Scintillation Process Research Articles

Addition of luminescence process in Monte Carlo simulation to precisely estimate the light emitted from water during proton and carbon-ion irradiation

Although luminescence of water lower in energy than the Cerenkov-light threshold during proton and carbon-ion irradiation has been found, the phenomenon has not yet been implemented for Monte Carlo simulations. The results provided by the simulations lead to misunderstandings of the physical phenomenon in optical imaging of water during proton and carbon-ion irradiation. To solve the problems, as well as to clarify the light production of the luminescence of water, we modified a Monte Carlo simulation code to include the light production from the luminescence of water and compared them with the experimental results of luminescence imaging of water. We used GEANT4 for the simulation of emitted light from water during proton and carbon-ion irradiation. We used the light production from the luminescence of water using the scintillation process in GEANT4 while those of Cerenkov light from the secondary electrons and prompt gamma photons in water were also included in the simulation. The modified simulation results showed similar depth profiles to those of the measured data for both proton and carbon-ion. When the light production of 0.1 photons/MeV was used for the luminescence of water in the simulation, the simulated depth profiles showed the best match to those of the measured results for both the proton and carbon-ion compared with those used for smaller and larger numbers of photons/MeV. We could successively obtain the simulated depth profiles that were basically the same as the experimental data by using GEANT4 when we assumed the light production by the luminescence of water. Our results confirmed that the inclusion of the luminescence of water in Monte Carlo simulation is indispensable to calculate the precise light distribution in water during irradiation of proton and carbon-ion.

Trends in Search for Bright Mixed Scintillators

The paper addresses the recent progress in study of light yield behavior in scintillators based on solid solutions (mixed crystals), which are among the brightest known scintillators. Following the prior work [O. Sidletskiy et al., Phys. Status Solidi A 2014, 211, 2384], the extended phenomenological analysis of the correlations between light yield behavior and physical properties of mixed crystals (band gaps of the components, ionic radii, and atomic weights of the substitutional atoms) is carried out. The trend to light yield enhancement is notable in systems with the substitution of atoms in two non‐equivalent crystallographic sites in the lattice leading to formation of planes and columns enriched with one of the substitutional atoms. This evidences an important role of nanoscale spatial inhomogeneity (domains or clusters) in scintillation process in mixed scintillators. A roadmap of search for new bright scintillators is proposed basing on the established trends.

Interstellar scintillation observations for PSR B0355+54

In this paper, we report our investigation of pulsar scintillation phenomena by monitoring PSR B0355$+$54 at 2.25 GHz for three successive months using \emph{Kunming 40-m radio telescope}. We have measured the dynamic spectrum, the two-dimensional correlation function, and the secondary spectrum. In those observations with high signal-to-noise ratio ($S/N\ge100$), we have detected the scintillation arcs, which are rarely observable using such a small telescope. The sub-microsecond scale width of the scintillation arc indicates that the transverse scale of structures on scattering screen is as compact as AU size. Our monitoring has also shown that both the scintillation bandwidth, timescale, and arc curvature of PSR B0355$+$54 were varying temporally. The plausible explanation would need to invoke multiple-scattering-screen or multiple-scattering-structure scenario that different screens or ray paths dominate the scintillation process at different epochs.

Structure, photoluminescence, and scintillation characteristics of a Gd1.9Y0.1SiO5:0.5%Ce (GYSO:Ce) single crystal scintillator

A single crystal of Gd1.9Y0.1SiO5:0.5%Ce (GYSO:Ce) was grown using the Czochralski method and its structure, photoluminescence, and scintillation properties were analyzed. Based on Rietveld structure refinement, we found that mixing with Y5at% changed the host structure very little. Two typical types of cerium emission, i.e., Ce1 and Ce2, were confirmed based on the photoluminescence excitation and emission spectra at 77 K. The energy transfer of Gd 6PJ→Ce1 probably explains the slow component in the photoluminescence and scintillation processes. The light yield of the GYSO:Ce sample was determined as 11200 ph/MeV and the energy resolution was 8.6%. The afterglow intensity of GYSO:Ce was relatively low.

