Gadolinium Aluminum Gallium Garnet Research Articles

Multi-chip front-end electronics LYRA for X and γ Ray detector for HERMES mission

We present the experimental results of the Application Specific Integrated Circuit (ASIC), called LYRA, specifically designed for the High-Energy Rapid Modular Ensemble of Satellites (HERMES) mission concept, a constellation of nano-satellites able to detect and localize high-energy rapid transient events (up to 2.2 MeV) as the Gamma Ray Bursts (GRBs) from the deep space. LYRA has been desied for the detection system composed by a combination of Gadolinium Aluminum Gallium Garnet (GAGG) scintillators for high-energy photons, coupled to a matrix of 120 silicon drift detectors (SDD), used for detecting both scintillation light and low-energy photons. The LYRA ASIC has been conceived with a multi-chip architecture: 120 LYRA Front-End chips (LYRA-FE) are placed in close proximity to the anodes of the SDD matrix for a first processing of the detector signals and trasmit them in current mode to four 32-channel LYRA Back-End chips (LYRA-BE) to complete the elaboration. The requirements that the LYRA ASIC have to fulfill for the HERMES project are challenging: the maximum input energy measured in Silicon must reach 120 keV — corresponding to 2.2 MeV on GAGG — with a linearity error below 1%, the electronic noise must be less then 30 el. r.m.s. and the power consumption less then 1 mW per channel in a system with 120 channels working in parallel. The characterization of LYRA has been carried out on a dedicated test board, coupling one channel of the ASIC with a 25 mm2 SDD. An input full scale range of 5.2 fC and an electronic noise of 22 el. r.m.s. have been measured at -33∘C with a power consumption of 745 µW per channel.

Tm:GGAG disordered garnet crystal for 2 µm diode-pumped solid-state laser

Lasing parameters and wavelength tunability of a new disordered solid-state active medium, Tm3+-doped gadolinium gallium aluminum garnet (Tm3+-doped , Tm:GGAG), were investigated. Five samples with thulium concentration ranging from 1.4 at.% to 9.2 at.% (Tm/Gd) were designed, grown and investigated. Basic spectroscopic characterization and measurement of a refractive index were performed. Each sample was tested as an active medium of a 2 µm laser, longitudinally diode-pumped at 786 nm. Tm3+ concentration dependence of laser parameters was investigated. The highest slope efficiency of 43% w.r.t. absorbed power in the pulsed regime and 35% in the CW regime was achieved with the 2.4 at.% (Tm/Gd) sample. All samples produced beams of good quality with estimated M. Concentration-dependent tunability was investigated and a smooth continuous range of wavelengths, up to 187 nm wide, was recorded.

Row–Column Readout Method to Mitigate Radiographic-Image Blurring From Multipixel Events in a Coded-Aperture Imaging System

In order to improve the image quality of a coded-aperture-based gamma-ray imaging apparatus, a method for removing Compton scattered and other multipixel events is presented that removes inaccurate position estimations from the data stream. In coded-aperture gamma-ray imaging detectors, scattering events that are shared between different partitions in the active medium can result in the inaccurate determination of the initial gamma-ray interaction position, the result of which is distortion in the angular reconstruction mapping. For coded-aperture system in which the rapid localization and identification of the emitting source is important, the distortion due to scattered radiation can increase the integration time of the system. We show, in both software and hardware implementations using a cerium-doped gadolinium aluminum gallium garnet pixelated scintillator array, that filtering the data to remove multipixel events can improve the image quality despite the efficiency loss associated with not utilizing all of the interactions within the detector.

High-performance coded aperture gamma camera based on monolithic GAGG:Ce crystal.

A 50 × 50 × 10mm3 monolithic gadolinium aluminum gallium garnet (Gd3Al2Ga3O12; GAGG):Ce crystal coupled to a 8 × 8 silicon photomultiplier (SiPM) array was developed; it showed very good system uniformity and a high energy resolution of 7.4% at 662 keV. By using a convolutional neural network-based positioning algorithm and a fan-beam calibration method, the detector achieved a position resolution of ∼1.4mm and a depth of interaction resolution of ∼2mm. Based on this high-performance monolithic detector, we developed a coded aperture gamma camera. A 1-mCi Cs-137 source centered at a 2-m distance from the mask could be reconstructed with a signal-to-noise ratio of 6.5 in 1s. Furthermore, the imaging ability of a low-energy Am-241 source and a low-activity Cs-137 source when the background-to-signal ratio was approximately 1:1 and a double low-activity source (Cs-137 and Na-22) was demonstrated. It is shown that the monolithic-crystal-based coded aperture gamma camera can achieve high performance and has a large potential for further improvement.

