Knocked-out Atoms Research Articles

Inelastic collision effects of high-energy neutrons in tungsten materials

Because the inelastic impact of high flux, high-energy neutrons on tungsten material in a commercial fusion reactor would drastically decrease the service life and steady-state functioning of the core plasma in the future, it is vital to investigate the damage process. In this paper, a particle sputtering theory approach is used to derive an equation for calculating the sputtering yield of high-energy neutrons by simulating the transport of primary knocked-out atoms (PKA) resulting from inelastic collisions of 14.1 MeV neutrons with tungsten materials at different depth levels, namely Y =∫E0Emaxf(E)*Y(E)dE, where f (E) is the energy probability distribution function of PKA, Y (E) is the sputtering yield at a given energy of PKA particles generated by inelastic scattering collisions, Emax is the maximum kinetic energy of PKA, and E0 is the minimum kinetic energy of PKA (generally taken as 0). According to the result, when the neutron wall load is 1 MW/m2, and the annual average equivalent material wastage is less than 1 nm, sputtering damage to tungsten material can be ignored. However, it must be kept in mind that the reflection of the PKA into the plasma will have an impact on the performance of the plasma confinement system and the damage caused by the PKA's internal energy.

ВПЛИВ ЛОКАЛЬНИХ МЕХАНІЧНИХ НАПРУЖЕНЬ НА КОЕФІЦІЄНТ РОЗПИЛЕННЯ КРЕМНІЮ ІОННИМ ПУЧКОМ

A review of scientific publications and modeling of the effect of mechanical stresses on the sputtering yield of silicon by an ion beam is carried out. It is shown that the flux of atoms (from the depth to the surface) through interstitial or vacancy mechanisms due to the stress gradient caused by the limiting bending of the plate is insufficient to explain the increase in the sputtering coefficient. Calculations show that even the limiting elastic deformations do not significantly change the energy of atom detachment from the site, and an increase in the drift velocity of atoms due to the enrichment of the near-surface region with vacancies is insufficient to increase the sputtering rate. Consequently, it is necessary that the elastic deformation is transformed into plastic with the formation of mobile weakly bound atoms. The calculated stress distribution in a loaded silicon wafer using the COMSOL Multiphysics software package showed that the key driving force behind the increase in the silicon sputtering coefficient is the concentration of compressive and tensile stresses in the vicinity of the simulated crater during sputtering. The created crater is a stress concentrator, the gradients of which significantly exceed the values obtained by bending a plate without a crater. It is demonstrated that the generated stresses exceed the ultimate strength of the material in the vicinity of the crater, which begins to relax due to the expulsion of "excess" atoms in the tension region. The appearance of additional deformation-stimulated fluxes of weakly bound surface atoms at the bottom and walls of the crater provides an increase in the concentration of knocked-out atoms in the process of ion sputtering. Simulations predict an increase in sputtering yield of up to 40%. It is also shown that closely spaced craters, due to elastic interaction with each other, compensate each other's elastic fields, which has an effect on the value of the sputtering coefficient.

Role of the Carbon Sublattice in n-SiС Conductivity Compensation

The formation of radiation defects in the silicon-carbide sublattice under irradiation with 15-MeV protons and 0.9-MeV electron is analyzed. Numerical simulation is carried out, and histograms of the distribution of the energy transferred to carbon recoil atoms are obtained. Two energy ranges are considered when analyzing the histograms. Single isolated Frenkel pairs with closely located components are produced in the low-energy range. The recoil atom energy is sufficient to produce a displacement cascade in the other range. As the energy of primary knocked-out atoms increases, the average distance between genetically related Frenkel pairs increases, and as a consequence, the fraction of pairs that do not recombine under irradiation increases. The recombination radius of the Frenkel pair in the carbon sublattice is estimated.

