arXiv subjects

M. Nikl

Publications and source records attributed to M. Nikl.

15 recordsLinked to original sources

arXiv2024

Exploring Scintillators and Cherenkov Radiators for MIP Timing Detectors

This article presents the timing performance of materials with fast light emission, tested as Minimum Ionizing Particle detectors using 150 GeV hadron beams in Monte Carlo simulations and at the CERN SPS North Area. Pixels of cross-section 2 x 2 mm2 or 3 x 3 mm2 and length of 3 or 10 mm were coupled to Hamamatsu SiPM and read out by fast high-frequency electronics. Materials whose timing performance relies on Cherenkov emission, namely BGSO, PWO, and PbF2, achieved time resolutions in the range 24-36ps. Scintillators as L(Y)SO:Ce, GAGG, and BaF2 reached below 15 ps, the best topping at 12.1 +/- 0.4 ps. These fast materials are compared to LYSO and their additional benefit is discussed. Given the promising results of BaF2, the study is completed with measurements of the scintillation properties of a set doped with yttrium to quench the slow light emission.

physics.ins-det↗

arXiv2022

Incorporation of the Ce3+ activator ions in LaAlO3 crystals: EPR and NMR study

This work reports the results of EPR and NMR study of the Ce3+ incorporation in LaAlO3 single crystals grown by the micro-pulling-down method in the range of the Ce concentrations from x=0.1 at.% up to 100 at.%. From EPR measurements, Ce3+ g tensor parameters were obtained as function of Ce concentration. The g tensor has orthorhombic symmetry even in the trigonal phase (x < 10 at.%) suggesting that the incorporation of Ce at La site lowers lattice symmetry near this ion. The local properties of the La1-xCexAlO3 crystals were further studied by 27Al and 139La high-resolution NMR measurements. It was found that 139La chemical shift has the Fermi contact interaction origin. It linearly increases with Ce concentration up to 165 ppm at x = 0.5. Due to this strong Fermi contact interaction, separated peaks corresponding to different Ce-O-La spin transfer passways are resolved in the 139La NMR spectra. On the other hand, no Fermi contact interaction is visible in 27Al NMR spectra. However, these spectra contain satellite peak which intensity linearly increases with increase of Ce concentration leaving position of this peak unchanged. This was interpreted as manifestation of crystal structure modification in vicinity of Ce ions in agreement with EPR data. Thus, optical properties of Ce3+ in LaAlO3 will be namely determined by the local crystal structure near this ion.

cond-mat.mtrl-sci↗

arXiv2021

Effect of W and Mo co-doping on the photo- and thermally stimulated luminescence and defects creation processes in Gd3(Ga,Al)5O12:Ce crystals

Photo- and thermally stimulated luminescence characteristics of Gd3(Ga,Al)5O12:Ce single crystals co-doped with W and Mo are investigated in the 85 - 510 K temperature range and compared with the corresponding characteristics of the undoped and Ce3+ - doped Gd3(Ga,Al)5O12 single crystals of similar composition. A strong effect of the W and Mo impurity ions appears in the photoluminescence spectra and temperature dependences of the photoluminescence intensity, afterglow intensity and decay kinetics, thermally stimulated luminescence (TSL) intensity and TSL glow curves, excitation spectra of the TSL glow curve peaks and activation energy of their creation. The obtained results are explained by the enhancement of the intrinsic emission contribution into the luminescence spectrum of Gd3(Ga,Al)5O12:Ce,W and Gd3(Ga,Al)5O12:Ce,Mo due to a large concentration of various W - and Mo - related electron traps in these crystals and, consequently, the O- - type hole centers, as well as intrinsic crystal lattice defects (e.g., cation vacancies needed for the excess positive charge compensation of the W6+ and Mo6+ ions). The influence of the co-doping with W and Mo ions on the scintillation characteristics of Gd3(Ga,Al)5O12:Ce is discussed.

cond-mat.mtrl-sci↗

arXiv2020

Undoped and Eu, Na co-doped LiCaAlF6 scintillation crystals: paramagnetic centers, charge trapping and energy transfer properties

