cs-137 gamma. Samples emitted at longer wavelengths than commercial blue scintillators such as EJ-212 but outperformed this standard when accounting for the variability of photomultiplier tube sensitivity.

Effect of low electric fields on alpha scintillation light yield in liquid argon

Measurements were made of scintillation light yield of alpha particles from the 222Rn decay chain within the DarkSide-50 liquid argon time projection chamber. The light yield was found to increase as the applied electric field increased, with alphas in a 200 V/cm electric field exhibiting a ∼2% increase in light yield compared to alphas in no field.

Piezoelectric effect on compensation of the quantum-confined Stark effect in InGaN/GaN multiple quantum wells based green light-emitting diodes

InGaN based light emitting diodes (LEDs) suffer from the adverse influence of band tilt due to the piezoelectric field induced quantum-confined Stark effect (QCSE). We demonstrate incremental recovering of QCSE induced band tilt along with its impact on the performance of III-nitride-based green LEDs (at 532.5nm) by applying external stress. The external tensile stress determined by peak shift in Raman spectroscopy ranges from 0 to 0.95GPa as the wafer curvature increases from 0 to 2.32m−1. Compared to the pristine LEDs wafer, the PL emission peak undergoes an increase in intensity of 61%, blue shift of 11.5nm and peak narrowing of 5.5nm through bending the wafer to the maximum curvature of 2.94m−1. These results all verify the adverse effects due to the presence of the QCSE effect in green LEDs, which can be effectively suppressed by the external tensile stress, thus improving both the radiative recombination rate and electron-hole wave function overlaps. Under optimal conditions, the light output power is enhanced by 12.4% in chips on epi-wafer (COW) form under the wafer to the maximum curvature of 2.94m−1 without affecting the voltage required to reach the same current density. This work sheds light on implementation of the facile design in real LEDs devices to achieve significant increase in light output without involving any complicated transferring process onto flexible substrates.

Alternative Geometries for Improved Light Output of Inorganic Scintillating Crystals

It is important to optimize the crystal-photodetector coupling for gamma detection since it improves the light extraction and is directly linked to the achieved time resolution. In this paper we present our work on altering the geometry of the crystal coupling face to enhance the light output. Simulations compare the light output of alternative geometries of the crystal extraction face with respect to that of the regular cuboid geometry. We machined two different crystal shapes from $2 \times 2 \times 15 \hbox{mm}^{3}$ LSO crystals and measured the light output for different wrapping and coupling conditions. We also measured the coincidence time resolution (CTR) of the shaped crystals to compare the effect of timing on shaped crystals with respect to the reference samples. Simulations predicted an increase of up to 35% in light transfer efficiency for certain geometries of the exit face with respect to that of a regular cuboid geometry. Experimental measurements performed on shaped LSO crystals ( $2 \times 2 \times 15 \hbox{mm}^{3}$ ) show that an increase in light output of up to 40% and a systematic improvement in CTR of at least 5% is possible for one of the tested geometries. This opens new windows for preparing crystals used in TOF-PET scanners and high energy physics experiments.

Fabrication and characterization of cubic SrI2(Eu) scintillators for use in array detectors

Strontium iodide (SrI2(Eu)) is a promising spectroscopic detector for use in both nuclear security and medical imaging owing to its excellent energy resolution and low internal background radiation. A cubic form is preferable when coupling with a silicon-based photosensor in order to build an array detector for use in applications such as Compton cameras. Here, cubic SrI2(Eu) crystals with 10mm sides were fabricated and evaluated. The cubic SrI2(Eu) samples coupled to an avalanche photodiode exhibited an energy resolution of approximately 3.6% at 662keV when using a shaping time of 3µs. An increase in light output and an improvement of energy resolution were also observed at lower temperatures. The excellent energy resolution of these devices indicates that these crystals are promising potential detectors for use in Compton cameras and other imaging detectors.

High-performance, single-pyramid micro light-emitting diode with leakage current confinement layer

A high-performance, electrically driven, single-pyramid, micro light-emitting diode (micro-LED) was demonstrated. An insulated SiO2 layer, which served as a leakage current confinement layer (LCC layer), was deposited at the lower part of the pyramid, before the deposition of the transparent conducting layer. The micro-LED with the LCC layer demonstrated a decrease in reverse leakage current by two orders of magnitude and a 190% increase in light output at 500 µA compared to a micro-LED without an LCC layer. The small leakage current enables the micro-LED to operate with an ultra-low injection current (<1 µA). The improved performance of the micro-LED is attributed to the presence of the LCC layer, which blocks the direct pathway of the leakage current via threading dislocations distributed at the lower part of pyramid and causes carriers to be injected more effectively into the active layers at the nearly defect-free apex of the pyramidal LED.

The role of cerium variable charge state in the luminescence and scintillation mechanism in complex oxide scintillators: The effect of air annealing

The influence of annealing in air at elevated temperatures on the absorption, luminescence and scintillation characteristics was studied for a set of Ce-doped aluminum garnet and perovskite single crystals. Positive effects consisting mainly in increase of light yield and decrease of afterglow were found to varying extent in all the materials. It is explained by the positive role of created stable Ce4+ center in scintillation mechanism and by decrease of deep trap concentration based on oxygen vacancies.