Influence Of Radiation Defects Research Articles

Effect of Radiation Defects on Thermo–Mechanical Properties of UO2 Investigated by Molecular Dynamics Method

Nuclear fuel performance is deteriorated due to radiation defects. Therefore, to investigate the effect of irradiation-induced defects on nuclear fuel properties is essential. In this work, the influence of radiation defects on the thermo-mechanical properties of UO2 within 600–1500 K has been studied using the molecular dynamics method. Two types of point defects have been investigated in the present work: Frenkel pairs and antisites with concentrations of 0 to 5%. The results indicate that these point defects reduce the thermal expansion coefficient (α) at all studied temperatures. The elastic modulus at finite temperatures decreases linearly with the increase in concentration of Frenkel defects and antisites. The extent of reduction (R) in elastic modulus due to two different defects follows the trend Rf > Ra for all studied defect concentrations. All these results indicate that Frenkel pairs and antisite defects could degrade the performance of UO2 and should be seriously considered for estimation of radiation damage in nuclear fuels used in nuclear reactors.

Degradation Under Influence of Radiation Defects of Detector Properties of CdTe and Cd0.9Zn0.1Te Irradiated by Neutrons

This work is devoted to the study by computer simulation of the mechanisms of the influence of radiation defects, arising under the influence of neutron irradiation, on the changes in electrical properties: resistivity ρ, electron mobility μn, lifetime of nonequilibrium electrons τn and holes τp in Cd0.9Zn0.1Te and charge collection efficiency η of uncooled ionizing radiation detectors based on this material. Radiation defects, which are corresponded by deep energy levels in the band gap, act as trapping centers of nonequilibrium charge carriers, noticeably affect the degree of compensation by changing ρ of the detector material, the recombination processes, decreasing τn and τp, and also the scattering of conduction electrons, decreasing μn, that ultimately can cause degradation of the charges collection efficiency η. The specific reasons for the deterioration of the electrophysical and detector properties of this semiconductor under the influence of neutron irradiation were identified, and the main factors affecting the increase in the resistivity of Cd0.9Zn0.1Te during its bombardment by low-energy and high-energy neutrons, leading to complete degradation of the recording ability of detectors based on this materials, were found. The recombination of nonequilibrium charge carriers is noticeably stronger than the decrease in μn affects the degradation of detector properties, therefore, the effect of recombination processes at deep levels of radiation defects on the degradation of τn, τp, and η of detectors based on Cd0.9Zn0.1Te was studied. A comparative analysis of the properties of Cd0.9Zn0.1Te with the previously studied CdTe:Cl was made. An attempt was made to explain the higher radiation resistance of Cd0.9Zn0.1Te compared to CdTe:Cl under neutron irradiation by the influence of the radiation self-compensation mechanism with participation of deep donor energy levels: interstitial tellurium and tellurium at the site of cadmium. In addition, the rate of recombination at defect levels in Cd0.9Zn0.1Te is, ceteris paribus, lower than in CdTe:Cl due to the smaller difference between the Fermi level and the levels of radiation defects in cadmium telluride. The relationship between the band gaps of Cd0.9Zn0.1Te and CdTe:Cl, the concentration of radiation defects, the Fermi level drift during irradiation, and the radiation resistance of the detectors were also noted. The important role of purity and dopant shallow donor concentration in initial state of the detector material is indicated.

Influence of radiation defects in a semiconductor structure on the electric field distribution in the air gap of the gas-discharge chamber

We have proposed and analyzed a theoretical model of the influence of a semiconductor structure both without radiation defects and with their presence on the EMI profile in the air gap of the gas-discharge chamber near the surface of the structure. We have carried out mathematical modeling and comparison of the electric field distribution in the gap for the “ideal” structure (without defects) and structures with a single defect at different depths of its occurrence. A transistor was chosen as the semiconductor structure. We have demonstrated a possibility in principle of using the simulated process for diagnosing radiation defects in electronic equipment under space conditions.

Physical properties of calcium fluoride nanopowder produced by the method of evaporation by pulsed electron beam in a low-pressure gas

Mesoporous CaF2 nanopowders with the specific surface area up to 88.7 m2/g have been produced by the evaporation by a pulsed electron beam in vacuum. The influence of thermal annealing in air on the size and morphology of CaF2 nanoparticles and their magnetic, luminescence, and texture characteristics has been studied. Ferromagnetism of CaF2 nanopowders was discovered for the first time. The influence of radiation defects (CaF2 dye centers) on magnetic properties of CaF2 nanoparticles has been examined.

