Radiation-induced Defect Centers Research Articles

TL, EPR, and optical properties of undoped and Eu-, Ce-, Tb-, Sm-, Cu-, Mn-, and Li- doped MgSiO3 phosphors, synthesized by sol-gel combustion technique

Polycrystalline MgSiO3 samples, undoped or doped with rare earth- (Eu, Tb, Ce, Li, Sm), transition- (Mn and Cu), and alkali metal- (Li) elements, were prepared by the sol-gel combustion method and characterized by thermoluminescence, electron spin resonance, and optical properties. Enstatite and protoenstatite phases are observed in undoped and doped MgSiO3. Better TL results have been observed for phosphors doped with Tb, Ce, and Li (0.5 % mol). Fading experiments and the GCD method have been performed. EPR signals of gamma-irradiated phosphors indicate the formation of radiation-induced defect centers. Cu-doped MgSiO3 shows an unusual feature of the Cu2+ ion. The Cu2+ ion occupies a tetragonally compressed octahedral site, presenting an unconventional attribute. Optical bandgap value Eg is higher for the Ce, Tb, Mn, Li, Sm, and Cu-doped MgSiO3 phosphors. Ce, Li, and Tb dopant ions were found suitable for the MgSiO3 sample regarding its thermoluminescent response and its possible application in radiation dosimetry.

Enhanced optical cross-section of radiation induced defect centers under plasmon resonance conditions: Shifting stimulation wavelength of optically stimulated luminescence dosimeters

Plasmon-enhanced luminescence is an important tool for development of advanced sensing technologies but the mechanisms originating these enhancements are still in debate. In this work, we investigate plasmon-enhanced luminescence mechanisms using the Optically Stimulated Luminescence (OSL) from defect centers in X-ray irradiated sodium chloride nanocrystals deposited on silver nanoparticle (AgNP) films with varying plasmon properties upon blue, green, and red OSL stimulation. Although the absolute OSL intensity for NaCl crystals is higher upon blue stimulation, under plasmon resonance conditions, higher absolute OSL intensity occurs upon green stimulation and the largest OSL enhancements are observed by employing red stimulation. These results suggest that, as long as the OSL stimulation wavelength is well tuned with the film plasmon band, plasmonic coupling does not depend on the trap depth and that the largest luminescence enhancements are achieved in spectral regions where the optical cross-sections of the defect centers are smaller. In terms of radiation dosimetry applications, this result allows to shift the stimulation wavelength farther from the OSL emission window, improving the signal to noise ratio and the dose assessment sensitivity and accuracy.

Deep Radiation-Induced Defect Centers Created by a Fast Neutron Flux in CdZnTe Single Crystals

The effect of a fast neutron flux (Φ = 1014–1015 cm–2) on the electrical and photoluminescence properties of p-CdZnTe single crystals is studied. Isothermal annealing is performed (T = 400–500 K), and the activation energy of the dissociation of radiation-induced defects is determined at ED ≈ 0.75 eV.

Radiation-induced photoluminescence enhancement of Bi/Al-codoped silica optical fibers via atomic layer deposition

The radiation-induced photoluminescence (PL) properties of Bi/Al-codoped silica optical fibers were investigated. The Bi/Al-related materials were doped into fiber core via atomic layer deposition. The pristine fiber samples were irradiated with different doses, and its absorption and PL properties were studied. A new absorption peak appeared at approximately 580 nm, and the intensity of absorption peaks is increased with the increasing of radiation doses. When the fiber samples were excited with a 532 nm pump, the intensity of the near infrared fluorescence decreased lightly. However, when the fiber samples were excited with a 980 nm pump the intensity of the fluorescence increased significantly with the increase of radiation doses (0-2.0 kGy). The intensity of fluorescence decreased when the radiation doses were increased up to 3.0 kGy. furthermore, the fluorescence intensity of the 1410 nm band increased much more than that the 1150 nm band. In addition, the microstructural characteristics of the Bi/Al-codoped silica optical fibers were analyzed using electron spin resonance (ESR). Many radiation-induced defect centers were present, and the intensity of the ESR signals also increased with the increase of radiation doses. The photoluminescence properties and microstructural characteristics were related in the radiated Bi-related silica optical fibers. A possible underlying mechanism for the radiation-induced photoluminescence enhancement process in the Bi/Al-doped silica fiber is discussed.

