X-ray Luminescence Efficiency Research Articles

Intramolecular charge transfer enables highly-efficient X-ray luminescence in cluster scintillators

Luminescence clusters composed of organic ligands and metals have gained significant interests as scintillators owing to their great potential in high X-ray absorption, customizable radioluminescence, and solution processability at low temperatures. However, X-ray luminescence efficiency in clusters is primarily governed by the competition between radiative states from organic ligands and nonradiative cluster-centered charge transfer. Here we report that a class of Cu4I4 cubes exhibit highly emissive radioluminescence in response to X-ray irradiation through functionalizing biphosphine ligands with acridine. Mechanistic studies show that these clusters can efficiently absorb radiation ionization to generate electron-hole pairs and transfer them to ligands during thermalization for efficient radioluminescence through precise control over intramolecular charge transfer. Our experimental results indicate that copper/iodine-to-ligand and intraligand charge transfer states are predominant in radiative processes. We demonstrate that photoluminescence and electroluminescence quantum efficiencies of the clusters reach 95% and 25.6%, with the assistance of external triplet-to-singlet conversion by a thermally activated delayed fluorescence matrix. We further show the utility of the Cu4I4 scintillators in achieving a lowest X-ray detection limit of 77 nGy s−1 and a high X-ray imaging resolution of 12 line pairs per millimeter. Our study offers insights into universal luminescent mechanism and ligand engineering of cluster scintillators.

Response of Lead Fluoride (PbF2) Crystal under X-ray and Gamma Ray Radiation

Background: In this research, the response of a 10 × 10 × 10 mm3 commercially available PbF2 crystal was experimentally assessed under X-ray and gamma ray radiation to verify the possible application of this material in X-ray medical imaging. Methods: The measurements were performed under X-ray from 50 to 130 kVp and gamma ray excitation (Tc-99m 140 keV and I-131 365 keV). The PbF2 response was experimentally assessed by the determination of the absolute luminescence efficiency (AE), X-ray luminescence efficiency (XLE), and the stopping power of this scintillating crystal in terms of the energy absorption efficiency (EAE). The results were compared with bismuth germanate (BGO) crystal, which is commonly used in medical imaging modalities. Results: The AE of PbF2 gradually decreased from 50 kVp up to 130 kVp. The maximum value was 0.61 efficiency units (EU) at 140 keV, and the minimum value was 0.03 EU at 71 keV (130 kVp). Similarly, low values appeared for the XLE, where the maximum value was 16.9 × 10−5 at 140 keV. Conclusions: These findings show that the PbF2 scintillator has unacceptably low luminescence efficiency. Although PbF2 can effectively absorb radiation, the scintillation light, at room temperatures, is negligible, and, thus, it could not be used in medical imaging applications in the examined energy range.

Luminescence Efficiency of Cerium Bromide Single Crystal under X-ray Radiation

A rare-earth trihalide scintillator, CeBr3, in 1 cm edge cubic monocrystal form, is examined with regard to its principal luminescence and scintillation properties, as a candidate for radiation imaging applications. This relatively new material exhibits attractive properties, including short decay time, negligible afterglow, high stopping power and emission spectrum compatible with several commercial optical sensors. In a setting typical for X-ray radiology (medical X-ray tube, spectra in the range 50–140 kVp, human chest equivalent filtering), the crystal’s light energy flux, absolute efficiency (AE) and X-ray luminescence efficiency (XLE) were determined. Light energy flux results are superior in comparison to other four materials broadly used in modern medical imaging (slope of the linear no-threshold fit was 29.5). The AE is superior from 90 kVp onwards and reaches a value of 29.5 EU at 140 kVp. The same is true for the XLE that, following a flat response, reaches 9 × 10−3 at 90 kVp. Moreover, the spectral matching factors and the respective effective efficiencies (EE) are calculated for a variety of optical sensors. The material exhibits full compatibility with all the flat-panel arrays and most of the photocathodes and Si PMs considered in this work, a factor that proves its suitability for use in state-of-the-art medical imaging applications, such as CT detectors and planar arrays for projection imaging.