Dual-Energy Micro-CT Using GAGG:Ce and LYSO:Ce Scintillators

Dual-energy computed tomography (CT) has a better material determination capability than traditional CT. In this article, we develop a dual-layer dual-energy micrometer CT (DEMCT) imaging system with micrometer spatial resolution. The detector of the DEMCT system uses cerium-doped gadolinium aluminum gallium garnet (GAGG:Ce) and cerium-doped lutetium yttrium oxyorthosilicate (LYSO:Ce) scintillators to absorb X-rays of different energies, which is a new application of GAGG:Ce scintillator. As GAGG:Ce and LYSO:Ce have high light yields and their emission wavelengths match well with the quantum efficiency of common charge-coupled device (CCD), thicknesses of the scintillators are allowed to be thin, which is of great significance for high-resolution imaging due to the limitation of the depth of field of objectives. We propose a mechanical adjustment method for registering high-energy and low-energy images, which saves the postprocessing time and avoids oscillating or nonconvergent iteration results of mathematical registration methods. Meanwhile, in order to increase the dual-energy spectra separation of materials detection in the DEMCT, a fast optimal kilovolt peak selection method is proposed. Results show that the DEMCT has the spatial resolution of 2 μm in low-energy imaging and 4 μm in high-energy imaging. In addition, materials of a silica mixture sample are clearly separated by the pixel selection of the energy map based on the DEMCT data. The proposed DEMCT system has the potential to separate materials that cannot be correctly determined on traditional CT.

Development of a Gamma Camera with a Diverging Collimator Using DMLS 3D Printing

The purpose of this study is to use a Monte Carlo simulation to derive optimal values for collimator design variables and to produce an optimized collimator for a gamma camera using a 3D printing direct metal laser sintering (DMLS) technique. For the optimization studies, GATE (Geant4 application for tomographic emission) simulation was used, and we tested lead, tungsten, and a full absorber. A total of 15 design variables were simulated using hole sizes of 0.5 mm, 0.7 mm, and 1.0 mm and slat heights from 10 mm to 20 mm at intervals of 2.5 mm. The scintillator used GAGG (gadolinium aluminum gallium garnet) and was set to a size of 25.8 × 25.8 mm². The radiation of the source used in the simulation was 37 MBq, and the source was a 140 keV point source. To obtain the diverging collimator optimization design variables, the point source was detected and an image was acquired to analyze the sensitivity and spatial resolution values. As a result, tungsten, which has good hardness, was used. If the source is located in the center of the diverging collimator, the FWHM is limited to 3.0 mm or less. The optimization value is obtained by considering the permeability, sensitivity, and spatial resolution at a height of 15 mm or higher. The results obtained by moving the source were also similar to those obtained when the source was located at the center. Based on the values of the optimized design variables, this study designed and produced a collimator using DMLS, which is a 3D printing technique. We believe this process can be applied in various fields, such as medical and industrial sectors; optimized collimators can be produced for different purposes while maintaining high precision.

Development of DOI Detector for PET using Different Kinds of Reflectors

We developed a depth of interaction (DOI) detector for positron emission tomography (PET) using different kinds of reflectors. The detector module consists of two layers of scintillator arrays, which was composed of 4 × 4 Gadolinium Aluminum Gallium Garnet (GAGG) crystals of size 3 mm × 3 mm × 10 mm, and 4 × 4 silicon photomultiplier (SiPM) arrays. The bottom layer optically coupled to the SiPM used a diffuse reflector and the top layer used a specular reflector. The layer interacted with gamma ray and crystal could be determined by analyzing the signal size because it obtained for each layer is different by using different reflectors for the crystals for each layer. The detector performance was analyzed by the flood image, energy spectrum with two photoelectric peaks, and energy resolution. In the experiment for the detector module performance, all pixels in the flood map were well decoded, and the energy spectrum of each pixel is measured with two photo peaks.

