Core-valence Luminescence Research Articles

Pulse-Shape Discrimination of SiPM Array-Coupled CLYC Detector Using Convolutional Neural Network

Cs2LiYCl6: Ce3+ (CLYC) is a dual-mode gamma-neutron scintillator with a medium gamma-ray resolution and pulse-shape discrimination (PSD) capability. The PSD performance of CLYC is greatly weakened when coupled with silicon photomultipliers (SiPMs) because of SiPMs’ low detection efficiency for the ultrafast Core-Valence-Luminescence (CVL) component under gamma excitation. In our previous work, the PSD Figure-of-Merit (FoM) value was optimized to 2.45 at the gamma-equivalent energy region of the thermal neutron by using the charge comparison method. However, this value was reduced to 1.37 at the lower gamma-equivalent energy region of more than 325 keV, and neutrons were difficult to distinguish from gamma rays. Hence, new algorithms should be studied to improve the PSD performance at low gamma-equivalent energy regions. Convolutional Neural Networks (CNNs) have excellent image recognition capabilities, and thus, neutron and gamma-ray waveforms can be discriminated by their characteristics through a known training set. In this study, neutron and gamma-ray waveforms were measured with a 137Cs source and moderated 252Cf source via an SiPM array-coupled CLYC detector and divided into two groups: training and PSD testing. The CNN training set comprised 137Cs characteristic gamma-ray waveforms and thermal neutron waveforms that were discriminated by the charge comparison method from the training group. A CNN with two convolution-pooling layers was designed to accomplish PSD with the test group. The PSD FoM value of the CNN method was calculated to be 37.20 at the gamma-equivalent energy region of more than 325 keV. This result was much higher than that of the charge comparison method, indicating that neutrons and gamma rays could be better distinguished with the CNN method, especially at low gamma-equivalent energy regions.

Diffusion of 5p-holes in BaF2 nanoparticles

The kinetic parameters of core-valence luminescence for BaF2 nanoparticles with different sizes were studied under synchrotron radiation excitation with photon energy of 18.8 eV at room temperature. In contrast to BaF2 single crystals the decay kinetics of nanoparticles is shown to be non-exponential indicating the luminescence quenching. We assume that the quenching of core-valence luminescence is caused by the diffusion of core holes to nanoparticle surface. The diffusion length of the core holes in BaF2 has been estimated as 2–3 nm using diffusion model of luminescence quenching in nanoparticles.

Growth and characterization of Cs2LiLaCl6:Ce single crystals

Cs2LiLaCl6:Ce (CLLC) is a promising scintillator material for dual gamma/neutron detection. The vertical Bridgman technique, with a LiCl-rich melt and high axial temperature gradient, was used to grow transparent CLLC crystals for the first time. The stoichiometric CLLC powder melted incongruently with a melting range of 343–442 °C. Additional LiCl suppressed the formation of Cs3LaCl6 second phase, which was identified in crystals grown from stoichiometric melts. CLLC was found to have a Stokes shift of about 1358 cm−1 and a photoluminescence decay time of 34.6 ns. Direct electron-hole capture by Ce3+ ions and core-valence luminescence were the main contributors to the X-ray induced luminescence spectrum. A ∅9 × 8 mm3 cylinder of the CLLC crystal with 0.5at% Ce had an energy resolution of 7.1% at 662 keV. The scintillation decay times of the CLLC crystal excited with 662 keV gammas were 79 ns and 1491 ns.

First-Principles Study of Electronic Structure of Cubic Perovskite-Like Fluorides

The electronic density of states of cubic perovskite-like fluorides has been evaluated by means of the projector augmented waves (PAW) within the local density approximation (LDA). For the first time the improved electronic energy bands have been calculated using the GW approximation. Ground-state and quasiparticle calculations have been done with the ABINIT code. Combined analysis of the densities of electronic states and obtained electron energy spectra makes it possible to draw conclusions regarding the possibility of the phenomenon of core-valence luminescence in the considered crystals.

Pulse shape discrimination of Cs2LiYCl6:Ce3+ scintillator from −30 °C to 180 °C

In this study, the scintillation performance of Ce doped Cs2LiYCl6 (CLYC) from −30°C to 180°C is presented. Pulse shape discrimination (PSD) of CLYC at various temperatures is investigated. Previously, it had been shown that PSD based on Core-Valence-Luminescence fades away at temperatures >120°C. However, it has been discovered that there is a secondary mechanism which can be utilized for neutron-gamma Pulse Shape Discrimination in addition to the well-known Core-Valence-Luminescence (CVL). In contrast to CVL, this mechanism is found to be insensitive to temperature change. Temperature dependent PSD algorithms are developed to enable reliable neutron-gamma PSD up to 180°C.

Optical and luminescence characteristics of BaF2 and BaF2:Tm in vacuum UV and UV spectral regions

The spectral characteristics of BaF2 and BaF2:Tm crystals and ceramics are compared. The vacuum UV (VUV) and UV absorption spectra and excitation spectra of core-valence and exciton luminescence are investigated. The main exciton and electron-hole processes responsible for the structure of recorded spectra are considered.