Atomistic simulations of dislocation dynamics in δ-Pu-Ga alloys

Molecular dynamics with the modified embedded atom model (MEAM) for interatomic interaction is applied to direct simulations of dislocation dynamics in fcc δ-phase Pu-Ga alloys. First, parameters of the MEAM potential are fitted to accurately reproduce experimental phonon dispersion curves and phonon density of states at ambient conditions. Then the stress-velocity dependence for edge dislocations as well as Pierls stress are obtained in direct MD modeling of dislocation motion using the shear stress relaxation technique. The simulations are performed for different gallium concentrations and the dependence of static yield stress on Ga concentration derived demonstrates good agreement with experimental data. Finally, the influence of radiation defects (primary radiation defects, nano-pores, and radiogenic helium bubbles) on dislocation dynamics is investigated. It is demonstrated that uniformly distributed vacancies and nano-pores have little effect on dislocation dynamics in comparison with that of helium bubbles. The results of the MD simulations evidence that the accumulation of the radiogenic helium in the form of nanometer-sized bubbles is the main factor affecting strength properties during long-term storage. The calculated dependence of static yield stress on helium bubbles concentration for fcc Pu 1wt.% Ga is in good agreement with that obtained in experiments on accelerated aging. The developed technique of static yield stress evaluation is applicable to δ-phase Pu-Ga alloys with arbitrary Ga concentrations.

Дослідження впливу радіаційних дефектів на оптичні та люмінесцентні властивості кристалів CaF2

Дослідження впливу радіаційних дефектів на оптичні та люмінесцентні властивості кристалів CaF2

Influence of radiation defects on diffusion of arsenic and antimony in implanted silicon

Diffusion of implanted Sb and As in silicon layers with various contents of radiation defects is investigated during furnace thermal annealing and rapid thermal annealing (RTA). The defect concentration was varied by the additional introduction of Si+ ions. As the defect concentration in the layer increases, the antimony diffusivity increases both during lamp annealing and during furnace thermal annealing, which is governed by diffusion over excess vacancies. The As diffusivity increases with an increase in the concentration of radiation defects in the layer during the lamp annealing and decreases during furnace thermal annealing. The increase in the As diffusivity during RTA is attributed to interstitial Si atoms. The observed decrease in the As diffusivity is caused by the impurity capture by excess vacancies as the traps. However, at defect concentrations much higher than the impurity concentration, the As diffusivity increases but remains substantially lower than the intrinsic value. It follows from the results that the rate of intrinsic As diffusion along the interstitial channel is higher than along the vacancy channel.

Peculiarities of influence of radiation defects on photoconductivity of silicon irradiated by fast neutrons

The possibility of creating photoresistors from silicon of the n-type with specific resistance of 100–170 Ω cm using irradiation by fast neutrons of a nuclear reactor is shown. An optimal regime is found for silicon radiation-heat treatment to obtain photosensitive structures. About tenfold variation in resistance is revealed at illuminance of ∼200 lx.

Influence of radiation defects on electrical losses in silicon diodes irradiated with electrons

Silicon diodes with a p+-n junction irradiated with 3.5-MeV electrons (the fluence ranged from 1015 to 4 × 1016 cm−2) have been studied. It is established that the dependence of the tangent of the angle of electrical losses tanδ on the frequency f of alternating current in the range f = 102−106 Hz is a nonmonotonic function with two extrema: a minimum and a maximum. Transformation of the dependences tanδ(f) as the electron fluence and annealing temperature are increased is caused by a variation in the resistance of n-Si (the base region of the diodes) as a result of accumulation (as the fluence is increased) or disappearance and reconfiguration (in the course of annealing) of radiation defects. The role of time lag of the defect recharging in the formation of tanδ(f) is insignificant.