Multiangle Spectrophotometric Methods of Reflection for Determining Refractive Coefficients

The features of the development and application of the procedures for measuring refractive coefficients based on multiangle spectrophotometric methods of reflection are considered. The effect of the shape, size, and surface treatment of the samples on their spectral dependences of the reflection is described. The possibility of determining the refraction coefficients by two spectrophotometric methods is demonstrated: using the spectrum of reflection from a single face at a small angle of light incidence close to normal and using the method of reflection of light incident at the Brewster angle. The method of reflection at an incidence angle close to normal can be applied in the case of a nonabsorbing sample characterized by an extinction coefficient not exceeding 10–6 to 10–4. This is a so-called express method, because it allows us to immediately obtain the dispersion dependence of the refraction coefficient. The method allows measuring the dispersion dependences of the refraction coefficients for the samples whose shape excludes multiple reflections: plates with one polished side, plates with a large thickness polished on two sides, and prisms or plates with non-plane-parallel faces. In measurements using the Brewster method, there are no requirements for the value of the extinction coefficient of the sample (absorption) and we can use a sample of any shape, including plates with a small thickness polished on both sides. However, the resulting values of the refraction coefficients are discrete, and a large array of measurement results must be accumulated. The determined measurement accuracy of both methods was Δ = ±0.001 at a confidence probability of P = 0.95. Applicability of the spectrophotometric methods of measurement is demonstrated for the samples of a gadolinium aluminum gallium garnet, which is a crystal of a cubic system and is characterized by a single refraction coefficient. It is shown that the values of the refraction coefficients obtained by these two methods agree well within the measurement accuracy.

Recovery of inter-detector and inter-crystal scattering in brain PET based on LSO and GAGG crystals

Gadolinium aluminum gallium garnet (GAGG) is a promising scintillator crystal for positron emission tomography (PET) detectors owing to its advantages of energy resolution, light yield, and absence of intrinsic radiation. However, a large portion of the incident photons undergoes Compton scattering within GAGG crystal because of its low stopping power compared to that of lutetium-based crystals such as Lu2SiO5 (LSO). Inter-detector scattering (IDS) and inter-crystal scattering (ICS) result in loss of sensitivity and image quality of PET, respectively. We performed a Monte Carlo simulation study to evaluate IDS recovery in our currently developing brain-dedicated PET, and extended the idea to ICS recovery. We also compared the impact of the recoveries on LSO- and GAGG-based PET scanners. We measured the sensitivity and spatial resolution of the brain PET, and analyzed the image quality using a lesion phantom, a hot-rod phantom, and a 2D Hoffman phantom with applying IDS or ICS recovery. IDS recovery increased the PET sensitivity and improved the noise level of the reconstructed images. ICS recovery enhanced the spatial resolution and the contrast of the images was improved. As the occurrence rates of IDS and ICS were higher in GAGG than in LSO, the overall impact of IDS or ICS recovery was significant in GAGG. In conclusion, we showed that the proportional method would be suitable for IDS and ICS recoveries of PET, and emphasized the importance of ICS and IDS recoveries for PET using crystals with low stopping power.

Study of electronic excitation relaxation at cerium ions in Gd3Al2Ga3O12 matrix using multipulse transient absorption technique

A method for investigation of excitation relaxation—the multipulse transient absorption technique—has been applied for studying carrier relaxation and recombination processes of excited Ce3+ ions in Ce-doped and Mg-codoped gadolinium aluminum gallium garnet (Gd3Al2Ga3O12, GAGG) single crystals with different codoping levels. The method enabled deeper insight into the complex dynamics of electron transitions between higher excited levels and the emitting state of Ce3+, which are difficult to observe in conventional pump–probe experiments because of overlapping absorption bands arising from different structural units. Fitting the experimental results to calculations based on the rate equations for the population levels of Ce3+ ions and traps enabled us to estimate the relaxation times from these levels. In particular, it was shown that the electron relaxation time between doublet eg and triplet t2g components of the split level 5d of Ce3+ is faster than 300 fs. Substantial influence of electron trapping is demonstrated, and the potential fluctuations due to intrinsic composition fluctuations in multicomponent GAGG crystals are shown to be the predominant origin of the traps.

Double- and Triple-Crystal X-Ray Diffraction Analysis of Face-Grown Gd3Ga3Al2O12:Ce Crystals

Crystals of gadolinium–gallium–aluminum garnet doped with cerium ions (Gd3Ga3Al2O12:Ce) are analyzed by X-ray diffractometry and X-ray topography. These crystals have a number of unique scintillation characteristics (including a high specific light yield, fast emission kinetics, and high energy and time resolutions), which make them promising for designing detectors of ionizing radiation in a wide energy range. It is shown that the methods used make it possible to carry out the further optimization of preparation processes of bulk crystals by revealing specific features of their growth (e.g., facet inhomogeneity, which is characteristic of Czochralski-grown single crystals).