Behaviour of Y203:Eu3+ Scintillator under Radiation used in Medical Applications

The Y2 O3:Eu3+ scintillator was studied for use in radiation detectors of medical imaging systems. Y2 O3 :Eu3+ was used in the form of laboratory prepared test screens. The x-ray luminescence efficiency of the screens was measured for tube voltages up 250 kVp. The intrinsic x-ray to light conversion efficiency (nc) and other optical parameters of the Y2 03:Eu3+ scintillator related to optical scattering, absorption, and reflection were determined. The light emission spectrum of Y2 03:Eu3+ was measured (λ=613 rim). The x-ray luminescence efficiency peaked at 180 mg/cm2 screen coating weight. The intrinsic x-ray to light conversion efficiency was found to be nc =0.095. Results indicated that Y2 03:Eu3+ is a medium to high overall performance material that could be used in medical imaging systems.

Effect of the Concentration on the X-ray Luminescence Efficiency of a Cadmium Selenide/Zinc Sulfide (CdSe/ZnS) Quantum Dot Nanoparticle Solution

In the current study preliminary results on the luminescence efficiency (LE) of toluene dissolved Cadmium Selenide/Zinc Sulfide (CdSe/ZnS, Sigma-Aldrich, Lumidot 694622) quantum dot samples (QDs) after exposure to X-rays of variable radiation flux are shown. The distinctive influence of the weight over volume (w/v) concentration of the samples in LE was investigated. The light emission of the QDs was additionally measured after UV irradiation. The distribution of the emitted light was symmetrical with a maximum at 590 nm. The w/v concentration of the QDs varied between 7.1×10-5 mg/mL to 28.4×10-5 mg/mL. The samples were handled in a cubic 12.5×12.5×45mm3 quartz cuvette. Each sample was excited under X-ray irradiation, in the energy range from 50 to 130 kVp using a BMI General Medical Merate tube with rotating Tungsten anode and inherent filtration equivalent to 2 mm Al. The X-ray LE, induced by the 28.4×10-5 mg/mL QDs found higher, however, the distinction was vague in the highly concentrated samples. The maximum efficiency was obtained at the 90 kVp for QDs with 21.3×10-5 mg/mL w/v concentration. In the high energy range (120-130 kVp) all concentration levels exhibited comparable X-ray induced LE. The luminescence properties of the investigated QDs appear promising for X-ray detection applications.

X-ray Luminescence Efficiency of GAGG:Ce Single Crystal Scintillators for use in Tomographic Medical Imaging Systems

The purpose of the present study was to evaluate different scintillator crystal samples, with a cross section of 3×3mm2 and various thicknesses ranging from 4mm up to 20mm, of the new mixed Gd3Al2Ga3O12:Ce (GAGG:Ce) scintillator material under X-ray irradiation, for potential applications in Tomographic Medical Imaging systems. Evaluation was performed by determining the X-ray luminescence efficiency (XLE) (emitted light energy flux over incident X-ray energy flux) in energies employed in general X-ray imaging. For the luminescence efficiency measurements, the scintillator samples were exposed to X-rays using a BMI General Medical Merate tube, with rotating Tungsten anode and inherent filtration equivalent to 2 mm Al. X-ray tube voltages between 50 to 130 kV were selected. An additional 20 mm filtration was introduced to the beam to simulate beam quality alternation equivalent to a human body. The emitted light energy flux measurements were performed using an experimental set up comprising a light integration sphere coupled to an EMI 9798B photomultiplier tube which was connected to a Cary 401 vibrating reed electrometer. The GAGG:Ce sample with dimensions 3×3×10 mm3 exhibited higher XLE values, in the whole X- ray energy range examined. XLE value equal to 0.013 was recorded for this crystal at 130 kVp - a setting frequently used in Computed Tomography applications.