Anomalous magnetic properties of electron-irradiated antiferromagnetic copper monoxide

The influence of radiation defects on the magnetic properties of polycrystalline CuO and a high density nanoceramic with crystallite sizes d = 5 and 15 nm has been studied in the temperature range T = 77–300 K. Electron irradiation at fluences Φ = 5 × 1018 cm−2 initiated an increase in the susceptibility χ ∼ 1/T below 150 K, a feature anomalous for 3D antiferromagnets. The nonlinear behavior of magnetization in weak fields, the increase in the magnetic moment with decreasing temperature, and observation of the spontaneous magnetic moment at temperatures T < 150 K can be attributed to local changes in the exchange parameters and the formation of clusters with uncompensated magnetic moments in the antiferromagnetic matrix near point defects.

Influence of radiation defects on formation of thermal donors in silicon irradiated with high-energy helium ions

Isothermal annealing at temperatures from 370 to 410 °C was used for investigation of radiation enhanced formation of thermal donors in oxygen-rich FZ silicon irradiated with 7 MeV helium ions. Results show that formation of these defects is caused by hydrogen atoms which are diffusing from the anode contact. Hydrogen enhances diffusivity of interstitial oxygen and allows it to react with radiation defects containing vacancies and oxygen to form a new type of thermal donors. Their distribution is then given by concentration profile of radiation damage and diffusion length of hydrogen atoms. This effect occurs only in oxygen-rich silicon annealed in air and is absent in oxygen–lean silicon and samples annealed in vacuum.

The Influence of Radiation Defects on the Charge Transport in SiC Nuclear Detectors in Conditions of Elevated Temperatures and Deep Compensation of the Conductivity

P+–n–n+-detector structures based on CVD films with an uncompensated donor concentration of 2×1014 cm-3 have been studied. The p+-region was created by implantation of Al ions. Preliminarily, the detectors were irradiated with 8 MeV protons at a fluence of 3×1014 cm-2 and then annealed in a vacuum at 600°C for 1 h and 700°C for 1 h. Nuclear spectrometric techniques with 5.4 MeV a-particles were employed to test the detectors. In measurements performed in the temperature range 20–150°C, the forward- and reverse-bias modes were compared. It is shown that the annealing leads to a higher collection efficiency of carriers generated by nuclear radiation and to a decrease in the amount of charge accumulated by traps in the course of testing. Despite the positive effect of the annealing, there remains a considerable amount of radiation defects, which is manifested, in particular, in the kinetics of the forward current.

The Influence of Radiation Defects in Meteoritic Nanodiamond on the Elemental Composition of the P3 Noble Gas Component

Radiation defects in meteoritic nanodiamond produced during noble gas implantation have been ignored so far. At the same time, since the density of the defects depends on the mass of implanted ions with the same energy, graphitization of diamonds with a high density of defects during their thermal metamorphism could be an additional cause of the elemental fractionation of noble gases in the nanodiamond remaining after metamorphism. It has been shown that this factor allows one to explain the variations observed in the elemental composition of one of the main components (P3) of noble gases in the nanodiamond of meteorites thermally metamorphosed to a variable degree. It is also supposed that the bimodal release of the P3 noble-gas component upon pyrolysis of the nanodiamond can be caused by a different density of the radiation defects produced in the crystal lattice of nanodiamond grains during noble gas implantation. In this case, a low-temperature peak of gas release results from the destruction of the grains with a defect density higher than the critical one, at which diamond graphitization occurs at a relatively low pyrolysis temperature, while a high-temperature peak of release arises from the destruction of the nanodiamond grains, where the radiation defects have a weak influence on their graphitization temperature.

Characterization of as-grown and heavily irradiated GaN epitaxial structures by photoconductivity and photoluminescence

The influence of radiation defects on photoconductivity transients and photoluminescence (PL) spectra have been examined in semi-insulating GaN epitaxial layers grown on bulk n-GaN/sapphire substrates. Defects induced by 10-keV X-ray irradiation with a dose of 600 Mrad and 100-keV neutrons with fluences of 5×10 14 and 10 16 cm −2 have been revealed through contact photoconductivity and microwave absorption transients. The amplitude of the initial photoconductivity decay is significantly reduced by the radiation defect density. A simultaneous decrease with radiation-induced defect density is also observed in the steady-state PL intensity of yellow, blue and ultraviolet bands peaked at 2.18, 2.85, and 3.42 eV, respectively. The decrease of the PL intensity is accompanied by an increase of asymptotic decay lifetime, which is due to excess carrier multi-trapping. The decay can be described by the stretched exponential approximation exp[ - ( t / τ ) α ] with different values of α in as-grown material ( α ≈ 0.7 ) and irradiated samples ( α ≈ 0.3 ). The value of the fracton dimension d s of the disordered structure, evaluated as d s = 2 α / ( 1 - α ) , changes from 4.7 to 0.86 for as-grown and irradiated material, respectively, implying percolative carrier motion on an infinite cluster of dislocations net in the as-grown material and cluster fragmentation into finite fractons after irradiation.