Photoluminescence spectra of 1 MeV electron beam irradiated In0.53Ga0.47As/InP quantum well and bulk materials

Minimizing the impact of radiation-induced degradation on optoelectronic devices is important in several applications. Satellites and other spacecraft that fly in near-earth orbits (below 3.8 earth radius) are extremely susceptible to radiation damage caused by the high flux of electrons trapped in the earth’s magnetosphere. Optoelectronic devices are particularly vulnerable to displacement damage caused by electrons and protons. Effects of 1 MeV electron beam irradiation on the photoluminescence properties of In0.53Ga0.47As/InP quantum well (QW) and bulk structures, which are grown by metal-organic vapor phase epitaxy, are investigated. Samples are irradiated at room temperature using an ELV-8II accelerator with 1 MeV electron at doses ranging from 5×1012 to 9×1014 cm-2, and a dose rate of 1.075×1010 cm-2·s-1. Photoluminescence measurements are made using a 532 nm laser for excitation and a cooled Ge detector with lock-in techniques for signal detection. Photoluminescence intensity of all the structures is degraded after irradiation, and its reduction increases with increasing total dose of irradiation. Electron beam irradiation causes a larger reduction in the photoluminescence intensity and carrier lifetime of the bulk than that of quantum well. Photoluminescence intensity of five-layer quantum wells degenerates to 9% that before irradiation as the fluence reaches 6×1014 cm-2. As the electron beams bombard into the sample, the destruction of the lattice integrity will cause the decrease in the number of excitons and intensity of photoluminescence. Electron beam irradiation introduces defects in the samples, increases the density of the nonradiative recombination centers, and results in the decrease of carrier mobility. In a quantum well structure, due to the two-dimensional confinement, the probability of carrier nonradiative recombination at radiation-induced defect centers will be reduced. The reduction of photoluminescence intensity in the bulk is severer than in the quantum well while the cross-sectional area which is sensitive to radiation is kept the same. The number of interface defects which are produced by electron irradiation will increase with the number of layers in quantum well and the heterojunction interface of quantum wells, so is the degration of photoluminescence intensity. The degration is mainly due to the increase of non-radiative centers in the samples. By comparing the different structures, the quantum well structure shows a better radiation resistance.

Influence of gamma-ray irradiation on Faraday effect of Cu-doped germano-silicate optical fiber

Influence of gamma-ray irradiation on the Faraday effect of the Cu-doped germano-silicate optical fiber was investigated. The Verdet constant of the gamma-ray irradiated optical fiber at 660nm was measured to be 3.07radT−1m−1, 1.46 times larger than that of before the irradiation at total dose of 1200Gy. Cu-related radiation-induced defect centers and Cu metal particles which were reduced from Cu2+ ions by the irradiation are thought to be responsible for the increase in the Verdet constant of the optical fiber.

In situ reactor radiation-induced attenuation in sapphire optical fibers heated up to 1000 °C

The purpose of this work was to determine the suitability of using instrumentation utilizing sapphire optical fibers in a high temperature nuclear reactor environment. For this, the broadband (500–2200nm, or 0.56–2.48eV) optical transmission in commercially available sapphire optical fibers was monitored in situ prior to, during, and after reactor irradiation. Portions of the sapphire fibers were heated to temperatures up to 1000°C during irradiation. The sapphire fibers were irradiated, mostly at a neutron flux of 5.0×1011n/cm2/s and a gamma dose rate of 28kGy/h (dose in sapphire), to a total neutron fluence of 6.4×1016n/cm2 and total gamma dose on the order of 1MGy. Results were generally consistent with the results of previous in situ measurements of the transmission in unheated sapphire fibers during reactor irradiation. Added attenuation at 850, 1300, and 1550nm, appears to be limited by the growth of radiation-induced defect centers that are located in the ultra violet to the visible range and is therefore less at 1300 and 1550nm than at 850nm. A linear increase in attenuation, due to displacement damage effects, was observed with increased irradiation time at constant reactor power. However, the rate of increase of the added attenuation during constant power reactor irradiation monotonically decreased with increasing temperature up to 1000°C, with the most significant decrease occurring between 300 and 600°C. Additional calculations predicted that the majority of (if not all of) the observed increases in attenuation during irradiation at 600 and 1000°C were due to effects in the unheated sections of the irradiated sapphire fibers. These results suggest that, for a reactor radiation environment similar to that tested in this work, heating sapphire fibers to temperatures of 600°C or greater during irradiation would significantly reduce (or possibly eliminate entirely) the rate of growth of the added attenuation in the sapphire fibers.