Single crystals of LiCaAlF6 undoped and Eu, Na co-doped were studied by electron paramagnetic resonance, radioluminescence and thermally stimulated luminescence techniques applied in a correlated manner. The undoped samples exposed to X-ray irradiation exhibited two hole-like charge trapping centers creation, the molecular ions of the form: ClF- and F2^- - F2^- dimer. Their trap depths and frequency factors were determined as follows: Et1=1.7 eV and Et2=1.1. eV for trap depths and f ~ 10^13 s-1 for frequency factor, respectively. It was found that the europium preferable charge state is 2+ in the LiCaAlF6:Eu,Na samples, however, some amount of the Eu3+ is also present. Moreover, there were two Eu2+ centers: the dominating Eu2+(Ca) and the low-content Eu2+(Li). The amount of the latter is easily governed by the sodium admixture while the former is insensitive to the Na co-doping. Eu and Na co-doping affected the defects distribution and incorporation in the LiCaAlF6 host.

cond-mat.mtrl-sci↗

arXiv2020

Rare-earth ions incorporation into Lu2Si2O7 scintillator crystals: Electron paramagnetic resonance and luminescence study

The present work reports results of the electron paramagnetic resonance (EPR), optical absorption, radio- and photoluminescence (RL and PL) complex study of the Lu2Si2O7:Pr and Lu2Si2O7:Ce pyrosilicate crystals. In both crystals, the EPR spectra demonstrate the presence of characteristic signals originating from the Yb3+, Er3+, Nd3+, Dy3+, Gd3+ and V3+ ions existing in the material as uncontrolled impurities. The corresponding spectra (except for Gd3+) have been analysed in detail and g- and hyperfine tensors are determined for all these ions in the lutetium pyrosilicate for the first time. Optical absorption, RL and PL measurements in the Lu2Si2O7:Pr crystal demonstrated presence of only characteristic Pr3+ transitions. In addition, the Pr3+- Pr3+ energy transfer was observed and confirmed experimentally.

cond-mat.mtrl-sci↗

arXiv2019

Oxygen-vacancy centers in Y3Al5O12 garnet crystals: electron paramagnetic resonance and dielectric spectroscopy study

F+ center, an electron trapped at oxygen vacancy (VO), was investigated in the oxygen deficient Y3Al5O12 (YAG) crystals by EPR. The measurements were performed at temperatures 5-450 K and frequencies 9.4-350 GHz with using both the continue wave and pulse EPR technique. The pulse electron-nuclear double resonance was applied to resolve the hyperfine interaction of the trapped electron with surrounding nuclei. The measurements show that at low temperatures, T < 50 K, EPR spectrum of the F+ center is anisotropic with g factors in the range 1.999-1.988 and originates from three magnetically inequivalent positions of the center in garnet lattice according to different directions of the Al(IV)-VO-Al(VI) chains, where Al(IV) and Al(VI) are the tetrahedral and octahedral Al sites, respectively. As the temperature increases, the EPR spectrum becomes isotropic suggesting a motional averaging of the anisotropy due to motion of electron between neighboring oxygen vacancies. With further increase of the temperature to T > 200 K, we observed delocalization of the electron into the conduction band with the activation energy about 0.4-0.5 eV that resulted in substantial narrowing of the EPR spectral line with simultaneous change of its shape from the Gaussian to Lorentzian due to diminish up to zero of the Fermi contact hyperfine field at 27Al and 89Y nuclei. Such temperature behavior of the F+-center electron in YAG is completely similar to behavior of a donor electron in a semiconductor. Our findings is further supported by measurements of the conductivity and dielectric properties. In particular, these data show that the conduction electrons are not homogeneously distributed in the crystal: there are high-conductive regions separated by poorly-conductive dielectric layers. This leads to the so-called Maxwell-Wagner dielectric relaxation with huge apparent dielectric constant at low frequencies.

cond-mat.mtrl-sci↗

arXiv2018

Vanadium in yttrium aluminum garnet: charge states and localization in the lattice

Vanadium ions charge states and their incorporation in the yttrium aluminum garnet Y3Al5O12 (YAG) lattice were studied by the correlated optical absorption and electron paramagnetic resonance (EPR) measurements. In as-grown crystals, the occupation of the V3+ at both the octahedral and tetrahedral aluminum sites was proven. The V3+ to V4+ charge transformation was observed after annealing in air, whereas annealing in the hydrogen atmosphere resulted exclusively in a slight weakening of the V3+ absorption bands due to partial recharge of these ions. Spin Hamiltonian parameters of the V3+ and V4+ ions at the tetrahedral sites including the zero field splitting and the 51V hyperfine constants have been determined using the high-frequency, up to 300 GHz, EPR measurements. From the analysis of the spin Hamiltonian parameters in the framework of the crystal field theory, the ground state energy levels splitting of the V3+ and V4+ ions were calculated. The charge distribution over the tetrahedral V3+ and its nearest oxygen surroundings was found to be strongly inhomogeneous whereas the tetrahedral V4+ ion concentrated the charge with very weak participation of surrounding ligands. Furthermore, the correlation of the optical and EPR data allowed the proper assignment of the optical absorption peaks in YAG:V crystals.