Light emission efficiency and imaging performance of Lu2O3:Eu nanophosphor under X-ray radiography conditions: Comparison with Gd2O2S:Eu

Nanocrystallic europium-activated lutetium oxide (Lu2O3:Eu) is a strong candidate for use in digital medical imaging applications, due to its spectroscopic and structural properties. The aim of the present study was to investigate the imaging and efficiency properties of a 33.3mg/cm2 Lu2O3:Eu scintillating screen coupled to a high resolution RadEye HR CMOS photodetector under radiographic imaging conditions. Since Lu2O3:Eu emits light in the red wavelength range, the light emission efficiency and the imaging performance were compared with results for a Gd2O2S:Eu phosphor screen. Parameters such as the Absolute Efficiency (AE), the X-ray Luminescence Efficiency (XLE), and the Detector Quantum Gain (DQG), were investigated. The imaging characteristics of Lu2O3:Eu nanophosphor screen were investigated in terms of the Modulation Transfer Function (MTF), the Normalized Noise Power Spectrum (NNPS) and the Detective Quantum Efficiency (DQE). It was found that Lu2O3:Eu nanophosphor has higher AE and XLE by a factor of 1.32 and 1.37 on average, respectively, in the whole radiographic energy range in comparison with the Gd2O2S:Eu screen. DQG was also found higher in the energy range from 50kVp to 100kVp and comparable thereafter. The imaging quality of Lu2O3:Eu nanophosphor coupled to the CMOS sensor was found to outmatch in any aspect in comparison with the Gd2O2S:Eu screen. These results indicate that Lu2O3:Eu nanophosphor could be considered for further research in order to be used in medical imaging applications.

X-ray luminescence efficiency and detector quantum gain of LuPO4:Eu nanophosphor

X-ray luminescence efficiency and detector quantum gain of LuPO4:Eu nanophosphor

On the response of GdAlO3:Ce powder scintillators

The aim of the present study was to investigate the luminescence efficiency (XLE) of gadolinium aluminum perovskite (GdAlO3:Ce) powder scintillator. This powder phosphor, also known as GAP:Ce scintillator, is a non-hygroscopic material, emitting blue light with short decay time. For the purposes of this study, five scintillating screens with coating thicknesses, 14.7, 31.0, 53.7, 67.2 and 121.1mg/cm2, were prepared in our laboratory from GdAlO3:Ce powder (Phosphor Technology, Ltd) by sedimentation on silica substrates. The light emitted by the phosphors under investigation was evaluated by performing measurements of the absolute luminescence efficiency (AE), X-ray luminescence efficiency and detector quantum gain (DQG) under X-ray exposure conditions with tube voltages ranging from 50 to 140kV. The quantum detection efficiency (QDE) and energy absorption efficiency (EAE) were also evaluated. The spectral compatibility of GdAlO3:Ce, with various existing optical detectors, was investigated after emission spectra measurements. A theoretical model, describing radiation and light transfer, was used to fit experimental AE data. This has allowed the estimation of optical attenuation coefficients of the scintillator. GdAlO3:Ce exhibited higher QDE and EAE values, compared to aluminium perovskite (YAlO3:Ce) but lower absolute efficiency values. Absolute efficiency was found to increase with increasing X-ray tube voltage, although for values higher than 120kVp a decrease was observed.

X-ray luminescence spectra of graded-gap AlxGa1−xAs structures irradiated by alpha particle

The influence of 241 Am alpha particle irradiation on X-ray luminescence spectra of the graded-gap Al x Ga 1− x As structures of different thicknesses is investigated. It is observed that the integral X-ray luminescence intensity of nonirradiated thin (15 μm) structure is 1.4 times less than that in the thick (32 μm) structure, and this difference increases to 3 times after 3×10 10 cm −2 dose of irradiation by alpha particle. The X-ray luminescence intensity of the energy hν <1.5 eV of thin nonirradiated structure is about 7 times less than that in thick one. The internal graded-gap electric field F gg is responsible of that large difference, because it shifts the X-ray generated carriers to the narrow-gap surface with great nonradiative surface recombination rate. The alpha particle irradiation increases nonradiative recombination rate and causes a decrease of the X-ray luminescence intensity of all spectra lines in the thin (15 μm) detector. The most significant drop in X-ray luminescence efficiency is observed from the region at narrow-gap surface after the initial stage (10 9 cm −2 dose) of alpha particle irradiation. In the 32 μm thick detector, the luminescence intensity of the energy hν =1.8 eV does not change up to 2×10 10 cm −2 of alpha particle irradiation dose. That means the high irradiation hardness of the thick graded-gap X-ray detector with optical response.