Strong Visible and Near-Infrared Cathodoluminescence in Alkali Feldspar

ABSTRACT In this study, cathodoluminescence (CL) spectroscopy at direct current and alternating current under the sample temperature condition of 40–293 K using different modulation frequencies is presented for alkali feldspar from the Dartmoor granite (UK). These feldspars contain strain-controlled lamellar crypto- and microperthites that are cross-cut by strain-free deuteric microperthites. The CL spectra of the alkali feldspar at room and low temperature confirm that the observed emission peaked at ∼460 nm could be associated with Al-O−-Al or Ti impurity centers, yellow emission ∼560 nm could be associated with the presence of the centers such as radiation-induced defect centers, and ∼756 nm emission could be associated with the Fe3+ impurity center on T1 and T2 sites. The consequence of their association is to produce different luminescence properties such as intensity, peak wavelength, and band shape.

Cathodoluminescence of alkali feldspars and radiation effects on the luminescent properties

Cathodoluminescence (CL) spectroscopy provides useful information about the existence of radiation-induced defect centers with a few micrometer resolutions and therefore has great potential to estimate the accumulated dose of natural radiation in micrometer-ordered mineral grains from radioactive decay. Although great scientific interest exists concerning the CL of various types of minerals, very few investigation have been conducted on the luminescence properties of radiation-induced alkali feldspars. This study, therefore, has sought a clarification of radiation effects on emission centers detected by CL analysis of alkali feldspar implanted with He+ ions at 4.0 MeV, which corresponds to the energy of an α particle derived from radioactive decay of 238U and 232Th. Panchromatic CL images of cross sections of sanidine, orthoclase, and microcline show a dark line with ~1 μm width on the bright luminescent background at 12–15 μm beneath the implanted surface, of which behavior may be corresponding to the electronic energy loss process of 4.0 MeV He+ ion. CL and Raman spectroscopy revealed that He+ ion implantation may leads to a partial destruction of the feldspar framework and Na+ migration, resulting in a quenching of CL emission from alkali feldspar, proportional to the radiation dose. CL spectra of unimplanted and He+-ion-implanted sanidine, orthoclase and microcline have emission bands at ~400–410 nm and at ~730 nm. Deconvolution of the CL spectra can successfully separate these emission bands into emission components at 3.05, 2.81, 2.09, 1.73, and 1.68 eV. These components are assigned to the Ti4+ impurity, Al-O−-Al/Ti defect, a radiation-induced defect center, and Fe3+ impurities on the T1 and T2 sites, respectively. The intensity at 3.05 eV negatively correlates with radiation dose owing to decreases in the luminescence efficiency. A slight Na+ diffusion and breaking of the linkage between Ti4+ and oxygen as a ligand might reduce the activation energy, which decreases the availability of radiative energy in the luminescence process of Ti4+ impurity centers. Furthermore, He+ ion implantation causes electron holes to be trapped at and released from Lowenstein bridges as a consequence of Na+ migration and leads to a partial destruction of Al-O bonds, which might be responsible for an increase and decrease in the intensity of emission component at 2.81 eV. With an enhanced radiation dose, there is a decrease in intensity at 1.73 eV and an increase in intensity at 1.68 eV. Deconvoluted CL spectra of the alkali feldspars reveal a positive correlation between intensity at 2.09 eV and the radiation dose, which may be due to the formation of a radiation-induced defect center. These correlations can be fitted by an exponential curve, where the gradients differ between the alkali feldspars studied, and are largest for the microcline, followed by the orthoclase and then the sanidine. The intensity at 2.09 eV has the potential to be used in geodosimetry and geochronometry.