Impact of Non-Uniformity in Light Collection on the Energy Resolution of the PANDA Electromagnetic Calorimeter at Photon Energies Below 1 GeV

The electromagnetic calorimeter (EMC) of the PANDA detector at the future FAIR facility comprises more than 15,000 lead tungstate (PWO) crystals. The barrel part will consist of 11 crystal geometries with different degree of tapering, which causes a non-uniformity in light collection as an interplay between the focusing and the internal absorption of the light. For the most tapered crystals the detected light is enhanced by 40%, if the scintillation process is created in the front part of the crystal. Due to the shower development and its fluctuations the non-uniformity leads to a reduction of the energy resolution. To reduce this effect, one lateral crystal side face has been de-polished to a roughness of 0.3 μm. Measurements confirm an increase of the light yield in the rear part of the crystal. In contrast, only a slight decrease can be observed in the front part. The overall non-uniformity is significantly reduced below 5%. This paper will discuss the experimental studies based on GEANT4 and optical simulations to understand the impact of a de-polished side face on the light collection. For consequences on the future performance, a 3×3 sub-array of de-polished crystals was directly studied using a tagged photon beam in the energy range from 50 MeV up to 800 MeV, respectively, performed at the tagged photon facility at MAMI, Mainz. The comparison to an array composed of polished crystals confirms a significant improvement of the constant term of the energy resolution from above 2 % down to 0.5 % and only a small increase of the statistical term. The results can be reproduced in GEANT4 simulations.

Studies on Structural, Optical, Thermal and Electrical Properties of Perylene-Doped p-terphenyl Luminophors.

A simple solid state reaction technique was employed for the preparation of polycrystalline luminophors of p-terphenyl containing different amounts of perylene followed by spectral characterization techniques viz. XRD, SEM, TGA-DSC, UV-Visible spectroscopy, thermo-electrical conductivity, fluorescence spectroscopy, fluorescence life time spectroscopy and temperature dependent fluorescence. X-ray diffraction profiles of the doped p-terphenyl reveal well-defined and sharp peaks indicate homogeneity and crystallinity. The SEM micrograph of pure p-terphenyl exhibit flakes like grains and then compact and finally gets separately with perylene amounts. The observed results indicate that closed packed crystal structures of doped p-terphenyl during crystal formation. The band gaps estimated from UV-visible spectroscopy decreased from 5.20 to 4.10eV, while thermo-electrical conductivity increases with perylene content. The fluorescence spectra showed partial quenching of p-terphenyl fluorescence and simultaneously sensitization of perylene fluorescence at the excitation wavelength of p-terphenyl (290nm) due to excitation energy transfer from p-terphenyl to perylene. The observed sensitization results are in harmony with intense blue color seen in fluorescence microscopy images and has high demand in scintillation process.

Opticks : GPU Optical Photon Simulation for Particle Physics using NVIDIA® OptiX™

Opticks is an open source project that integrates the NVIDIA OptiX GPU ray tracing engine with Geant4 toolkit based simulations. Massive parallelism brings drastic performance improvements with optical photon simulation speedup expected to exceed 1000 times Geant4 when using workstation GPUs. Optical photon simulation time becomes effectively zero compared to the rest of the simulation. Optical photons from scintillation and Cherenkov processes are allocated, generated and propagated entirely on the GPU, minimizing transfer overheads and allowing CPU memory usage to be restricted to optical photons that hit photomultiplier tubes or other photon detectors. Collecting hits into standard Geant4 hit collections then allows the rest of the simulation chain to proceed unmodified. Optical physics processes of scattering, absorption, scintillator reemission and boundary processes are implemented in CUDA OptiX programs based on the Geant4 implementations. Wavelength dependent material and surface properties as well as inverse cumulative distribution functions for reemission are interleaved into GPU textures providing fast interpolated property lookup or wavelength generation. Geometry is provided to OptiX in the form of CUDA programs that return bounding boxes for each primitive and ray geometry intersection positions. Some critical parts of the geometry such as photomultiplier tubes have been implemented analytically with the remainder being tessellated. OptiX handles the creation and application of a choice of acceleration structures such as boundary volume hierarchies and the transparent use of multiple GPUs. OptiX supports interoperation with OpenGL and CUDA Thrust that has enabled unprecedented visualisations of photon propagations to be developed using OpenGL geometry shaders to provide interactive time scrubbing and CUDA Thrust photon indexing to enable interactive history selection.

