Isotropic G-value Research Articles

γ-radiation-induced centers in irradiated perovskite SrCeO3 and their role in thermally stimulated reactions: Fluorescence, thermally stimulated luminescence, electron paramagnetic resonance and shielding studies

One promising host for actinide radioactive wastes is orthorhombic perovskite-type oxide. Perovskite SrCeO3 ceramic was synthesized by nitrate-fuel combustion, which includes the organic fuel glycine. Powder X-ray diffraction was utilized to determine the structural features, and γ-radiation-induced changes and shielding properties were investigated in perovskite SrCeO3. In a mixed-phase sample, a bright sky-blue luminescence was observed, and two thermoluminescence (TL) peaks were seen in irradiated SrCeO3 ceramic. Electron paramagnetic resonance (EPR) spectrum in γ-irradiated SrCeO3 ceramic had contributions from five defect centers. Center I having an isotropic g-value equal to 2.0283, is ascribed to an O− ion, while center II with an axial g-tensor with principal values g|| = 2.0224 and g⊥ = 2.0068 is determined as an O2− ion. O− ion relates to the TL peak at 215 °C. Center III with a g-value equal to 2.0009 is identified as an F+ center and is related to the 185 °C TL peak. The defect center associated with center IV is also identified as an F+ center. An additional defect center in the higher field region of the spectrum is assigned to an F+ center, and the center results from an F-center (oxygen vacancy with two electrons). The mass attenuation coefficients, effective atomic numbers, and half-value layer thicknesses concerning shielding property effectiveness were computed and it was found that the perovskite SrCeO3 ceramic provided superior γ-shielding properties.

Combined TL and EPR study to elucidate the origin of luminescent emission from La-doped CaF2 phosphor

This paper presents the results of the combined study of the thermoluminescence (TL) and electron paramagnetic resonance (EPR) properties of La-doped CaF2 (CaF2: La) phosphor. X-ray diffraction (XRD) results confirmed that CaF2 has been successfully synthesized. Scanning electron microscopy (SEM) results and energy dispersive spectrometer (EDS) spectra show that the synthesized phosphor is pure and has a good overall morphology. The glow curve of CaF2: La phosphor shows an emission band between 230 and 500 nm centered at 340 nm. The Tm-Tstop method and the deconvolution technique were used to find the number of overlapping TL peaks in the 50–300 °C region and their kinetic parameters. A linear dose-response curve was obtained for the range 1–9 Gy, with the onset of a slight supralinear behavior for doses higher than 9 Gy. Gamma-irradiated CaF2: La phosphor has been studied using the technique of EPR to identify the defect centers and also to infer the centers involved in the TL process. Thermal annealing studies indicate the presence of three defect centers. One of the centers (center I) is identified as the O2− ion and is characterized by an axial g-tensor with principal values g∥ = 2.013 and g⊥ = 2.0053. This center correlates with the TL peak at 200 °C. Center II with an isotropic g-value of 2.0005 is attributed to an F center and also relates to the 200 °C TL peak. The third center (center III) is assigned to an F center.

Study of the centers responsible for the TL emission by EPR and PL analysis of Eu-doped CaSiO3 phosphors synthesized by the devitrification method

CaSiO3 doped at different ppm of Eu were synthesized by the devitrification method. Polycrystals exhibit a prominent high-temperature TL peak that increases in intensity and shifts toward higher temperatures with increasing amounts of dopant. TL emission spectrum of CaSiO3: Eu displays two very broad bands centered at 400 and 440 nm corresponding to the temperatures of 229.5 and 373.5 °C, respectively. PL measurements indicate the presence of Eu2+ and Eu3+ ions. EPR spectrum arises from two defect centers. One of the centers with principal g-values 2.016, 2.0091, and 2.0051 is attributed to O− ion and the center correlates with the TL peak at 236 °C. Center II with an isotropic g-value of 2.0018 is attributed to an F+ center and the center relates to the TL peak at 120 °C. Center III is also tentatively identified as the F+ center which is associated with the TL peak at 365 °C.