Investigation of Radiation Damage in Trombay Nuclear Waste Glasses by ESR and Photoluminescence Techniques

Electron spin resonance (ESR) and photoluminescence (PL) techniques were utilized to investigate the gamma radiation induced changes occurring in the Trombay research reactor nuclear waste glasses. Fe and Eu were doped in the glass samples which acted as local probes. From these studies it was seen that upon irradiation various types of defect centers namely boron oxygen hole centers, E’ centers, silicon hole centers, oxygen deficient centers etc. were formed in the glass. Further, contrary to the other waste glass compositions reported, no change was observed in the Eu oxidation state after irradiation. In case of the Fe doped glass, it was observed that, for glasses irradiated with same radiation dose, the radiation induced defect centers decreased with increase in the Fe content in the glass. Although the radiation lead to formation of various defect centers, the overall glass network remained unaffected to a large extent.

Response of cathodoluminescence of alkali feldspar to He+ ion implantation and electron irradiation

Abstract Cathodoluminescence (CL) of minerals such as quartz and zircon has been extensively studied to be used as an indicator for geodosimetry and geochronometry. There are, however, very few investigations on CL of other rock-forming minerals such as feldspars, regardless of their great scientific interest. This study has sought to clarify the effect of He+ ion implantation and electron irradiation on luminescent emissions by acquiring CL spectra from various types of feldspars including anorthoclase, amazonite and adularia. CL intensities of UV and blue emissions, assigned to Pb2+ and Ti4+ impurity centers respectively, decrease with an increase in radiation dose of He+ ion implantation and electron irradiation time. This may be due to decrease in the luminescence efficiencies by a change of the activation energy or a conversion of the emission center to a non-luminescent center due to an alteration of the energy state. Also, CL spectroscopy of the alkali feldspar revealed an in-crease in the blue and yellow emission intensity assigned to Al-O−-Al/Ti defect and radiation-induced defect centers with the radiation dose and the electron irradiation time. Taken together these results indicate that CL signal should be used for estimation of the α and β radiation doses from natural radionuclides that alkali feldspars have experienced.

Optical spectroscopy and microscopy of radiation-induced light-emitting point defects in lithium fluoride crystals and films

Broad-band light-emitting radiation-induced F2 and F3+ electronic point defects, which are stable and laser-active at room temperature in lithium fluoride crystals and films, are used in dosimeters, tuneable color-center lasers, broad-band miniaturized light sources and novel radiation imaging detectors. A brief review of their photoemission properties is presented, and their behavior at liquid nitrogen temperatures is discussed. Some experimental data from optical spectroscopy and fluorescence microscopy of these radiation-induced point defects in LiF crystals and thin films are used to obtain information about the coloration curves, the efficiency of point defect formation, the effects of photo-bleaching processes, etc. Control of the local formation, stabilization, and transformation of radiation-induced light-emitting defect centers is crucial for the development of optically active micro-components and nanostructures. Some of the advantages of low temperature measurements for novel confocal laser scanning fluorescence microscopy techniques, widely used for spatial mapping of these point defects through the optical reading of their visible photoluminescence, are highlighted.

Radiation response behaviour of Al codoped germano-silicate SM fiber at high radiation dose

Radiation response behaviour of Ge+Al doped SM fiber fabricated by the solution doping process has been studied at room temperature with respect to 1310nm transmission wavelength under three different dose rates of 200, 400 and 600Rad/min to compare with that of standard Er doped as well as Ge doped SM fibers. Their radiation sensitivity has been observed with variation of dose rates, transmission wavelength along with their recovery nature. Radiation response behaviour of Al doped SM fiber is found to be slightly non-linear in nature with very low dose rate dependency. No saturation level was found upto 13Krad cumulative dose. Thermobleaching as well as photobleaching phenomena have also been studied. Gamma irradiated Al doped preform shows an absorption peak at around 300nm due to generation of Al (E′) defect center and gets annihilated after thermobleaching process. Gamma irradiated Al doped SM fiber shows prominent photobleaching effect on their optical attenuation with respect to the 850nm transmission wavelength. From ESR study resonance signals for Al3+ related radiation-induced defect centers are not clearly observed in this study. A very weak hyperfine pattern has been observed for gamma irradiated Al doped preform sample. The high radiation sensitivity along with linear response behaviour, low recovery and almost dose rate independence behaviour of the material system of Ge+Al codoped SM core optical fiber under gamma radiation shows their potential for application as fiber optic radiation sensor in comparison to the universal standard erbium doped SM fiber.