cond-mat.mtrl-sci↗

arXiv2018

Aluminum and Gallium Distribution in the Lu3(Al5-xGax)O12:Ce Multicomponent Garnet Scintillators Investigated by the Solid-State NMR and DFT calculations

Distribution of aluminum and gallium atoms over the tetrahedral and octahedral sites in the garnet structure was studied in the mixed Lu3Al5-xGaxO12 crystals using the 27Al and 71Ga MAS NMR together with the single crystal 71Ga NMR. The experimental study was accompanied by theoretical calculations based on the density functional theory in order to predict the tendency in substitutions of Al by Ga in the mixed garnets. Both experimental and theoretic results show a non-uniform distribution of Al and Ga over the tetrahedral and octahedral sites in the garnet structure, with strong preferences for Ga, having larger ionic radius than Al, to occupy the tetrahedral site with smaller volume in the garnet structure. The quadrupole coupling constants and chemical shift parameters for Al and Ga nuclei have been determined for all the studied compounds as well as electric field gradients at Al and Ga nuclei were calculated in the framework of the density functional theory.

cond-mat.mtrl-sci↗

arXiv2017

Influence of gallium content on Ga3+ position and photo- and thermally stimulated luminescence in Ce3+ - doped multicomponent (Y,Lu,)3GaxAl5-xO12 garnets

Photoluminescence, thermally stimulated luminescence (TSL) and EPR characteristics of the Ce3+ doped single crystals of multicomponent Y1Lu2GaxAl5-xO12 and Lu3GaxAl5-xO12 garnets with different Ga contents (x = 0, 1, 2, 3, 4, 5) excited in the Ce3+ - related absorption bands are investigated in the 9 - 500 K temperature range. The distribution of Ga3+ and Al3+ ions in the crystal lattice is determined by the NMR method. The relative number of Ga3+ ions in the tetrahedral crystal lattice sites, the maxima positions of the TSL glow curve peaks and the corresponding trap depths are found to decrease linearly with the increasing Ga content. At the same time, the reduction of the activation energy Ea of the TSL glow curve peaks creation under irradiation in the 4f - 4d1 absorption band of Ce3+ is strongly nonlinear. To explain this effect, the suggestion is made that Ea is the energy distance between the excited 5d1 level of Ce3+ and a defect level located between the 5d1 level and the bottom of the conduction band and arising from the Ga3+ ion perturbed by the nearest neighboring Ce3+ ion. The electrons thermally released from the excited Ce3+ ions are suggested to be trapped at the perturbed Ga3+ ions resulting in the appearance of electron Ga2+ centers. In spite of the fact that the paramagnetic Ga2+ ions were not detected by EPR, the described above process was found for Fe3+ impurity ions, namely the electron transfer from the 5d1 excited levels of Ce3+ to Fe3+ was directly detected by EPR.

cond-mat.mtrl-sci↗

arXiv2017

Hole self-trapping in the Y3Al5O12 and Lu3Al5O12 garnet crystals

The processes of hole localization in the Y3Al5O12 and Lu3Al5O12 single crystals were investigated by electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL). It was found that holes created by x-ray irradiation at 77 K are predominantly self-trapped at regular oxygen ions forming O- hole center. This self-trapped hole (STH) center is thermally stable to about 100 K in both YAG and LuAG crystals. At higher temperatures, thermally liberated holes are retrapped at oxygen ions in the vicinity of an acceptor ion such as Mg2+ and Al_{Y} or Al_{Lu} antisite ion that leads to increase of the thermal stability of the trapped hole to app. 150 K. TSL measurements show two composite glow peaks in the temperature range of 77 - 280 K, the temperature positions of which well correlate with the thermal stability of the O- centers. The hole thermal ionization energy was determined from a numerical fit of the TSL peaks within the model of second order kinetics. It is in the range of 0.25 - 0.26 eV for the O- STH center, and increases to 0.41 - 0.45 eV for O- center stabilized by the acceptor. Revealed O- centers can be attributed to O- small polarons formed mainly due to the hole stabilization by short-range interaction with the surrounding lattice.