Identification of defect centers responsible for thermoluminescence emission in sol-gel synthesized calcium silicate phosphor

The thermoluminescence (TL) of calcium silicate phosphor (CSO) prepared by the sol-gel method and sintered at 1200 °C were investigated. From Tm-Tstop curve, TL emission spectrum and computer deconvolution using electron traps with discrete and continuous distributions, the glow curves were found to be composed of four TL peaks (117, 190, 250 and 275 °C) with a single emission band centered at 370 nm. Electron paramagnetic resonance (EPR) investigation has been carried out to identify the defect centers formed in the CSO phosphor by γ-irradiation and find the centers related to the TL process in the phosphor. At room temperature, three defect centers were observed. The first center, characterized by the principal g-values of 2.014, 2.011, and 2.0080 was assigned to an O− ion. The second center with g-values 2.015, 2.013, and 2.010 is also attributed to an O− ion and is associated with the TL peak at 280 °C. The third center, with an isotropic g-value of 2.0011 was identified as the F+ center (singly ionized oxygen vacancy) and relates to the TL peak at 280 °C.

TL, OSL, EPR, and defect centers in Tb-doped strontium silicate phosphor synthesized by a solid-state reaction method

Strong thermoluminescence (TL) was observed in Tb-doped strontium silicate phosphor prepared by the solid-state reaction method. Two glow peaks are observed at 100 and 190 °C. TL emission spectra indicate three recombination centers. The number of peaks and the kinetic parameters associated with the TL glow peaks of Tb-doped Sr2SiO4 phosphor after gamma irradiation were analyzed by Tm-Tstop and deconvolution methods. A study of defect centers induced by γ-irradiation and identification of defect centers responsible for the observed TL glow peaks has been carried out using the electron paramagnetic resonance (EPR) technique. The observed spectrum at room temperature is a superposition of at least three defect centers. Center I is characterized by an axially symmetric g-tensor with principal values g|| = 2.019, and g⊥ = 2.0013 and is ascribed to a O2− ion. Center II has a rhombic g-tensor with principal values 2.0072, 2.0024, and 1.995 and is assigned to an F+ center (singly ionized oxygen vacancy) and correlates with the 190 °C TL peak. The third center (center III) has an isotropic g-value of 1.991 and is identified as an F+ center.

Structural characterization, continuous-wave optically stimulated luminescence, and correlation between thermoluminescence and EPR of Ce-doped Ca2Al2SiO7 phosphor synthesized by a solid-state reaction method

A calcium aluminum silicate (Ca2Al2SiO7) phosphor doped with different concentrations of cerium has been prepared by a solid-state reaction method. From X-ray diffraction (XRD) data and using the Rietveld refinement method, the phase, structure, and average crystalline size have been determined. Luminescence emissions have been investigated by optically stimulated luminescence (OSL) and thermoluminescence (TL) methods. TL studies reveal a broad peak centered at 220 °C with an emission band at 410 nm. The TL intensity of a sample doped with 0.1 mol% Ce proved to be 115 times stronger than that of an undoped sample. Electron paramagnetic resonance (EPR) has been utilized to detect the defect centers responsible for the TL process in Ce-doped Ca2Al2SiO7 phosphor. The observed EPR spectrum results from a superposition of three defect centers. Center I with a g-value of 2.0090 is assigned to an O− ion and exhibits hyperfine interaction with a nearby nucleus with spin 3/2. This center relates to the TL peak at 220 °C. Center II with an isotropic g-value of 2.0073 is also ascribed to an O− ion and correlates with the dominant 220 °C TL peak. Center III has a g-value of 2.0076 and is identified as an F+ center (singly-ionized oxygen vacancy).

Thermoluminescence and electron paramagnetic resonance correlation studies in lithium silicate phosphor

Lithium silicate phosphor, synthesized by the solid-state reaction method, displays three thermoluminescence (TL) peaks at 155 °C, 240 °C and 430 °C. Activation energy, frequency factor and kinetic order associated with the TL peaks have been determined using E-Tstop and glow curve deconvolution (GCD) methods. Electron Paramagnetic Resonance (EPR) studies have been carried out to identify the defect centers induced in the phosphor by gamma irradiation and also to find the centers responsible for the TL process in the system. The observed EPR spectrum arises from a superposition of three defect centers. One of the centers (center I) with a g-value 2.0097 is identified as the O− ion. Center II with an isotropic g-value 2.0002 is assigned to a F+-type center (singly ionized oxygen vacancy). The F+ center is likely to be associated with the TL peak at 156 °C. Center III characterized by a rhombic g-tensor with g1 = 1.9831, g2 = 1.9676, and g3 = 1.9208 is identified as a Ti3+ center. The Ti3+ center relates to the low temperature TL peak at 155 °C.