Preparation and structural properties of Lu 2O 3:Eu 3+ submicrometer spherical phosphors

Monodisperse non-agglomerated Lu 2O 3:Eu 3+ submicrometer spheres were obtained by the homogeneous precipitation technique with subsequent annealing for spheres crystallization. The morphological and structural parameters of the Lu 2O 3:Eu 3+ crystalline spheres prepared were investigated by the electron microscopy methods, thermal analysis (TG-DTA), X-ray diffractometry (XRD), X-ray photoelectron (XPS) and FT-IR spectroscopy. The influence of the annealing temperature on the morphology and sphericity was shown. Eu 3+-doped lutetium oxide spheres were characterized by effective luminescence under X-ray excitation in the λ = 575–725 nm range corresponding to 5D 0 → 7F J transitions ( J = 0–4) of Eu 3+ ions. It was shown that the X-ray luminescence efficiency of the Lu 2O 3:Eu 3+ spherical phosphors prepared strongly depend on annealing temperature and dopant concentration.

The features of morphology and crystalline structure of the monodisperse Lu 2O 3:Eu 3+ submicrospheres prepared by sol–gel method

Monodisperse Lu 2O 3:Eu 3+ submicrospheres with the diameters ranging from 40 to 400 nm were obtained by the urea-based homogeneous precipitation technique with subsequent spheres crystallization during annealing. The inner structure, morphology of Lu 2O 3:Eu 3+ polycrystalline spheres and thermal stability of the particles shape were investigated by means of transmission electron microscopy (TEM), X-ray diffractometry (XRD) and electron paramagnetic resonance (EPR). Eu 3+-doped lutetium oxide spheres were characterized by effective luminescence under X-ray excitation in the λ=575–725 nm range corresponding to 5D 0→ 7F J transitions ( J=0–4) of Eu 3+ ions. Experimental data showed that the X-ray luminescence efficiency increases with the increase of the crystallites size, which occurred by the annealing temperature and particles diameter.

Evaluation of the Luminescence Efficiency of YAG:Ce Powder Scintillating Screens for Use in Digital Mammography Detectors

In the present study scintillating screens prepared from Y3Al5O12:Ce (YAG:Ce) powder phosphor were evaluated for use in digital mammography. YAG:Ce has never previously been used in X-ray medical imaging, however since it emits green light (i.e peak at 550 nm), it is expected to match well the spectral sensitivities of most photodetectors (photodiodes, CCDs and amorphous silicon sensors) incorporated in various digital mammography detectors. YAG:Ce was purchased in powder form and was used in order to prepare test screens in the laboratory. Screens were evaluated by determining the quantum detection efficiency (QDE), the energy absorption efficiency (EAE), the absolute luminescence efficiency, the X-ray luminescence efficiency (XLE), the light emission spectrum, the X-ray to light intrinsic conversion efficiency and the spectral compatibility with various photodetectors. Results were compared with phosphor materials commercially employed in X-ray imaging such as CsI:Tl and Gd2O2 S:Tb. Maximum YAG:Ce emission efficiency was observed for the 63 mg/cm2 screen at 49 kVp. The spectral compatibility with amorphous silicon photodiodes (0.93) and CCDs (0.95) was found to be very high, better than the corresponding compatibility of the CsI:Tl screen, mostly used in current digital radiography detectors. Taking into account the YAG:Ce overall performance, its short decay time as well as its spectral compatibility with amorphous silicon detectors and CCDs, YAG:Ce could be of interest for further investigation for applications in digital mammography.