Simulation of argon response and light detection in the DarkSide-50 dual phase TPC

A Geant4-based Monte Carlo package named G4DS has been developed to simulate the response of DarkSide-50, an experiment operating since 2013 at LNGS, designed to detect WIMP interactions in liquid argon. In the process of WIMP searches, DarkSide-50 has achieved two fundamental milestones: the rejection of electron recoil background with a power of ∼107, using the pulse shape discrimination technique, and the measurement of the residual 39Ar contamination in underground argon, ∼3 orders of magnitude lower with respect to atmospheric argon. These results rely on the accurate simulation of the detector response to the liquid argon scintillation, its ionization, and electron-ion recombination processes. This work provides a complete overview of the DarkSide Monte Carlo and of its performance, with a particular focus on PARIS, the custom-made liquid argon response model.

Structure and luminescence properties of single crystal scintillator (Gd0.9Lu0.1)2Si2O7:0.1%Ce

A novel mixed crystal scintillator of (Gd0.9Lu0.1)2Si2O7:0.1%Ce (Lu-GPS:Ce) was grown by floating zone method. The structure of as grown sample was determined through XRD to be triclinic. Its photoluminescence (PL) and scintillation properties, were evaluated in this paper. The PL properties were studied through Vacuum ultra-violet (VUV) excitation, emission and PL decay results. Scintillation properties were investigated through X-ray excited luminescence (XEL), scintillation decay curves and pulse spectrum under γ-ray excitation. The absolute light yield of as grown Lu-GPS:Ce sample is 16,600ph/MeV. It is found that non-radiative energy transfer from Gd3+ to Ce3+ occurs, leading slow component in the PL and scintillation process.

Characterizing and simulation the scintillation properties of zinc oxide nanowires in AAO membrane for medical imaging applications

In this work, a new method is proposed for extracting some X-ray detection properties of ZnO nanowires electrodeposited on Anodized Aluminum Oxide (AAO) nanoporous template. The results show that the detection efficiency for 12μm thickness of zinc oxide nano scintillator at an energy of 9.8 keV, near the K-edge of ZnO (9.65 keV), is 24%. The X-rays that interact with AAO can also generate electrons that reach the nano scintillator. The scintillation events of these electrons are seen as a low energy tail in the spectrum. In addition, it is found that all the X-rays that are absorbed in 300 nm thickness of the gold layer on the top of the zinc oxide nanowires can participate in the scintillation process with an efficiency of 6%. Hence, the scintillation detection efficiency of the whole detector for 9.8 keV X-ray energy is 30%. The simulation results from Geant4 and the experimental detected photons per MeV energy deposition are also used to extract the light yield of the zinc oxide nano scintillator. The results show that the light yield of the zinc oxide nanowires deposited by the electrochemical method is approximately the same as for single crystal zinc oxide scintillator (9000). Much better spatial resolution of this nano scintillator in comparison to the bulk ones is an advantage which candidates this nano scintillator for medical imaging applications.