X-BAND EPR STUDIES OF GAMMA IRRADIATED A NEW ISOQUINOLINE SULFONAMIDE: C17H20BrNO3S

A new epoxyisoquinoline sulfonamide as a fused heterocycle compound (I), (4aS,7S,8aR)-8a-bromo-7-methyl-2-tosyl-2,3,4,7,8,8a-hexahydro-1H-4a,7-epoxyisoquinoline was synthesized using green and cascade progress which was then irradiated by 60Co-gamma () ray source with a dose speed of 0.981 kGy/h at the room temperature for 72h. EPR spectra of I compound were measured at different orientations in magnetic field with X-band EPR spectrometer along the temperatures between 150-340 K. Isotropic spectrum was obtained without depending on temperature and radical structure, C ̇H_3 H was observed. Radical was simulated using the Bruker software WinEPR-Simfonia and the experimental isotropic g-value with hyperfine coupling constant of radical was calculated.

Elucidation of the centers responsible for the TL peaks in the anhydride crystal

The mineral anhydride after gamma irradiation displays four thermoluminescence (TL) peaks at 150, 200, 268 and 338 °C. The kinetic parameters, namely activation energy, frequency factor and kinetic order associated with the main TL peaks have been determined using initial rise and glow curve deconvolution (GCD) methods. Defect centers induced in the mineral by gamma irradiation are studied by electron spin resonance (ESR) with a view to identify the centers responsible for the TL process. ESR spectrum at room temperature is observed to be a superposition of at least six defect centers. Center I with principal g-values g|| = 2.011 and g⊥ = 2.012 is identified as (SO4)− radical and the center relates to the TL peak at 200 °C. Centers II, III and IV are characterized by axial g-tensors and all these three centers are assigned to (SO4)− radicals. These centers correlate with the 268 °C TL peak. Center V with an isotropic g-value equal to 1.9953 is attributed to (SO3)− radical and the radical is also associated with the 268 °C TL peak. Center VI having an isotropic g factor 2.015 is assigned to an intrinsic O− type center and the center correlates with the TL peak at 268 °C. Center VII displays a rhombic g-tensor and is ascribed to (SO3)− radical.

Investigations of BaCl2:Eu2+ nanophosphor using electron paramagnetic resonance, structural analysis and thermoluminescence

Structural analysis of the Eu2+-doped BaCl2 nanocrystals and the doping process was monitored and characterized via electron paramagnetic resonance spectroscopy. Structural analysis has shown a slight distortion of the cell which is reflected in the low value of microstrain and the Eu2+-doping effect is limited to the first order chlorine ions neighbors of Ba2+. Electron paramagnetic resonance measurements have indicated the presence of the Eu2+-dopant ions in the BaCl2 host matrix with a solubility limit of about 2%. From the spectra simulation, the isotropic g-value giso = 1.9951(7), the isotropic hyperfine coupling constant Aiso = 42.4 MHz and the high-order zero-field splitting parameters from the crystal field B20 = 21 MHz and B22 = -493 MHz were obtained. During X-ray irradiation, defects are produced and stabilized by the Eu2+ dopant ions. The single dominant thermoluminescence peak at 132 °C (activation energy E = 1.1 eV) was assigned to the recombination of the F(Cl)-center with Eu2+ related hole centers.

Thermoluminescence and defect centers in β-CaSiO3 polycrystal

βCaSiO3 polycrystal was synthesized by the devitrification method. The polycrystal exhibits three thermoluminescence (TL) peaks at 124 °C, 250 °C and 306 °C. Electron paramagnetic resonance (EPR) spectroscopy was used to study the defect centers induced in the polycrystal by gamma irradiation and to identify the centers responsible for the TL process. Three defect centers contribute to the observed spectrum at room temperature. Center I with principal g-values 2.0135, 2.0094 and 2.0038 is attributed to O− ion and the center appears to be the recombination center for 124 °C, 147 °C and 306 C TL peaks. Center II exhibiting an isotropic g-value of 2.00025 is identified as an F+-center (singly ionized oxygen vacancy). F+-center is also observed to be a recombination center for several TL peaks. Center III is assigned to a Ti3+ center displaying an orthorhombic g-tensor with principal values g1 = 1.9830, g2 = 1.9741 and g3 = 1.9046. This center is associated with 124 °C and 147 °C TL peaks. TL emission spectrum of β-CaSiO3 shows two emission bands at 370 and 520 nm.