Comparative evaluation of single crystal scintillators under x-ray imaging conditions

The present study is a comparative investigation of the luminescence properties of (Lu,Y)2SiO5: Ce (LYSO: Ce), YAlO3: Ce (YAP: Ce), Gd2SiO5: Ce (GSO: Ce) and (Bi4Ge3O12) BGO single crystal scintillators under x-ray excitation. Results will be of value in designing dual modality tomographic systems (PET/CT, SPECT/CT) based on a common scintillator crystal. All scintillating crystals have dimensions of 10 × 10 × 10 cm3 are non-hygroscopic exhibiting high radiation absorption efficiency in the energy range used in medical imaging applications. The comparative investigation was performed by determining the x-ray luminescence efficiency (emitted light flux over incident x-ray energy flux) in the range of x-ray energies employed in: (i) general x-ray imaging (40–140 kV, using a W/Al x-ray spectrum) and (ii) x-ray mammography imaging (22–49 kV, using a Mo/Mo x-ray spectrum). Additionally, light emission spectra of crystals at various x-ray energies were measured, in order to determine the intrinsic conversion efficiency and the spectral compatibility to optical photon detectors incorporated in medical imaging systems. The light emission performance of LYSO:Ce scintillator studied was found very high for x-ray imaging.

Luminescence Emission Properties of $({\rm Lu},{\rm Y})_{2}{\rm SiO}_{5}$:Ce (LYSO:Ce) and $({\rm Lu},{\rm Y}){\rm AlO}_{3}$:Ce (LuYAP:Ce) Single Crystal Scintillators Under Medical Imaging Conditions

LYSO:Ce and LuYAP:Ce are single crystal non-hygroscopic scintillators of high density, high light yield and short decay time, which have been successfully used in small animal PET imagers. In the present study, the luminescence emission properties of (Lu0.9, Y0.1)2SiO5:Ce (LYSO:Ce) and (Lu0.7, Y0.3)AIO3:Ce (LuYAP:Ce) crystals were investigated for use in X-ray medical imaging. Both crystals had dimensions of 2 times 2 times 8 mm3, with all surfaces polished. Evaluation was performed by determining the X-ray luminescence efficiency (XLE) (emitted light energy flux over incident X-ray energy flux) and the detector optical gain (DOG) (emitted light photons per incident x-ray photon) in a wide range of X-ray energies employed in mammography (22-49 kVp) and in general X-ray imaging (50-140 kVp). Measurements were performed using an experimental set-up based on a photomultiplier coupled to an integration sphere. The emission spectrum under X-ray excitation was measured using an optical grating monochromator to determine the spectral compatibility to various optical photon detectors incorporated in medical imaging detectors. Optical characteristics such as transmission and absorption spectra were investigated in addition to the scintillation properties. The light emission performance of the two scintillation materials studied was found adequately high for X-ray imaging.

A systematic study of the performance of the CsI:Tl single-crystal scintillator under X-ray excitation

The light emission performance of the X-ray excited CsI:Tl single-crystal scintillator was investigated as a function of X-ray tube voltage and crystal thickness. Four CsI:Tl single-crystal layers (CRYOS Ltd., Ukraine) with thickness from 1 to 7 mm were irradiated employing two X-ray tube voltage ranges: (i) the 22–45 kV (molybdenum anode–molybdenum filter (Mo/Mo)) range, employed in mammographic imaging and (ii) the 40–140 kV (tungsten anode–aluminum filter) tube voltage range, used in general X-ray projection and tomographic imaging. The X-ray luminescence efficiency (light emission spectrum over incident X-ray fluence) of the crystals was determined by performing light emission spectrum and X-ray exposure measurements. In addition, the intrinsic conversion efficiency (fraction of the absorbed X-ray converted into light) and the spectral compatibility to various optical detectors were estimated from these measurements. The luminescence efficiency was found to be a nonlinear function of crystal thickness and of X-ray tube voltage. Peak efficiency (29.5 μWm −2/mRs) was observed for the 5 mm thick crystal at 140 kV. A secondary efficiency peak was observed at 42 kV (Mo anode) probably due to the effect of the K-photoelectric absorption edge (at 33 and 35 keV for Cs and I, respectively). For the thicker (7 mm) crystal, the efficiency was found to decrease due to light attenuation effects within the scintillator mass.