TDC Array Tradeoffs in Current and Upcoming Digital SiPM Detectors for Time-of-Flight PET

Radiation detection used in positron emission tomography (PET) exploits the timing information to remove background noise and refine position measurement through time-of-flight information. Fine time resolution in the order of 10 ps full-width at half-maximum (FWHM) would not only improve contrast in the image, but would also enable direct image reconstruction without iterative or back-projected algorithms. Currently, PET experimental setups based on silicon photomultipliers (SiPMs) reach 73 ps FWHM, where the scintillation process plays the larger role in spreading the timing resolution. This will change with the optimization of faster light emission mechanisms (prompt photons), where readout optoelectronics will once more have a noticeable contribution to the timing resolution limit. In addition to reducing electronic jitter as much as possible, other aspects of the design space must also explored, especially for digital SiPMs. Unlike traditional SiPMs, digital SiPMs can integrate circuits like time-to-digital converters (TDCs) directly with individual or groups of light sensing cells. Designers should consider the number of TDCs to integrate, the area they occupy, their power consumption, their resolution, and the impact of signal processing algorithms and find a compromise with the figure of merit and the coincidence timing resolution (CTR). This paper presents a parametric simulation flow for digital SiPM microsystems that evaluates CTR based on these aspects and on the best linear unbiased estimator (BLUE) in order to guide their design for present and future PET systems. For a small $1.1\times 1.1\times3.0$ mm3 LYSO crystal, the simulations indicate that for a low jitter digital SiPM microsystem with 18.2% photon detection efficiency, fewer than four timestamps with any multi-TDC configuration scheme nearly obtain the optimal CTR with BLUE (just below 100 ps FWHM), but with limited 5% improvement over only using the first observed photon. On the other hand, if a similar crystal but with 2.5% prompt photon fraction is considered, BLUE provides an improvement between 80% and 200% (depending on electronic jitter) over using only the first observed photon. In this case, a few tens of timestamps are required, yielding very different design guidelines than for standard LYSO scintillators.

Growth of LiF/LiBaF3 eutectic scintillator crystals and their optical properties

Li-containing materials can be applied as neutron scintillators, and LiBaF3 can discriminate neutron and gamma rays. Moreover, LIF/LiBaF3 can have higher cross section of thermal-neutron capture compared with LiBaF3. In this study, LiF (82.5 mol%) and (Ba1−x RE x )F2 (17.5 mol%, RE = Ce and Eu, x = 0.002) eutectic crystals, LiF/RE:LiBaF3, were grown by the micropulling down method with different pulling rates (growth rate) in order to observe the eutectic structure. Lamellar microstructure was formed for each pulling rate. LiF/Ce:LiBaF3 excited by 5.5-MeV alpha rays had a broad peak at ~350 nm corresponding to 5d–4f transition of Ce3+. On the other hand, LiF/Eu:LiBaF3 had two scintillation processes; a sharp emission was originated from 6P7/2 → 8S7/2 transitions in the 4f electronic configuration of Eu2+ at 360 nm, and a broad one was attributed to Eu2+ trapped exciton recombination at 400–450 nm. Since scintillation light was observed for these materials, these scintillators are sensitive to neutrons.

Non-proportional response of scintillators to alpha particle excitation

In this paper, we investigated the relation between gamma nonproportionality and alpha particle nonproportionality. First, we collected literature data on the $\alpha/\beta$ ratio of almost all commonly used scintillators. Second, we extended the literature review with our own measurements of LaBr3:Ce $\alpha/\beta$ ratio in function of Ce3+ concentration. A strong influence of codoping on the $\alpha/\beta$ ratio of LaBr3:Ce has been observed. Finally, we conclude that the gamma nonproportionality, gamma intrinsic energy resolution, and $\alpha$ particle response are closely related to each other, and they are due to quenching in the high density part of ionization tracks. Though it still needs to be tested for a larger group of scintillators, $\alpha/\beta$ can be used as a single value characterizing intensity of light quenching processes in scintillators.