cond-mat.mtrl-sci↗

arXiv2012

Deep trapping states in Cerium doped (Lu,Y,Gd)3(Ga,Al)5O12 single crystal scintillators

We study deep trapping states in Ce3+-doped garnet crystals with the composition (Lu,Y,Gd)3(Ga,Al)5O12, recently shown as having remarkably high light yield. We use thermally stimulated luminescence (TSL) technique above room temperature and determine the composition Gd3Ga3Al2O12 as the host showing the lowest concentration of traps. This host consistently manifest very low afterglow comparable to that of the standard BGO crystal. We also perform TSL glow peak analysis based on the initial rise technique to evaluate trap depth and other characteristics associated with TSL peaks.

cond-mat.mtrl-sci↗

arXiv2011

ESR and TSL study of hole capture in PbWO_4:Mo,La and PbWO_4:Mo,Y scintillator crystals

The processes of hole localization in double-doped PbWO_4:Mo,La and PbWO_4:Mo,Y single crystals have been studied by continuous wave and pulse electron spin resonance (ESR) and thermally stimulated luminescence (TSL) methods. We show that the holes created by the UV irradiation are preferably trapped at lattice oxygen ions in the vicinity of perturbing defects such as lead vacancies, impurity ions (La, Y, Mo), and other lattice imperfections. This leads to a variety of O^- centers, which differ both by thermal stability (from about 170 K up to 240 K) and ESR parameters. The hole centers of this type were not observed neither in PbWO_4:Mo nor in PbWO_4:La(Y) crystals. The recombination processes of thermally released holes with electrons stored at different traps, including Pb^+ - WO_3 and (MoO_4)^3- centers, are systematically studied by TSL. Thermal stability parameters are defined by ESR and TSL techniques for different O^- type defects.

cond-mat.mtrl-sci↗

arXiv2011

Time-resolved spectroscopy of exciton states in single crystals, single crystalline films and powders of YAlO_3 and YAlO_3:Ce

Luminescence characteristics of single crystals (SC), single crystalline films (SCF), powders and ceramics of YAlO_3 and YAlO_3:Ce have been studied at 4.2-300 K under photoexcitation in the 4-20 eV energy range and X-ray excitation. The origin and structure of defects responsible for various exciton-related emission and excitation bands have been identified. The ~5.6 eV emission of YAlO_3 SCF is ascribed to the self-trapped excitons. In YAlO_3 SC, the dominating 5.63 eV and 4.12 eV emissions are ascribed to the excitons localized at the isolated antisite defect Y^{3+}_Al and at the Y^{3+}_{Al} defect associated with the nearest-neighbouring oxygen vacancy, respectively. Thermally stimulated release of electrons, trapped at these defects, takes place at 200 K and 280 K, respectively. The formation energies of various Y^{3+}_{Al}-related defects are calculated. The presence of Y_{Al} antisite-related defects is confirmed by NMR measurements. The influence of various intrinsic and impurity defects on the luminescence characteristics of Ce^{3+} centers is clarified.

cond-mat.mtrl-sci↗

arXiv2009

Electron Spin Resonance investigation of undoped and Li-doped CdWO_4 scintillator crystals

Electron spin resonance (ESR) spectra of Fe3+ and Mn2+ ions have been studied in the nominally pure and 0.05% Li-doped single crystals of CdWO4. The zero-field splitting parameters are determined with a high precision for both of the impurities. The result suggest that the Li-doping leads to the increase of the ionic charge of iron from 3+ to 4+ and of manganese, from 1+ to 2+.

cond-mat.mtrl-sci↗

arXiv2001

Slow relaxation, confinement, and solitons

Millisecond crystal relaxation has been used to explain anomalous decay in doped alkali halides. We attribute this slowness to Fermi-Pasta-Ulam solitons. Our model exhibits confinement of mechanical energy released by excitation. Extending the model to long times is justified by its relation to solitons, excitations previously proposed to occur in alkali halides. Soliton damping and observation are also discussed.

cond-mat.mtrl-sci↗