Scintillation Yield Research Articles

Tailoring the scintillation properties of β-Ga2O3 by doping with Ce and codoping with Si

Measurements of pulse height spectra and scintillation time profiles performed on Czochralski-grown β-Ga2O3, β-Ga2O3:Ce, and β-Ga2O3:Ce,Si crystals are reported. The highest value of scintillation yield, 7040 ph/MeV, was achieved for pure β-Ga2O3 at a low free electron concentration, nevertheless Ce-doped crystals could also approach high values thereof. Si-codoping, however, decreases the scintillation yield. The presence of Ce, and the more of Ce and Si, in β-Ga2O3 significantly increases the contribution of the fastest components in scintillation time profiles, which makes β-Ga2O3 a very fast scintillator under γ-excitation.

A particle detector that exploits Liquid Argon scintillation light

A number of innovative experiments dedicated to neutrino oscillations and rare event physics, such as direct searches for dark matter particles or neutrinoless double beta decays, are using liquefied noble-gases, particularly Liquid Argon (LAr), as detection media. Among many advantages of noble liquids, from the detection point of view, the most important ones are high scintillation and ionisation yields, possible long drift paths of ionisation electrons and feasible large detector masses. LAr is sufficiently dense and relatively cheap and therefore is the only adequate noble liquid for huge volumes.This paper describes a particle detection system that exploits the prompt signals from the scintillation light produced by ionising particles in LAr. The detector has been exposed to cosmic rays. The system performance in terms of trigger efficiency and timing resolution, with a view to its application in neutrino detectors, is presented.

PETALO: Time-of-Flight PET with liquid xenon

Liquid xenon offers several features, which make it suitable for applications in nuclear medicine, such as high scintillation yield and fast scintillation decay time. Moreover, being a continuous medium with a uniform response, liquid xenon allows one to avoid most of the geometrical distortions of conventional detectors based on scintillating crystals. In this paper, we describe how these properties have motivated the development of a novel concept for positron emission tomography scanners with Time-Of-Flight measurement, which combines a liquid xenon scintillating volume and silicon photomultipliers as sensors. A Monte Carlo investigation has pointed out that this technology would provide an excellent intrinsic time resolution, down to 70 ps. Also, the transparency of liquid xenon to UV and blue wavelengths opens the possibility of exploiting both scintillation and Cherenkov light for a high-sensitivity positron emission tomography scanner with Time-Of-Flight capabilities. Monte Carlo simulations point to a time resolution of 30-50 ps obtained using Cherenkov light. A prototype is being built to demonstrate the high resolution in energy, time and reconstruction of spatial coordinates of this concept, using a ring of 30 cm internal diameter and a depth of 3 cm instrumented with VUV-sensitive silicon photomultipliers.

Scintillation response of Europium and Indium-co-doped CsI(Na) single crystal under the exposure of gamma-ray

The traditional CsI(Na) single crystal scintillator was studied to improve its scintillation properties, by co-doping Sodium with Europium and Indium ions. The growth was done by the Czochralski technique. Inductively coupled plasma optical emission spectroscopy and UV spectra including the absorption, excitation, and photoluminescence of the cut ingots were obtained in the region of 200 to 800 nm. The results show that the luminescence intensity of Europium-co-doped CsI(Na) is significantly increased compared to CsI(Na) single crystal, however Indium co-dopant has reduced the CsI(Na) luminescence intensity. Eu’s high quantum emission efficiency due to its 4f → 5d internal transitions and the radiative energy transfer between Na + and Eu 2 + centers are the possible reason for this enhancement. While decreasing the CsI(Na) luminescence intensity, by adding the In co-dopant, is probably due to non-constructive spectral overlap between emission transition of In + and absorption transition of Na + which results in non-radiative energy transfer between Na + and In + . In this regard, the pulse height spectrum of these crystals was determined under the excitation of the gamma-ray sources. It is observed that Eu co-dopant strongly increases the scintillation light yield of the conventional CsI(Na) detector and improves its energy resolution, however In co-doped CsI(Na) reduces scintillation yield.

On the stabilization of Ce, Tb, and Eu ions with different oxidation states in silica-based glasses

We investigated the influence of the silica-based glass composition and its synthesis conditions on the oxidation states of luminescent rare earths like Ce, Tb, and Eu dopant ions. It was found that the repeated melting of the glass led to the stabilization of rare-earth ions in higher oxidation states; in the case of Ce doping, this caused a significant decrease in the scintillation yield, whereas for Tb doping no significant modifications were noticed. Furthermore, it was concluded that the glass matrix influences the rare-earth oxidation state depending on the ions modifying the glass structure. This effect was clearly observed for the Eu-doped series of Ca, Sr and Ba di-silicate glasses, where the conversion of Eu3+ to Eu2+ was found to be dependent on the distortion of the silica network. The influence of glass crystallization on the rare-earth oxidation state stabilization is also discussed.