Multi-frequency GNSS robust carrier tracking for ionospheric scintillation mitigation

Ionospheric scintillation is the physical phenomena affecting radio waves propagating from the space through the ionosphere to earth. The signal distortion induced by scintillation can pose a major threat to some GNSS application. Scintillation is one of the more challenging propagation scenarios, particularly affecting high-precision GNSS receivers which require high quality carrier phase measurements; and safety critical applications which have strict accuracy, availability and integrity requirements. Under ionospheric scintillation conditions, GNSS signals are affected by fast amplitude and phase variations, which can compromise the receiver synchronization. To take into account the underlying correlation among different frequency bands, we propose a new multivariate autoregressive model (MAR) for the multi-frequency ionospheric scintillation process. Multi-frequency GNSS observations and the scintillation MAR are modeled in state-space, allowing independent tracking of both line-of-sight phase variations and complex gain scintillation components. The resulting joint synchronization and scintillation mitigation problem is solved using a robust nonlinear Kalman filter, validated using real multi-frequency scintillation data with encouraging results.

Theory and experiment studies of the 1,4-bis(4-methoxylstyryl)benzene as a wavelength shifter of liquid scintillator

A novel wavelength shifter of the 1,4-bis(4-methoxylstyryl)benzene (bis-4-MOSB) was synthesized by employing the classical Horner-Wadsworth-Emmons reaction. Feasible analysis of the bis-4-MOSB as the wavelength shifter in a ternary liquid scintillator, in which p-xylene (PX) was as the solvent and 2,5-diphenyloxazole (PPO) was as the primary fluor, was carried out. The optimum prescription with 3.5g/L PPO and 25mg/L bis-4-MOSB was obtained with regard to the light yield. A series of characterization tests based on the optimal formulation were performed. Compared with the 1,4-bis(2-methylstyryl)benzene (bis-MSB), the maximum absorption peak at 356nm and maximum emission peak at 421nm in n-hexane with the red shift of 10nm and 3nm, respectively, were measured accordingly. The light yield characterized by using a relative measurement method achieved as high as 75.85% of the anthracene crystal. A brief density functional calculation was conducted to have an insight into the electronic structure characteristic of the bis-4-MOSB in the scintillation process.

Intense emission of Ba2MgSi2O7:Eu2+ under X-ray excitation for potential detecting applications

A series of Ba2-xEuxMgSi2O7 phosphors was prepared by a solid-state reaction method at high temperature. The theoretical density of the optimal Ba1.93Eu0.07MgSi2O7 material was calculated from the Rietveld refinement result. Eu L3-edge X-ray absorption near edge structure (XANES) spectrum was measured to confirm the valence of Eu ions in Ba2MgSi2O7. The X-ray excited radioluminescence and the thermoluminescence after β-ray irradiation were investigated based on the VUV-vis photoluminescence. The light yield of the optimal Ba1.93Eu0.07MgSi2O7 sample under X-ray excitation was estimated to be ~29,000±6000 photons/MeV. So the temperature-dependent luminescence properties of this sample under X-ray and 344nm excitation were further studied, and the charge traps in the scintillation process were discussed through thermoluminescence spectra. The high scintillation intensity together with an appropriate intrinsic decay time and its non-hygroscopicity endow the further optimized phosphor Ba1.93Eu0.07MgSi2O7 a promising scintillation material for X-ray detection.

Examination of scintillator-photovoltaic cell-based spent fuel radiation energy conversion for electricity generation

Using computational models, this research examined electricity generation from spent nuclear fuel and its possible uses. The proposed approach was based on converting gamma radiation energy into electricity using scintillators and photovoltaic cells. The work includes performing gamma radiation environment analysis around spent fuel, scintillated photon analysis, and photovoltaic cell analysis for electricity generation. The OrigenArp code was used for gamma radiation environment analysis and the MCNPX 2.7.0 code was used for analyzing scintillation process. For the scintillated photon analysis and photovoltaic cell analysis, a new simulation model was developed and validated based on comparison with experimental data. The effect of self-absorption and radiation damage within the scintillator was described by using experimental data. Based on using 14 energy conversion system units in a spent fuel storage pool in a PWR with CdWO4 as scintillator and SiO2 as photovoltaic cell, generation of electric energy was estimated to range between a few hundred watts and a few watts depending on the cooling time. The estimated amount of electric power generation from spent fuel energy conversion was not enough for large scale applications. But the converted electric power could be utilized as emergency power source in an operating nuclear power plant for various detection and monitoring purposes and for the support of spent fuel pool cooling pump operations.