Self-trapped exciton and core-valence luminescence in BaF2 nanoparticles

The influence of the BaF2 nanoparticle size on the intensity of the self-trapped exciton luminescence and the radiative core-valence transitions is studied by the luminescence spectroscopy methods using synchrotron radiation. The decrease of the self-trapped exciton emission intensity at energies of exciting photons in the range of optical exciton creation (hν ≤ Eg) is less sensitive to the reduction of the nanoparticle sizes than in the case of band-to-band excitation, where excitons are formed by the recombination way. The intensity of the core-valence luminescence shows considerably weaker dependence on the nanoparticle sizes in comparison with the intensity of self-trapped exciton luminescence. The revealed regularities are explained by considering the relationship between nanoparticle size and photoelectron or photohole thermalization length as well as the size of electronic excitations.

Fast inorganic scintillators – status and outlook

Scintillation detectors based on inorganic materials represent one of the most widely applied instrumentation techniques in the fields of nuclear and high-energy physics as well as medical or industrial applications. Driven by experimental requirements the research has focused onto a faster response, shorter decay times and higher compactness implementing high-Z ions. The discovery of the fast core-valence luminescence in BaF2, the allowed electric dipole transitions in Ce3+, and the short radiation lengths of BGO and PbWO4 have set important milestones. However, excellent time resolution is affected as well by the density of photoelectrons produced at the early stage of the signal generation and the integral light output. The paper will give a detailed overview of the present status on fast inorganic scintillators, their performance and the theoretical limitations on achievable energy and time resolutions. The results are illustrated by various applications in research. The state of the art scintillator material will be discussed based on new materials such as LaBr3 or LaCl3, which were doped with rare earth ions such as Ce3+-centers to reach decay times between 20 and 40ns, respectively.

Further performance tests of a picosecond X-ray and laser induced streak camera system with fast scintillation materials

A further performance test of a fluorescence lifetime spectroscopy system with 2D-photon counting based on a streak camera was carried out. This system offers three different excitation light sources. It includes a pulsable X-ray tube with 40 kV and a laser. A pulsed UV-LED has been installed for sample excitation. The table top measurement device has a temporal resolution down to 20 ps after deconvolution and fluorescence/scintillation decay times can be evaluated in a wavelength selectable range between 200 and 850 nm. Cesium fluoride, cesium chloride and cesium bromide as fast core-valence luminescence emitters with a relatively low light yield were used in the presented performance test. The easily manageable system is able to detect low light levels and to determine the decay times in agreement with literature in most instances. Beside mentioned scintillators PbCO3, SrI2 and Lu2SiO5:Ce as a high light yield material were also measured.

Electronic energy structure and core-valence luminescence of CsCl crystal

Electronic energy structure and core-valence luminescence of CsCl crystal

VUV Luminescence With Nd Doped KCaF$_{3}$ Under X-Ray Excitation

Radioluminescence properties of Nd doped KCaF3 single crystals were studied. X-ray excited luminescence spectra were measured in the vacuum ultraviolet (VUV) region by using a custom-made device filled with nitrogen atmosphere. As a result, VUV luminescence was observed at 190 nm. Radioluminescence intensity due to the core-valence luminescence (CVL) of KCaF3 decreased with increasing of Nd3+ concentration.

Nd concentration dependence on the optical and scintillation properties of Nd doped BaF 2

Nd concentration dependence on the scintillation properties of Nd x Ba 1− x F 2+ x ( x = 0.001–0.5) crystals were investigated. From X-ray induced emission spectra, the intensity from an intrinsic luminescence of BaF 2 including core-valence luminescence and self-trapped-exciton (STE) emission were decreased with an increase of Nd concentration. Decay times under 241Am α-ray excitation were evaluated by the Photomultiplier Tube (PMT) Hamamatsu R8778 that was sensitive across the vacuum ultra-violet (VUV) and visible wavelength range. The average decay times indicated that Nd doped BaF 2 with lower Nd concentration has obviously slower decay times than that of samples with higher Nd concentration because the decay time of STE emission was slower than that of the core–valence luminescence in BaF 2 and the emission of 5d–4f transition on Nd 3+. The light yield of fast Nd 3+ 5d–4f luminescence was quite lower than STE emission of BaF 2.

Inorganic Scintillators—A Never Ending Story

For decades, scintillation detectors based on inorganic materials have become one of the most widely applied instrumentation techniques in physics—in particular in the fields of nuclear and high-energy physics. Their discovery and development are strongly correlated with the experimental needs in basic research and technology in physics. Visual counting of the discovered X-rays or natural radioactivity became possible with CaWO4 or ZnS at the end of the 19th century. The first construction of scintillation detectors, made possible by the development of the photomultiplier tube, started with the discovery of activated and pure alkali halide crystals. NaI(Tl) and CsI(Tl), introduced by Hofstadter [1, 2], have provided for 60 years efficient photon and particle detection. The continuous increase of the energy range of the probes to be detected directed crystal development toward faster response, shorter decay times, and higher compactness implementing high-Z ions. In particular, the discovery of the fast core-valence luminescence in BaF2 [3, 4], the allowed electric dipole transitions in Ce3+, and the short radiation lengths of BGO [5] or PbWO4 [6] set important milestones in the last decades of the 20th century to find or even engineer the ideal scintillator.