Mechanisms of forming the Cr–N composite in the unsteady-state stage of ion beam-assisted deposition process

The paper presents the experimental and theoretical study of Cr–N films deposition by the IBAD method. The investigations were performed at various deposition rates. The ion energy was 30keV.Computer simulation methods were used to calculate the parameters of film irradiation with nitrogen ions for obtaining homogeneous chemical composition.The electrophysical properties of films produced were investigated experimentally. It is found that in the initial stages of film deposition the electrical resistivity of films irradiated with nitrogen ions is about 200 times higher than the one in unirradiated films. As the film thickness increases, this difference falls down to a factor of 6.The X-ray diffraction and electron microscopy analysis have revealed that in the films under irradiation Cr2O3 and Cr2N precipitates are formed. The present results are discussed in the framework of the concepts about the influence of radiation defects on the structure of resulting films.

Study of radiation defects for AlN ceramics under O+ irradiation

The behavior of radiation defects in AlN ceramics under O+ ion irradiation has been studied with in situ luminescence method. The intensity of luminescence emission had close relationship with absorbed dose. The aggregation of F-type defects in AlN has also been confirmed because of the occurrence of 470 nm emission peak. After O+ irradiation, the behavior of F-type defects was investigated by ESR measurement at 77 K. Influence of radiation defects on the characteristics of emission and ESR was discussed.

Defects and phase transformation in TGSe and DTGSe crystals

The specific heats of TGSe and DTGSe crystals, pure and γ-irradiated, were measured. The influence of radiation defects on the phase transition in ferroelectric crystals was studied by calorimetric experiments. The Landau free energy expansion coefficients were measured for various doses of γ-irradiation (0 < D < 5 MR). The order of the phase transition changed from the first to the second through the tricritical point in DTGSe crystal.

Radiation-induced defects in -activated yttrium vanadates

In recent years much attention has been paid to the study of radiation-induced defects in laser and luminescent materials, since they affect the optical and stimulated emission properties. Defects in : powder prepared by solid state diffusion have been studied by thermoluminescence (TL) and fluorescence measurements. The defects were accumulated and/or generated by exposing the powder to gamma rays. The influence of radiation defects was studied by comparing the fluorescence spectra of unirradiated and gamma-irradiated samples. The fluorescence output was found to decrease with the presence of defects. Broad and weak characteristic bands at 425 nm due to were observed along with emission lines at 575, 594, and 616 nm due to transitions (j = 0,1 and 2) respectively. Two TL peaks were obtained in the doped powder, whereas only a single peak appeared in the pure samples. The TL emission was also highly enhanced in the doped powder and this increase was attributed to the presence of impurities. Defect accumulation was noted to be strongly dependent on the level of gamma exposure. A suitable explanation of these results is suggested.

Radiation defects and phase transition in some glycine-containing ferroelectrics

The influence of radiation defects on the thermodynamic properties in glycine-containing ferroelectrics (TGSe, DTGSe) was studied in the three regions, small (D < 1 MR), middle (1 MR < D < 5 MR) and large (D > 5 MR) doses of irradiation, by calorimetric experiments. The systematic change of Landau free energy coefficients was detected for the system TGSe-DTGSe and the order of phase transition was changed from the first to the second by small radiation defects concentration in DTGSe crystal. The dose dependence of the basic phase transition parameters was revealed and compared with the result of microscopic theory of defected crystals phase transitions for small defect concentration (independent defects approximation).

Influence of radiation defects on the energy gap in YBa 2Cu 3O 7−δ as measured with the help of Andreev reflection

The influence of radiation-induced defects on the energy gap Δ in YBa 2Cu 3O 7−δ, as measured with the help of Andreev reflection, was investigated. It was found that Δ decreases with irradiation fluence F increasing at a much lower rate than T c. It is shown that δ remains different from zero for F>F c, where F c is the fluence corresponding to T c=0. The results are interpreted as a transition of a Bose condensate of Cooper pairs into a localized state.