Evaluation of the GSO:Ce scintillator in the X-ray energy range from 40 to 140 kV for possible applications in medical X-ray imaging

The purpose of the present study was to evaluate, under X-ray medical imaging conditions, the X-ray luminescence efficiency (XLE) and the optical quantum gain (OQG) of the Gd 2SiO 5:Ce scintillator in single crystal form, suitable for tomographic applications. Intrinsic physical properties and light emission characteristics of the Gd 2SiO 5:Ce scintillator, were also studied. Both experimental and Monte Carlo techniques were used. Various X-ray tube voltages (40–140 kV), currently employed in X-ray imaging applications, were used. XLE was found to vary slowly with X-ray tube voltage from (0.021±0.003) to (0.017±0.003). OQG varied from (317±18) to (466±23) light photons per incident X-ray. These values were adequately high for imaging applications using the particular energy range. Additionally, it was found by Monte Carlo simulations that for crystal thicknesses higher than 0.5 cm both XLE and OQG reached saturation levels, indicating that higher thickness crystals are of no practical use in X-ray medical imaging.

X-ray luminescence of ZnSCdS:Au,Cu phosphor using X-ray beams for medical applications

The purpose of the present study was to evaluate the X-ray luminescence and imaging performance of phosphor screens prepared from ZnSCdS:Au,Cu. Absolute efficiency, X-ray luminescence efficiency, detector optical gain, and gain transfer function were experimentally determined. Theoretical models were also employed to fit experimental data and to determine optical properties of the phosphor material. Additionally, the emission spectrum of ZnSCdS:Au,Cu was measured and its compatibility with the spectral sensitivity of radiographic optical detectors (films, photodiodes) was determined. Results showed that ZnSCdS:Au,Cu is an efficient phosphor exhibiting high intrinsic X-ray to light conversion efficiency (0.17) and an excellent spectral compatibility (0.9) with amorphous silicon photodiodes, used in optical detectors of modern digital radiography systems.

Assessment of the gain transfer function of phosphors for application in medical imaging radiation detectors

Objective: to study various phosphors used in detectors of medical imaging systems by the gain transfer function (GTF), defined in terms of X-ray luminescence efficiency, light spectrum and modulation transfer function. Materials and methods: four phosphor materials, La 2O 2S:Tb, Y 2O 2S:Tb, Y 2O 2S:Eu and Y 2O 3:Eu were used in the form of fluorescent layers prepared in the laboratory. The GTF was determined at 30 kVp and 80 kVp X-ray tube voltages for various phosphor coating weights. Results: La 2O 2S:Tb, which was the highest density and effective atomic number phosphor used, was found to exhibit the best GTF performance at 80 kVp. At 30 kVp, the yttrium based phosphors were found of increased performance. This is mainly due to the proximity of the X-ray energy to the K-absorption edge of yttrium at 17 keV. Europium activated phosphors were found to perform very well when combined with the red sensitive film and the silicon photodiode. Conclusion: The GTF may be a useful method for comparing and selecting phosphor materials for use in various medical imaging applications.

Signal-to-noise-ratio (SNR) of X-ray imaging scintillators determined by luminescence measurements

A new method for studying the signal-to-noise-ratio (SNR) of X-ray imaging scintillators was developed. SNR was expressed in terms of the X-ray luminescence efficiency (XLE) the X-ray to emitted light conversion efficiency and the mean light photon energy. Laboratory prepared Gd 2O 2S:Tb, La 2O 2S:Tb and Y 2O 2S:Tb scintillator screens were tested and best results were obtained for the 80 mg/cm 2 Gd 2O 2S:Tb screen. The method proposed allows for scintillator comparison on the basis of XLE and wavelength measurements.