Molecular oxygenates from the thermal degradation of tobacco and material characterization of tobacco char

Abstract This contribution investigates the formation of selected oxygenated molecular compounds with smoking temperatures, particulates and free radicals from the burning of tobacco selected from two cigarettes (SM1 and ES1), in a poor oxygen environment - an oxidative atmosphere (5% oxygen in N2), in the temperature region 200–700 °C at 2 s contact time. Molecular carbonyls were determined using a Gas chromatography (GC) hyphenated to a mass selective detector (MSD). Fourier transform infrared (FTIR) spectroscopy afforded the determination of functional groups adsorbed on tobacco particulates. Free radicals in tobacco smoke particulates were investigated using an electron paramagnetic resonance spectrometer (EPR) whereas particulate size of tobacco smoke was estimated using image J. Aztec (UK) software coupled to an Oxford detector (UK) facilitated elemental analysis of tobacco char. At ∼ 400 °C, 2-methylcyclopentenone gave a maximum yield of 7.95 μ g while 2-methyl-2-propanol had a maximum yield of 14.75 μ g . The absorption band at 1740 cm−1 confirms the presence of carbonyl groups ( − C = O ) adsorbed on tobacco smoke particulates. Tobacco particulates generated at 600 °C showed an isotropic g-value of 2.0028 and a particle size distribution between 18 and 30 nm. It was noted that SM1 cigarette had very high radical, oxygen and nitrogen content and therefore considerably more toxic than ES1 cigarette. Tobacco char was largely carbon (∼66–69%) – Energy dispersive X-ray (EDX) data hence the major reason carbon based radicals dominated the oxygen centered radicals at elevated temperatures. Generally, tobacco free radicals are very reactive intermediates and therefore precursors for detrimental ailments associated with cigarette smoking. These findings support the fact that research based policy framework is essential in minimizing the burden associated with tobacco abuse.

A new fluorene derived Schiff-base as a dual selective fluorescent probe for Cu2+ and CN.

A new fluorene based fluorogenic chemosensor, 2-[(9H-Fluoren-2-ylmethylene)-amino]-phenol (L), has been designed, synthesized, and characterized by CHN analyses and different spectroscopic methods. This turn-on fluorogenic chemosensor shows high selectivity and sensitivity toward Cu2+ and CN- with low detection limits of 1.54 × 10-9 M and 1.83 × 10-7 M, respectively. The stoichiometry ratio of L-Cu2+ in solution is 1:1, by the method of Job's plot and ESI-MS. The microcrystalline solid product of the chemosensor reaction with copper is characterized as CuL2. The χT value for CuL2 is temperature independent at a value of 0.403 cm3 K mol-1, which is in agreement with a mononuclear copper(II) complex with an isotropic g-value of 2.075. The fluorescence turn-on recognition process for detection of Cu2+ is attributed to the restricted imine isomerization and blocking of intramolecular charge transfer (ICT) quenching process in the analyte-bound sensor. The selectivity of L for Cu2+ is based on the chelation-enhanced fluorescence effect (CHEF) mechanism. Other interfering ions such as Na+, K+, Ca2+, Mg2+, Ag+, Fe2+, Fe3+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Mn2+, Pb2+ and Al3+, show no change in the fluorescence intensity of L in the presence of Cu2+. Furthermore, the compound L can be used as a fluorescence and colorimetric sensor for selective detection of CN- over a number of other anions based on the nucleophilic addition to the imine CN bond, with consequent hydrogen bond formation and electrostatic interaction of the resulting product with K+. The sensing mechanism for CN- was theoretically supported by DFT calculations.

Total Ionizing Dose Effects on TiN/Ti/HfO2/TiN Resistive Random-Access Memory Studied via Electrically Detected Magnetic Resonance.