Scintillation properties of GAGG:Ce ceramic and single crystal

Recently developed ceramic and single crystal Cerium-doped gadolinium aluminum gallium garnet (GAGG:Ce) scintillators were studied. We compared their scintillation properties with samples of the same 5 × 5 × 5 mm3 size. In photoluminescence emission spectra, ceramic and single crystal GAGG:Ce showed intense emission peak at 555 nm and 515 nm, respectively, ascribed to 5d–4f transitions of Ce3+. And the single crystal GAGG:Ce had a weak emission peak appearing at 380 nm. The light yield induced by 137Csγ-ray of ceramic was higher than that of single crystal. The afterglow of ceramic GAGG:Ce was shorter than that of single crystal after illumination. At room temperature, single crystal GAGG:Ce was more severely affected by thermoluminescence than ceramic GAGG:Ce. The primary decay time of ceramic and single crystal under γ-ray was 182.9 ns and 50.1 ns, respectively. Furthermore, the fast component of decay time was found in single crystal. This research can lay the foundation for the preparation of better GAGG:Ce scintillators in the future.

Simulation of novel scintillator crystals for brain PET

In the last decade, different positron emission tomography (PET) crystals have been proposed for brain PET detectors. Brain PET cameras with restricted field of view to a smaller size to cover the brain can both exhibit significantly higher performance and lower the cost when compared to conventional whole-body PET scanners. The existing designs based on scintillation crystals employ the most commonly known materials such as lutetium oxyorthosilicate and bismuth germanate. This is due to their relatively good parameters such as high light yield and density. The main drawback of using scintillation crystals is that they have a modest energy resolution. In this study, a brain PET scanner was designed and simulated using a Geant4 Application for Tomographic Emission (GATE) toolkit. The performances of five crystals, namely strontium hafnate, gadolinium aluminium gallium garnet, gadolinium yttrium gallium aluminium garnet, gadolinium lutetium gallium aluminium garnet, and lutetium oxyorthosilicate for comparison, were evaluated in terms of sensitivity, spatial resolution, and count rate. The performance evaluation results showed that among the suggested scintillators, strontium hafnate can be considered as a promising alternative detector for high performance brain PET systems.

Development of Single-Ended Readout DOI Detector With Quadrisected Crystals

In this article, we developed the novel single-end readout depth-of-interaction (DOI) detector with quadrisected crystals comparable in size to a single photo sensor. The proposed DOI detector consists of 784 gadolinium aluminum gallium garnet (GAGG) crystals, which are arranged in four layers of $14\times14$ arrays coupled with an $8\times8$ multipixel photon counter (MPPC) array. The size of each GAGG crystal is $1.45\times 1.45\times4.5$ mm3. Also, each MPPC has a surface area of $3.0\times3.0$ mm2. One pair of four-layered crystals coupled with a partial optical window is arranged across two MPPCs. Boundaries without the partial optical window are covered with optical reflectors. Light sharing is done between a pair of four-layered crystals optically coupled at one end. Each pair is coupled with paired silicon photomultipliers of different patterns. Crystal identification from each pair is obtained from the output rate of two MPPCs. From 511-keV uniform irradiation, responses of all crystal elements in the four layers can be separated clearly on a 2-D position histogram after applying the Anger calculation. Also, DOI information can be mapped to each cluster on curved lines. The energy resolution for all crystals is 9.1% after event reduction of intercrystal scatter.

Optimizing the Sensitivity of a GAGG:Ce-Based Thermal Neutron Detector

The neutron sensitivity of the cerium-doped gadolinium aluminum gallium garnet (GAGG:Ce) has been determined for five different scintillator volumes. The good $\gamma $ ray detection performance of GAGG:Ce potentially results in a background signal that hides the low-lying neutron-capture $\gamma $ rays from de-excitation of 156Gd and 158Gd when the detector volume, and therefore the total $\gamma $ ray absorption, is large. We have investigated the competition between the detection of the neutron signature and discrimination of the background $\gamma $ rays. A significant improvement in the neutron response has been demonstrated with a reduction in the scintillator volume. The peak-to-background ratio of the key neutron signature increased with decreasing scintillator thickness to a maximum value of 14 ± 3 for 50 $\mu \text{m}$ . The limit of this correlation was not determined since the thinnest scintillator available (50 $\mu \text{m}$ ) is greater than the mean free path of a neutron in 157Gd (15.54 ± 0.09 $\mu \text{m}$ ).