Unraveling the Critical Role of Site Occupancy of Lithium Codopants in Lu2SiO5:Ce3+ Single-Crystalline Scintillators.

Lithium codoping has emerged as an effective strategy to enhance the light yield of oxide scintillators for radiation detection applications, but the understanding of the actual role played by Li+ remains unclear. In this work, we comprehensively study the effects of Li codoping on optical and scintillation properties of Lu2SiO5:Ce (LSO:Ce) single crystals and reveal the critical role of site occupancy of Li. High-quality LSO:Ce single crystals codoped with 0.05, 0.1, and 0.3 at. % Li ions were grown by the Czochralski method. The optical absorption spectra confirm nonconversion of stable Ce3+ to Ce4+ in Li-codoped LSO:Ce regardless of the Li codoping concentration. The photoluminescence decay kinetics suggest an enhanced ionization of the excited 5d1 state of Ce3+ centers in highly codoped samples. A simultaneous improvement of scintillation light yield, decay time, and afterglow is achieved in LSO:Ce codoped with low concentrations of Li. The preferential occupation of Li at interstitial spaces and lutetium sites is proven to rely on its codoping concentration by using the 7Li nuclear magnetic resonance technique. The concentration-dependent site occupancy of Li alters the defect structures of LSO:Ce, in particular resulting in a distinct change in the number of cerium spatially correlated oxygen vacancies confirmed by thermoluminescence and afterglow measurements.

Scintillation properties and increased vacancy formation in cerium and calcium co-doped yttrium aluminum garnet

Co-doping with divalent elements is known to improve the light yield and decay times of some cerium-activated scintillators, despite the stabilization of tetravalent Ce4+, previously believed to be non-luminescent, from the Ce3+ state. Ce4+ stabilization is a charge compensation mechanism which results from divalent ion substitution of a 3 + site. To elucidate the underlying mechanisms, which remain poorly understood, we have grown three Ce,Ca:YAG crystals with different amounts of calcium co-dopants by the Czochralski method and characterized their scintillation and defect properties. Calcium co-doping reduces the decay times and stabilizes the formation of Ce4+ as expected. Interestingly, X-ray fluorescence analysis reveals a decreased concentration of cerium within the YAG crystal for the sample doped with high levels of Ca, which could contribute to the observation that scintillator properties are improved only for low levels of Ca co-doping. Additionally, positron annihilation spectroscopy reveals an increase in the concentration of vacancies with increasing Ca concentration, while thermoluminescence is observed to show no detectable signal. Furthermore, room temperature photoluminescence of the Ce 4f to 5d1 transition demonstrates decreased emission with increasing Ca co-doping. These data suggest that Ca co-doping decreases the decay time by creating defects with non-radiative decay pathways.

Scintillation response to gamma-rays measured at wide temperature range for Tl doped CsI with SiPM readout

A custom design cryostat was constructed to study the temperature dependence of relative light yield and non-proportionality trends of scintillators between −182 °C and ＋152 °C. CsI:Tl crystal response to γ-rays and X-rays was investigated between 14 keV and 662 keV. Scintillation light was detected by a SiPM device, which was installed on a copper frame separated from the crystal and the cooling rod to enable operating the device at room temperature. The scintillation efficiency of CsI:Tl is peaked at about room temperature. The light yield of CsI:Tl at temperature close to liquid nitrogen boiling point is reduced by a factor of 15 in comparison to room temperature conditions. The non-proportionality of CsI:Tl scintillation response is high at low temperatures and is getting more proportional with increasing temperature.

Effects of neutron radiation on the optical and structural properties of blue and green emitting plastic scintillators

We report on the optical and structural properties of plastic scintillators irradiated with neutron beams produced by the IBR-2 reactor of the Frank Laboratory of Neutron Physics in JINR, Dubna. Blue UPS-923A and green plastic scintillators were irradiated with neutron fluence ranging from 1013 to 1017 n/cm2. Discolouring in the plastic scintillators was observed after irradiation. The effects of radiation damage on the optical and structural properties of the samples were characterized by conducting light yield, light transmission, light fluorescence and Raman spectroscopy studies. The results showed that neutron radiation induced damage in the material. The disappearance of the Raman peak features in green scintillators at frequencies of 1165.8, 1574.7 and 1651.2 cm−1 revealed significant structural alterations due to neutron bombardment. Losses in fluorescence intensity, light yield and light transmission in the plastic scintillators were observed.