We observe a gamma-irradiation induced change in electrically detected magnetic resonance (EDMR) in TiN/Ti/HfO2/TiN resistive random access memory (RRAM). EDMR measurements exclusively detect electrically active defects which are directly involved in the transport mechanisms within these devices. The EDMR response has an isotropic g-value of 2.001 ± 0.0003. The response increases dramatically with increased gamma-irradiation. We tentatively associate this EDMR response with spin dependent trap assisted tunneling (SDTAT) events at centers coupled to hafnium ions. Although our study cannot fully identify the role of these defects in electronic transport, the study does unambiguously identify changes in transport defects caused by the ionizing radiation on defects involved in electronic transport in RRAM devices. This work also contributes more broadly to the RRAM field by providing direct, though incomplete, information about atomic scale defects involved in electronic transport in leading RRAM systems.

Point defects in MoS2: Comparison between first-principles simulations and electron spin resonance experiments

Several native point defects in MoS2 are theoretically studied, using first-principles simulations. The isotropic g-values of these defects are computed, and compared to recently reported experimental results, obtained from electron spin resonance experiments performed on MoS2 layers synthesized by the sulfurization of Mo films. We tentatively assign the observed electron spin resonance signal with isotropic g-value of 2.002 to a sulfur antisite defect. This defect presents energy levels in the MoS2 band-gap, and could hamper the proper functioning of MoS2-based field effect transistors. We next study the interaction of H2 molecules with the sulfur antisite defect, using first-principles molecular dynamics simulations and the nudged elastic band method. Our results predict that this defect can be passivated by hydrogen, with a computed activation energy of about 1.5eV.

Studies of defects and optical properties of CaAl12O19:Ho3+ phosphor material

Phosphor CaAl12O19 doped with holmium was synthesized by the low temperature solution combustion method. Powder X-ray diffraction (XRD) technique was used to find the phase formation and phase purity. The optical properties were studied using diffuse reflectance and photoluminescence (PL) spectroscopy. CaAl12O19:Ho exhibits three thermoluminescence (TL) peaks at approximately 140 °C, 260 °C and 440 °C. Electron Spin Resonance (ESR) studies were carried out to study the defect centers induced in the phosphor by gamma irradiation and also to identify the centers responsible for the TL process. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of at least four distinct centers. One of the centers (center I) with an isotropic g factor 2.0115 is tentatively assigned to an intrinsic O− type center. This center relates with the TL peak at 140 °C. Center II with an axially symmetric g-tensor with principal values g|| = 2.0283 and g⊥ = 2.0058 is identified as anO2− ion and appears to correlate also with the 140 °C TL peak. Center III with an isotropic g-factor 2.0011 is assigned to an F+ -type center (singly ionized oxygen vacancy) and is the likely recombination center for the two low temperature TL peaks. Another center with an isotropic g-value 2.0043 is visible after 300 °C thermal anneal and is also attributed to an F+ center (center IV). This center appears to be associated with the high temperature TL peak at 440 °C.

Optical and magnetic resonance studies of Be-doped GaN bulk crystals

Low temperature photoluminescence (PL) and optically-detected magnetic resonance (ODMR) at 24GHz have been performed on a set of Be-doped GaN bulk crystals grown by the high pressure nitrogen solution method. Most notably, these crystals exhibit an intense 2.25eV “yellow” PL band that is currently of high interest for use as a converter in white light-emitting-diode applications. In addition, the samples investigated in this work are found to emit a broad “violet” PL band of varying intensity with peak energy near 3.16eV that is shown to be associated with a high concentration of residual Mg acceptor impurities as supported by secondary ion mass spectroscopy measurements. ODMR angular rotation studies obtained on the “yellow” and “violet” emission bands revealed two strong signals in each case and helped to provide information on the nature of these recombination processes. One feature, common to the ODMR of each PL band, has Zeeman splitting g-values of ~1.95 and is ascribed to residual Si and O shallow donor impurities. The second ODMR signal observed on the 2.25eV “yellow” PL exhibits nearly isotropic g-values of ~2.0 characteristic of deep centers in GaN and is tentatively associated with Be-related deep acceptors. In contrast, the second ODMR feature found on the 3.16eV “violet” PL has a larger degree of anisotropy with g║~2.1 and g⊥~2.0 and are similar to the g-values assigned previously to Mg shallow acceptors from ODMR studies of highly Mg-doped GaN heteroepitaxial layers with large degree (≥10%) of compensation.