Study of the electronic structures of oxygen doped in LiBaF 3 crystal

The most likely substituting positions of impurity oxygen ions in LiBaF 3 crystals are studied using the general utility lattice program (GULP). The calculated results indicate that the main defect model is [ O F ′ + V F · ] in the O:LiBaF 3 crystal. The electronic structures of the LiBaF 3 crystal with the defect [ O F ′ + V F · ] are calculated using the DV-X α method. It can be concluded from the electronic structures that the LiBaF 3 crystal with the defect [ O F ′ + V F · ] will exhibit a 217–280 nm absorption band and the impurity oxygen will decrease core-valence luminescence yield.

Features of the core–valence luminescence and electron energy band structure ofA1−xCsxCaCl3 (A = K,Rb) crystals

From luminescence spectroscopy ofCsCaCl3,Rb1−xCsxCaCl3 and K1−xCsxCaCl3 crystals, we have found evidence for intrinsic and impurity core–valence luminescence dueto the radiative recombination of valence electrons with the holes of intrinsic or impurity5p Cs+ core states. The structural similarity of core–valence luminescence spectra has been revealed for theA1−xCsxCaCl3 (A = K,Rb) crystals investigated. The electron energy structure of theCsCaCl3 crystal has been calculated using the pseudopotential approach taking into account the gradientcorrections for the exchange–correlation energy. The calculated density of the electronic states ofCsCaCl3 has been compared with corresponding parameters obtained from the analysis ofcore–valence luminescence spectra.

Electronic energy band parameters of CsCl evaluated on core Bloch states and plane waves

Electronic energy bands of CsCl crystal have been calculated within the mixed basis approach with using the core Bloch states and plane waves. The calculated energy parameters of the crystal are in the satisfactory agreement with the experimental data obtained from the analysis of the core–valence luminescence spectra. The obtained results form a base for calculation of CVL spectra parameters.

Electronic energy structure and core-valence luminescence of ABX3 (A = K, Rb, Cs; B = Ca; x = F) crystals

Electronic energy structure and core-valence luminescence of <i>ABX</i>₃ (<i>A</i> = K, Rb, Cs; <i>B</i> = Ca; <i>x</i> = F) crystals

Optical and scintillation properties of Cs/sub 2/LiYCl/sub 6/:Ce/sup 3+/ and Cs/sub 2/LiYCl/sub 6/:Pr/sup 3+/ crystals

In this paper we report on the optical and scintillation properties of Cs/sub 2/LiYCl/sub 6/:Ce/sup 3+/ and Cs/sub 2/LiYCl/sub 6/:Pr/sup 3+/ crystals. Crystals of Cs/sub 2/LiYCl/sub 6/ doped with different Ce/sup 3+/ and Pr/sup 3+/ concentrations were grown and studied under optical, X-ray, gamma ray, and thermal neutron excitation. These scintillators exhibit core-valence and self-trapped exciton luminescence in addition to fast emission from Ce/sup 3+/ and Pr/sup 3+/. Efficient thermal neutron detection was observed for both scintillators. The position of the neutron peak in the pulse height spectra was found at about 5 times the channel number of the position of the /sup 137/Cs 662 keV gamma peak.

Scintillation and anomalous emission in elpasolite Cs 2LiLuCl 6:Ce 3+

The luminescence and scintillation properties of Cs 2LiLuCl 6:0.5%Ce 3+ are presented. Special attention is devoted to a 9.4 ns fast emission at 275 nm that can only be excited via the highest cubic field 5d e state of Ce. Contrary to Cs 3LuCl 6 and Cs 2LiYCl 6, where the same type of fast emission was observed, the emission in Cs 2LiLuCl 6 is still observed at room temperature. Assuming that the 5d e state is located inside the host conduction band (CB), we propose that the emission originates from a mixed state at or just below the bottom of the CB and ends at the 4f ground state of Ce 3+. To proof this model we studied the thermal quenching of the anomalous luminescence and performed X-ray photoelectron spectroscopy. A model for a temperature-activated energy transfer from the anomalous state to the lowest 5d t excited state of Ce 3+ explains most of the results. Besides the 275 nm emission, the material shows 5d t–4f Ce 3+ emission at 370 and 406 nm and 2 ns fast core-valence luminescence when excited with 16–22 eV photons. The scintillation properties of Cs 2LiLuCl 6:Ce are briefly discussed.

Xeol studies of impurity core-valence luminescence in mixed rubidium caesium chloride crystals

The impurity core valence luminescence (CVL) of Rb0.82Cs0.18Cl mixed crystals was monitored at the Cl K- and Rb L3-absorption edges using X-ray excited optical luminescence (XEOL) technique. It is found that XEOL spectrum at the chlorine K-edge is identical with the Cl K-absorption spectrum of pure RbCl. It is demonstrated that investigation of the XEOL yield spectra of impurity CVL can deliver essential site sensitive information about the structure of the CVL emission centre.