CATHODOLUMINESCENCE AND THERMOLUMINESCENCE STUDIES OF CLAY MINERALS

Cathodoluminescence (CL) measurements of clay minerals were performed at both room temperature and low temperature. Panchromatic CL images of kaolinite and dickite show blue emission at room temperature. Their CL spectra exhibit an intense broad band peak at around 375 nm. This emission band related to radiation induced defect centers (RID) disappears at low temperature. Effect of sample temperature on CL intensity of kaolinite and dickite can not be interpreted on the basis of a temperature quenching theory. In halloysite, serpentine minerals, sericite and sepiolite, the intensity of a broad band between 400 and 450 nm is higher at low temperature than at room temperature. CL spectra of serpentine minerals are characterized by a broad band centered at 720 nm, which can be assigned to Fe 3+ impurity center. Natural TL glow curve of sepiolite presents two glow peaks around at 210 and 260°C in the high temperature region, of which intensities are weak. This implies that it is rather a possibility of applying TL of sepiolite to the dating. The other minerals show no emission in natural TL measurements. X-ray induced TL of kaolin minerals exhibit intense several TL glow peaks below room temperature. Serpentine minerals have glow curves with a relative large peak between 70 and 90°C. Sericite, montmorillonite and sepiolite show a combination of emission peaks in their TL glow curves, while their intensities are weak compared to kaolin and serpentine minerals. Clay minerals might have characteristic TL related to their crystal structure.

Proton-induced damage in gallium nitride-based Schottky diodes

Proton irradiation decreases the doping concentration and increases the ideality factor and series resistance, but has very little effect on the Schottky barrier height in n-Gallium nitride Schottky diodes. 1.0-MeV protons cause greater degradation than 1.8-MeV protons because of their higher nonionizing energy loss. The displacement damage recovers during annealing. Comparison between Schottky diodes and high electron-mobility transistors suggests that the degradation in both types of devices is predominantly due to carrier removal and mobility degradation caused by radiation-induced defect centers in the crystal lattice, with interface disorder playing a relatively insignificant part in overall device degradation.

/sup 60/Co gamma irradiation effects on n-GaN Schottky diodes

The effect of /spl gamma/-ray exposure on the electrical characteristics of nickel/n-GaN Schottky barrier diodes has been investigated using current-voltage (I-V), capacitance-voltage (C-V), and deep-level transient spectroscopy (DLTS) measurements. The results indicate that /spl gamma/-irradiation induces an increase in the effective Schottky barrier height extracted from C-V measurements. Increasing radiation dose was found to degrade the reverse leakage current, whereas its effect on the forward I-V characteristics was negligible. Low temperature (/spl les/50) post-irradiation annealing after a cumulative irradiation dose of 21 Mrad(Si) was found to restore the reverse I-V characteristics to pre-irradiation levels without significantly affecting the radiation-induced changes in C-V and forward I-V characteristics. Three shallow radiation-induced defect centers with thermal activation energies of 88 104 and 144 meV were detected by DLTS with a combined production rate of 2.12 /spl times/ 10/sup -3/ cm/sup -1/. These centers are likely to be related to nitrogen-vacancies. The effect of high-energy radiation exposure on device characteristics is discussed taking into account possible contact inhomogeneities arising from dislocations and interfacial defects. The DLTS results indicate that GaN has an intrinsically low susceptibility to radiation-induced material degradation, yet the effects observed in the Schottky diode I-V and C-V characteristics indicate that the total-dose radiation hardness of GaN devices may be limited by susceptibility of the metal-GaN interface to radiation-induced damage.