Electronic structure of the Mn-cofactor of modified bacterial reaction centers measured by electron paramagnetic resonance and electron spin echo envelope modulation spectroscopies

The electronic structure of a Mn(II) ion bound to highly oxidizing reaction centers of Rhodobacter sphaeroides was studied in a mutant modified to possess a metal binding site at a location comparable to the Mn4Ca cluster of photosystem II. The Mn-binding site of the previously described mutant, M2, contains three carboxylates and one His at the binding site (Thielges et al., Biochemistry 44:389-7394, 2005). The redox-active Mn-cofactor was characterized using electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) spectroscopies. In the light without bound metal, the Mn-binding mutants showed an EPR spectrum characteristic of the oxidized bacteriochlorophyll dimer and reduced quinone whose intensity was significantly reduced due to the diminished quantum yield of charge separation in the mutant compared to wild type. In the presence of the metal and in the dark, the EPR spectrum measured at the X-band frequency of 9.4 GHz showed a distinctive spin 5/2 Mn(II) signal consisting of 16 lines associated with both allowed and forbidden transitions. Upon illumination, the amplitude of the spectrum is decreased by over 80 % due to oxidation of the metal upon electron transfer to the oxidized bacteriochlorophyll dimer. The EPR spectrum of the Mn-cofactor was also measured at the Q-band frequency of 34 GHz and was better resolved as the signal was composed of the six allowed electronic transitions with only minor contributions from other transitions. A fit of the Q-band EPR spectrum shows that the Mn-cofactor is a high spin Mn(II) species (S = 5/2) that is six-coordinated with an isotropic g-value of 2.0006, a weak zero-field splitting and E/D ratio of approximately 1/3. The ESEEM experiments showed the presence of one (14)N coordinating the Mn-cofactor. The nitrogen atom is assigned to a His by comparing our ESEEM results to those previously reported for Mn(II) ions bound to other proteins and on the basis of the X-ray structure of the M2 mutant that shows the presence of only one His, residue M193, that can coordinate the Mn-cofactor. Together, the data allow the electronic structure and coordination environment of the designed Mn-cofactor in the modified reaction centers to be characterized in detail and compared to those observed in other proteins with Mn-cofactors.

First-principles study of Ge dangling bonds in GeO2 and correlation with electron spin resonance at Ge/GeO2 interfaces

The g-tensors of dangling bonds at defective Ge atoms in GeO2 are computed using density functional theory. The isotropic g-values of these defects are found to increase with the number of Ge backbonds. By comparing these calculations with the isotropic g-value of a Ge-related defect at Ge/GeO2 interfaces, recently observed by electron spin resonance (ESR) experiments, we tentatively identify this defect as a Ge2O≡Ge• center, i.e., a dangling bond on a Ge atom backbonded to two Ge atoms and one O atom, likely present near the Ge/GeO2 interface. The interaction of this defect with molecular hydrogen is investigated using first-principles molecular dynamics simulations. Our simulations predict that the Ge dangling bond can be hardly passivated by H2 molecules, in agreement with the electron spin resonance study.

Theoretical Study of Ge Dangling Bonds in GeO2 and Correlation with ESR Results at Ge/GeO2 Interfaces

The g-tensors of dangling bonds at defective Ge atoms in GeO2 are computed using density functional theory. The isotropic g-values of these defects are found to increase with the number of Ge backbonds. By comparing these calculations with the isotropic g-value of a Ge-related defect at Ge/GeO2 interfaces, recently observed by electron spin resonance and electrically detected magnetic resonance experiments, we tentatively identify this defect as a Ge2O≡Ge• center, i.e. a dangling bond on a Ge atom backbonded to two Ge atoms and one O atom, likely present near the Ge/GeO2 interface. The interaction of this defect with molecular hydrogen is investigated using first-principles molecular dynamics simulations. Our simulations predict that the Ge dangling bond can be hardly passivated by H2 molecules, in agreement with the electron spin resonance study.