Proton light yield in organic scintillators using a double time-of-flight technique

Recent progress in the development of novel organic scintillators necessitates modern characterization capabilities. As the primary means of energy deposition by neutrons in these materials is n-p elastic scattering, knowledge of the proton light yield is paramount. This work establishes a new model-independent method to continuously measure the proton light yield in organic scintillators over a broad energy range. Using a deuteron breakup neutron source at the 88-in. Cyclotron at Lawrence Berkeley National Laboratory and an array of organic scintillators, the proton light yield of EJ-301 and EJ-309, commercially available organic liquid scintillators from Eljen Technology, was measured via a double time-of-flight technique. The light yield was determined using a kinematically over-constrained system in the proton energy range of 1–20 MeV. The effect of the pulse integration length on the magnitude and shape of the proton light yield relation was also explored. This work enables accurate simulation of the performance of advanced neutron detectors and supports the development of next-generation neutron imaging systems.

Improvements in Light Yield and Energy Resolution by Li+ Codoping (Lu0.75Y0.25)3Al5O12:Pr3+ Single Crystal Scintillators

In this study, we demonstrated a drastic improvement in energy resolution of (Lu0.75Y0.25)3Al5O12:Pr (LuYAG) single crystal scintillators from 4.8% to 4.1% at 662 keV upon Li+ codoping, which is the best value ever reported in the class of oxide single crystal scintillators. Li+ codoping can also significantly enhance its scintillation yield from 16 000 to 25 000 photons MeV−1, but slightly prolong the scintillation decay time. As Li+ codoping was found to have a negligible effect on the light yield non‐proportionality, we attributed the significant improvement in energy resolution to the reduced statistical variance in the number of photoelectrons produced in the photomultiplier tube (Rstat).

A dual-phase xenon TPC for scintillation and ionisation yield measurements in liquid xenon

A small-scale, two-phase (liquid/gas) xenon time projection chamber (Xurich II) was designed, constructed and is under operation at the University of Zürich. Its main purpose is to investigate the microphysics of particle interactions in liquid xenon at energies below 50 keV, which are relevant for rare event searches using xenon as target material. Here we describe in detail the detector, its associated infrastructure, and the signal identification algorithm developed for processing and analysing the data. We present the first characterisation of the new instrument with calibration data from an internal ^{83mathrm {m}}Kr source. The zero-field light yield is 15.0 and 14.0 photoelectrons/keV at 9.4 and 32.1 keV, respectively, and the corresponding values at an electron drift field of 1 kV/cm are 10.8 and 7.9 photoelectrons/keV. The charge yields at these energies are 28 and 31 electrons/keV, with the proportional scintillation yield of 24 photoelectrons per one electron extracted into the gas phase, and an electron lifetime of 200 upmu s. The relative energy resolution, sigma /E, is 11.9 and 5.8% at 9.4 and 32.1 keV, respectively using a linear combination of the scintillation and ionisation signals. We conclude with measurements of the electron drift velocity at various electric fields, and compare these to literature values.

Scintillation Yield Estimates of Colloidal Cerium-Doped LaF3 Nanoparticles and Potential for “Deep PDT”

A hybrid of radiotherapy and photodynamic therapy (PDT) has been proposed in previously reported studies. This approach utilizes scintillating nanoparticles to transfer energy to attached photosensitizers, thus generating singlet oxygen for local killing of malignant cells. Its effectiveness strongly depends upon the scintillation yield of the nanoparticles. Using a liquid scintillator as a reference standard, we estimated the scintillation yield of Ce0.1La0.9F3/LaF3 core/shell nanoparticles at 28.9 mg/ml in water to be 350 photons/MeV under orthovoltage X-ray irradiation. The subsequent singlet oxygen production for a 60 Gy cumulative dose to cells was estimated to be four orders of magnitude lower than the "Niedre killing dose," used as a target value for effective cell killing. Without significant improvements in the radioluminescence properties of the nanoparticles, this approach to "deep PDT" is likely to be ineffective. Additional considerations and alternatives to singlet oxygen are discussed.