Sensitivity of GAGG based scintillation neutron detector with SiPM readout

Here we report on the first results of sensitivity evaluation of the gadolinium-aluminum-gallium- garnet (GAGG) scintillation detector with SiPM readout to fast and slow neutrons and, to the natural background and Co-60 γ-radiation as well. Data on sensitivity were obtained using certified dosimetry benches, so it can be utilized in the calculation of detection limits of neutron flux with such type of detectors. It was concluded that use of GAGG scintillator has a good prospect for neutron monitoring in different parts of nuclear research reactors and power plants.

Diode-pumped laser and spectroscopic properties of Yb,Ho:GGAG at 2 µm and 3 µm

In this paper, spectroscopy and laser results obtained with new active medium Yb,Ho:GGAG (gadolinium–gallium–aluminum–garnet) emitting laser radiation at 2.09 m and 2.94 m are shown. The absorption, emission cross-section, and radiative decay time for several energy levels Yb3+ (2F5/2), Ho3+ (5I6 and 5I7) are presented. Using longitudinal diode pumping the laser action at 2 m and 3 m together with the wavelength tunability at several spectral bands 2063–2113 nm, 2860 nm, and 2912–2942 nm were reached.

A Compton polarimeter using scintillators read out with MPPCs through Citiroc ASIC

In recent years, a number of purpose-built scintillator-based polarimeters have studied bright astronomical sources for the first time in the hard X-ray band (tens to hundreds of keV). The addition of polarimetry can help data interpretation by resolving model-dependent degeneracies. The typical instrument approach is that incident X-rays scatter off a plastic scintillator into an adjacent scintillator cell. In all missions to date, the scintillators are read out using traditional vacuum tube photo-multipliers (PMTs). The advent of solid-state PMTs ("silicon PM" or "MPPC") is attractive for space-based instruments since the devices are compact, robust and require a low bias voltage. We have characterised the plastic scintillator, EJ-248M, optically coupled to a multi-pixel photon counter (MPPC) and read out with the Citiroc ASIC. A light-yield of 1.6 photoelectrons/keV has been obtained, with a low energy detection threshold of $\lesssim$5 keV at room temperature. We have also constructed an MPPC-based polarimeter-demonstrator in order to investigate the feasibility of such an approach for future instruments. Incident X-rays scatter from a plastic-scintillator bar to surrounding cerium-doped GAGG (Gadolinium Aluminium Gallium Garnet) scintillators yielding time-coincident signals in the scintillators. We have determined the polarimetric response of this set-up using both unpolarised and polarised $\sim$50 keV X-rays. We observe a clear asymmetry in the GAGG counting rates for the polarised beam. The low-energy detection threshold in the plastic scintillator can be further reduced using a coincidence technique. The demonstrated polarimeter design shows promise as a space-based Compton polarimeter and we discuss ways in which our polarimeter can be adapted for such a mission.

GAGG:Ce scintillation fibers for high energy physics applications

Radiation detectors based on multiple single crystalline scintillating fibers are promising for next generation of heterogeneous electromagnetic calorimeters to be exploited in high-luminosity colliders for high-energy physics experiments. These novel fiber-based detectors consist of long single crystalline rods placed in heavy metal absorber, thus, the light propagation conditions in these fiber-like structures are of crucial importance for obtaining uniform response to electromagnetic shower. In this work, the uniformity of Czochralski-grown cerium-doped gadolinium aluminum gallium garnet fibers was studied by employing confocal optical microscopy. Growth related defects were observed on a micrometer scale. Light attenuation length was estimated and found to vary in nominally identical fibers. The influence of crystal growth defects and surface polishing quality on light propagation is discussed.

Investigation into the potential of GAGG:Ce as a neutron detector

In this work we investigate the potential use as a thermal neutron detector of cerium-doped gadolinium aluminium gallium garnet (GAGG:Ce) coupled to a silicon photomultiplier (SiPM). The response to thermal neutrons has been measured, with two strong low energy neutron-indicative peaks clearly identifiable below 100 keV and additional γ peaks at higher energies. The neutron-related peaks are produced by a combination of contributions from excited states of the two isotopes 156Gd and 158Gd which can be clearly resolved in our GAGG scintillation detector. In particular, two peaks due to neutron-induced γ-ray emission are observed at approximately 82 keV and 260 keV, with best achieved energy resolutions of 24.1 ± 0.2% and 22.7 ± 0.7% respectively. Three different scintillator volumes (0.1 cm3, 0.4 cm3, and 1 cm3) were investigated and the respective results for each configuration will be presented. Our findings show that a GAGG-SiPM based detector can be used as a compact, efficient thermal neutron detector in a low γ-ray contamination environment.