Kinetics of UV laser radiation defects in high performance glasses

High purity fluoride phosphate glasses are attractive candidates as UV transmitting materials. The calculated values for the ultraviolet resonance wavelength are comparable with those of pure silica glass or fluoride single crystal CaF2. The formation of radiation-induced defect centers leads to additional absorption bands in the VUV–UV–vis range. The damage and the healing behavior by lamps and lasers are investigated in dependence on phosphate content and the content of impurities, mainly transition metals. Experiments were carried out using pulsed lasers with a duration of femto- and nanoseconds at a wavelength of 248 nm. The initial slope of the induced absorption shows a nonlinear dependence on the pulse energy density. Resonant and non-resonant two-photon mechanisms were observed. Two-photon-absorption coefficients at 248 nm for samples with different phosphate contents were measured. Models of the kinetics of the radiation-induced defects were developed. The inclusion of energy transfer was necessary to explain the difference in the damage behavior for nanosecond (248 nm, 193 nm) and femtosecond (248 nm) laser pulses.

Reducibility and Adsorbate Interactions of Ti in Titanosilicate Molecular Sieve TS-1

Electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopy are used to study reducibility and adsorbate interaction of Ti in titanosilicate TS-1 molecular sieve. Various reduction methods were used to reduce Ti(IV) in TS-1 to Ti(III) which was then monitored by ESR. When TS-1, after dehydration and oxygen treatment at high temperature followed by evacuation (activation), is γ-irradiated at 77 K; an ESR signal withg‖=1.970 andg⊥=1.906 is observed for isolated Ti(III) centers. Radiation induced defect centers known as V centers are also observed after γ-irradiation. When activated TS-1 is treated with CO or H2at 673 K, an axial ESR signal withg⊥=1.968 andg‖=1.933 is observed which is suggested to be Ti(III)(CO)nand Ti(III)(H2)ncomplexes. When activated TS-1 is exposed to D2O at room temperature and subsequently γ-irradiated at 77 K, a new ESR signal withg=1.924 is observed. This species is identified as Ti(III)-(OD)1from2D ESEM data. Adsorption of CH3OD on activated TS-1 produces a new Ti(III) species withg=1.931. This species is identified as Ti(III)-(CH3OD)1from2D ESEM data. Adsorption of C2D4on activated TS-1 produces a new Ti(III) species withg⊥=1.968 andg‖=1.910. This species is identified as Ti(III)-(C2D4)1from2D ESEM data. Possible coordination geometries of the various Ti(III)-adsorbate complexes are discussed.

Damage production in silicon irradiated with 112 MeV Ar ions

Silicon samples were irradiated below 50 K with 112 MeV Ar ions. Defect production was investigated at room temperature by EPR and infrared optical absorption techniques. The EPR measurements reveal the presence of amorphous zones and neutral tetravacancies (Si–P3 center) in the as-irradiated samples. The infrared optical absorption measurements indicate the formation of divacancy. The concentration of divacancy increases with ion fluence and saturates at high fluence. The isochronal annealing behaviors of the radiation induced defect centers are found to be fluence dependent. For 1.0 × 10 14/cm 2 irradiated sample, the disappearance of the Si–P3 center is accompanied by the appearance of Si–P1 and Si–B2 centers and the growth of Si–A11 center. The Si–P1 center and the isolated amorphous regions are annealed out between 500°C and 550°C. The Si–A11 center is stable up to 600°C. For 8.0 × 10 14/cm 2 irradiated sample, only Si–P1 center and the line due to the continued amorphous layer are detected above 200°C. The recrystallization of the continued amorphous layer occurs at temperature higher than 600°C.

Radiation-induced defect centers in 4H silicon carbide

Abstract Deep level transient spectroscopy (DLTS) and low temperature photoluminescence (LTPL) were applied to investigate radiation-induced defect centers and their thermal stability in 4H silicon carbide (SiC) epilayers grown by chemical vapor deposition (CVD). The epilayers were implanted with He+ ions and annealed at different temperatures. Several deep defect levels were monitored with DLTS in the 4H polytype. The correlation of these centers with photoluminescence lines is discussed with respect to appropriate annealing steps.