Defect-Controlled Scintillation Process in Undoped Y3Al5O12Crystals

Optical absorption, luminescence spectra, scintillation decay curves, and scintillation light yield were measured in a series of undoped yttrium–aluminum garnet (YAG) crystals of different origins. This paper reveals a correlation between luminescent properties and scintillation efficiency of undoped YAG crystals. All the samples were separated into three groups, Type I, Type II, and Type III, based on their optical properties and scintillation efficiency. The intrinsic ultraviolet luminescence band peaking around 300 nm is found in the samples with high scintillation yield (Type I). Luminescence intensity of this band depends on the crystal purity and on the stoichiometry of the garnets. The UV emission may be related to e–h recombination in vicinity of vacancies. The garnets of Type II contain more impurities which quench the 300-nm emission. The samples of Type III do not exhibit any significant radioluminescence. Light yield of the finest undoped YAG samples in the series, Type I, is similar to the yield of Ce-doped YAG. This Type of crystals can potentially be used in scintillation detectors.

Boron codoping of Czochralski grown lutetium aluminum garnet and the effect on scintillation properties

Many single crystal scintillators, such as Lu3Al5O12, have intrinsic defects that impede their performance. In addition to doping with activators such as cerium, codoping can be used to improve the scintillation properties of a variety of scintillators. In particular, boron has been shown to improve the light yield, energy resolution, and self-absorption of other garnet scintillators, such as GGAG, when incorporated into the lattice via codoping. In this study, single crystals of LuAG: 0.2 at.% Ce codoped with varying concentrations of boron were grown via the Czochralski method at a rate of 1.2 mm/h. Results will show the effect boron codoping has on the scintillation properties of LuAG: Ce, including light yield, decay time, and self-absorption.

Radiation imaging with optically read out GEM-based detectors

Modern imaging sensors allow for high granularity optical readout of radiation detectors such as MicroPattern Gaseous Detectors (MPGDs). Taking advantage of the high signal amplification factors achievable by MPGD technologies such as Gaseous Electron Multipliers (GEMs), highly sensitive detectors can be realised and employing gas mixtures with strong scintillation yield in the visible wavelength regime, optical readout of such detectors can provide high-resolution event representations. Applications from X-ray imaging to fluoroscopy and tomography profit from the good spatial resolution of optical readout and the possibility to obtain images without the need for extensive reconstruction. Sensitivity to low-energy X-rays and energy resolution permit energy resolved imaging and material distinction in X-ray fluorescence measurements. Additionally, the low material budget of gaseous detectors and the possibility to couple scintillation light to imaging sensors via fibres or mirrors makes optically read out GEMs an ideal candidate for beam monitoring detectors in high energy physics as well as radiotherapy. We present applications and achievements of optically read out GEM-based detectors including high spatial resolution imaging and X-ray fluorescence measurements as an alternative readout approach for MPGDs. A detector concept for low intensity applications such as X-ray crystallography, which maximises detection efficiency with a thick conversion region but mitigates parallax-induced broadening is presented and beam monitoring capabilities of optical readout are explored. Augmenting high resolution 2D projections of particle tracks obtained with optical readout with timing information from fast photon detectors or transparent anodes for charge readout, 3D reconstruction of particle trajectories can be performed and permits the realisation of optically read out time projection chambers. Combining readily available high performance imaging sensors with compatible scintillating gases and the strong signal amplification factors achieved by MPGDs makes optical readout an attractive alternative to the common concept of electronic readout of radiation detectors. Outstanding signal-to-noise ratios and robustness against electronic noise allow unprecedented imaging capabilities for various applications in fields ranging from high energy physics to medical instrumentation.

Neutron/gamma discrimination enhancement: plastic scintillators high dose irradiation and recovery time