A promising new mechanism of ionizing radiation detection for positron emission tomography: modulation of optical properties

Using conventional scintillation detection, the fundamental limit in positron emission tomography (PET) time resolution is strongly dependent on the inherent temporal variances generated during the scintillation process, yielding an intrinsic physical limit for the coincidence time resolution of around 100 ps. On the other hand, modulation mechanisms of the optical properties of a material exploited in the optical telecommunications industry can be orders of magnitude faster. In this paper we borrow from the concept of optics pump-probe measurement to for the first time study whether ionizing radiation can produce modulations of optical properties, which can be utilized as a novel method for radiation detection. We show that a refractive index modulation of approximately is induced by interactions in a cadmium telluride (CdTe) crystal from a 511 keV photon source. Furthermore, using additional radionuclide sources, we show that the amplitude of the optical modulation signal varies linearly with both the detected event rate and average photon energy of the radiation source.

Scintillation and luminescence characteristics of Ce3+doped in Li2O–Gd2O3–BaO–B2O3 scintillating glasses

Ce3+ activated Li2O–Gd2O3–BaO–B2O3glass scintillator containing neutron-capture elements (7Li, 11Band 158Gd)were developed by conventional melt-quenching technique. Luminescence spectra under UV and X-ray excitation showed Ce3+ion emission due to 5d→4f transition at 391nm. Energy transfer from the host glass to Ce3+ions were confirmed by VUV–UV and XEL spectra. The highest emission intensity of Ce3+ions were observed at 0.5mol%of CeF3. For the same concentration the decay time was obtained to be 19.7ns and their mean critical distance was calculated about 22.33Å. The observed decay constants revealed that direct electron-hole capture was a dominant scintillation process in the present glass matrix.

Enhancement Effect of Fused Double Benzene-Ring Structure on the Light Output of Carborane-Loaded Toluene- and Pseudocumene-Based Scintillators

Four sample groups were prepared by adding different concentrations of naphthalene (NP) or of 2,6-diisopropylnaphthalene (DIN) as a secondary solvent ( ${{\rm S}_2}$ ) in carborane-loaded toluene (TL)- and pseudocumene (PC)-based scintillators. The pulse-height spectra of the samples in response to $^{137}{\rm Cs} \gamma $ -rays and to thermal neutrons were collected to study the light output ( $L$ ) enhancement effect of ${{\rm S}_2}$ . It is found that for all sample groups, $L$ increases to a plateau with the concentration of secondary solvent ([ ${{\rm S}_2}$ ]). As [ ${{\rm S}_2}$ ] is increased from 0 to saturation concentration, $L$ increases by 31–45% and 34–53% in response to $^{137}{\rm Cs} \gamma $ -rays and to thermal neurons, respectively. A first-order approximation model is proposed to fit to the experimental data. The enhancement factor ( ${k_h}$ ) and maximum $L$ of each sample group are obtained from curve fitting. The Birks factor ( kB ) and electron equivalent energy ( keVee ) of each sample are calculated by a numerical method based on Birks formula. The $L$ enhancement is discussed according to the percent changes of these parameters. In conclusion, the enhancement effect is attributed mainly to the fused double benzene-ring structure of NP and DIN, which has more delocalized electrons, and provides faster Forster resonance energy transfer paths than the single benzene-ring structure of TL and PC. Adding NP or DIN leads to a significant increase in absolute scintillation efficiency $(S)$ and a slight decrease in kB for both scintillators. It is believed that DIN has slightly larger enhancement effect than NP. A so-called “dynamic quenching” is confirmed to exist in the scintillation process based on the ${k_h}$ values in response to $^{137}{\rm Cs} \gamma $ -rays and to thermal neutrons.