In the context of high-energy physics experiments, particle accelerators create a significant dose of radiations, up to 106 Gy for example at the Large Hadron Collider (LHC, Cern). Control detectors, such as scintillators, are affected by these high irradiation levels. The literature mentions that the light yield of organic scintillators dramatically drops when strongly irradiated. However, the tested scintillators can recover some of their light output with time. To the best of our knowledge, only the luminescent properties of plastic scintillators were analyzed under high radiation fluxes. But, plastic scintillators are also able to discriminate fast neutrons from gamma rays. In this work, we characterized neutron/gamma discriminating as well as emissive properties of plastic scintillators after high dose irradiations. Two identical lab-made plastic scintillators containing a polystyrene-based matrix and two fluorophores were analyzed before and after high dose irradiation. Irradiation was performed using a Gamma-Cell 220 Excel with twelve 60Co sources and a 53.7 Gy/min dose rate at isocenter. These two scintillators were irradiated to reach a cumulative dose equal to 104 Gy. In order to measure their neutron/gamma discrimination ability, scintillators were coupled to a Hamamatsu R7724-100 photomultiplier tube and placed in front of a 252Cf source (Activity 580 kBq). The anode signal fed a digitizer. Scintillation pulses were then recorded and post-processed. A charge-comparison method was implemented and FoM was evaluated. Offline treatment allowed the estimation of fast and slow decay times of neutron pulses, as well as their relative intensities. In parallel, characterization of radioluminescence properties of irradiated and non-irradiated samples was performed. Results indicate a strong modification in the neutron/gamma discrimination capability before and after high dose irradiations and a redshift of the radioluminescence spectrum. In fact, a 103 Gy irradiation increases the FoM of both sensors by a factor 2.5 at least. Further, as recovery time passes after a 104 Gy cumulative dose, FoM improves for scintillators and is multiplied by 6 compared to the value at the zero dose. Thanks to evaluated decay times and relative intensities, we could infer that a strong irradiation does not produce a higher yield of triplet excited states but it impacts the slow decay time of the tested scintillator. This leads to a better neutron/gamma discrimination than at the zero dose. These observations lead us to believe that the intrinsic nature of the plastic material is modified under high dose irradiation. In this paper we present the current iteration of this ongoing work. The neutron/gamma discrimination properties are presented for both plastic scintillators and first characterization results are discussed. We show that material modification under high dose irradiation manages to an improvement in neutron/gamma discrimination opening the field to applications of very high dose measurements.

Liquid xenon in nuclear medicine: state-of-the-art and the PETALO approach

Liquid xenon has several attractive features, which make it suitable for applications to nuclear medicine, such as high scintillation yield and fast scintillation decay time, better than currently used crystals. Since the '90s, several attempts have been made to build Positron Emission Tomography scanners based on liquid xenon, which can be divided into two different approaches: on one hand, the detection of the ionization charge in TPCs, and, on the other one, the detection of scintillation light with photomultipliers. PETALO (Positron Emission Tof Apparatus with Liquid xenOn) is a novel concept, which combines liquid xenon scintillating cells and silicon photomultipliers for the readout. A first Monte Carlo investigation has pointed out that this technology would provide an excellent intrinsic time resolution, which makes it possible to measure the Time-Of-Flight with high efficiency. Also, the transparency of liquid xenon to UV and blue wavelengths opens the possibility of exploiting both scintillation and Cherenkov light for a high-sensitivity TOF-PET.

Live event reconstruction in an optically read out GEM-based TPC

Combining strong signal amplification made possible by Gaseous Electron Multipliers (GEMs) with the high spatial resolution provided by optical readout, highly performing radiation detectors can be realized. An optically read out GEM-based Time Projection Chamber (TPC) is presented. The device permits 3D track reconstruction by combining the 2D projections obtained with a CCD camera with timing information from a photomultiplier tube. Owing to the intuitive 2D representation of the tracks in the images and to automated control, data acquisition and event reconstruction algorithms, the optically read out TPC permits live display of reconstructed tracks in three dimensions. An Ar/CF4 (80/20%) gas mixture was used to maximize scintillation yield in the visible wavelength region matching the quantum efficiency of the camera. The device is integrated in a UHV-grade vessel allowing for precise control of the gas composition and purity. Long term studies in sealed mode operation revealed a minor decrease in the scintillation light intensity.

Structural and optical properties of rare-earths doped barium bismuth borate glasses

Recently, great importance has been devoted to different glass systems doped with rare-earth ions because of their peculiar properties, in particular in the field of high-energy physics for particle energy measurement. The purpose of the present study was to investigate the optical and physical properties of Dy3+, Er3+, Nd3+ doped glasses belonging to the 20BaO-20Bi2O3-60B2O3 system in which several rare-earths oxide concentrations were added to encounter the requirements for particle energy measurement. High density, low refractive index, high emission intensity (or high scintillation yield) are required for this purpose. Moreover, molar volume, glass transition and melting temperatures, X-ray diffraction and Raman spectra were measured and discussed in order to characterize the glass state. All the properties measured have shown a non-linear trend moving from 1mol% to 10mol% of rare-earths content. At the same time comparison between the trend derived by samples with same stoichiometry but containing different rare earths highlight different behaviors. In particular the highest density has been reached with the glass where Dy2O3 is at 2,5mol%.