First-principles investigation of neutron-irradiation-induced point defects in B4C, a neutron absorber for sodium-cooled fast nuclear reactors

Boron carbide (B4C) is a leading candidate neutron absorber material for sodium-cooled fast nuclear reactors owing to its excellent neutron-capture capability. The formation and migration energies of the neutron-irradiation-induced defects, including vacancies, neutron-capture reaction products, and knocked-out atoms were studied by density functional theory calculations. The vacancy-type defects tend to migrate to the C–B–C chains of B4C, which indicates that the icosahedral cage structures of B4C have strong resistance to neutron irradiation. We found that lithium and helium atoms had significantly lower migration barriers along the rhombohedral (111) plane of B4C than perpendicular to this plane. This implies that the helium and lithium interstitials tended to follow a two-dimensional diffusion regime in B4C at low temperatures which explains the formation of flat disk like helium bubbles experimentally observed in B4C pellets after neutron irradiation. The knocked-out atoms are considered to be annihilated by the recombination of the close pairs of self-interstitials and vacancies.

Formation of Radiation Defects by Proton Braking in Lightly Doped n- and p-SiC Layers

Mathematical simulation of the cascade of displacements in SiC is used to consider the specific features of Frenkel-pair generation upon the scattering of 8- and 15-MeV protons. The distribution histograms of energies acquired not only by primary knocked-out atoms, but also by recoil atoms generated in displacement cascades, are calculated. An analysis of the histograms considers two energy ranges. In the first range of “low” energies, the spontaneous recombination of genetically related Frenkel pairs is dominant. Recoil atoms in the second range have a higher energy, which enables these atoms to leave the spontaneousrecombination zone and dissociate into isolated components. The compensation of lightly doped n- and p-4H-SiC samples grown by gas-phase epitaxy is experimentally studied under irradiation with 8- and 15-MeV protons. The carrier removal rates are measured. The calculated and experimental data are compared and estimates are obtained for the size of the spontaneous-recombination zone.

Образование радиационных дефектов в слабо легированных слоях n- и p-SiC при торможении протонов

AbstractMathematical simulation of the cascade of displacements in SiC is used to consider the specific features of Frenkel-pair generation upon the scattering of 8- and 15-MeV protons. The distribution histograms of energies acquired not only by primary knocked-out atoms, but also by recoil atoms generated in displacement cascades, are calculated. An analysis of the histograms considers two energy ranges. In the first range of “low” energies, the spontaneous recombination of genetically related Frenkel pairs is dominant. Recoil atoms in the second range have a higher energy, which enables these atoms to leave the spontaneousrecombination zone and dissociate into isolated components. The compensation of lightly doped n - and p -4 H -SiC samples grown by gas-phase epitaxy is experimentally studied under irradiation with 8- and 15-MeV protons. The carrier removal rates are measured. The calculated and experimental data are compared and estimates are obtained for the size of the spontaneous-recombination zone.

Effect of the energy of bombarding electrons on the conductivity of n-4H-SiC (CVD) epitaxial layers

The electrical characteristics of epitaxial layers of n-4H-SiC (CVD) irradiated with 0.9 and 3.5MeV electrons are studied. It is shown that the donor removal rate becomes nearly four times higher as the energy of impinging electrons increases by a factor of 4, although the formation cross section of primary radiation defects (Frenkel pairs in the carbon sublattice) responsible for conductivity compensation of the material is almost energy independent in this range. It is assumed that the reason for the observed differences is the influence exerted by primary knocked-out atoms. First, cascade processes start to manifest themselves with increasing energy of primary knocked-out atoms. Second, the average distance between genetically related Frenkel pairs grows, and, as a consequence, the fraction of defects that do not recombine under irradiation becomes larger. The recombination radius of Frenkel pairs in the carbon sublattice is estimated and the possible charge state of the recombining components is assessed.

Dynamic model of the threshold displacement energy

A dynamic (cascade-probability) model for calculating the threshold displacement energy of knocked-out atoms (Ed) was proposed taking into account the influence of the instability zone (spontaneous recombination). General expression was recorded for Ed depending on the formation energy of interstitial atoms Ef and vacancies Ei, on the energy transfer coefficient α and the number of interactions i needed to move the atom out of the instability zone. The parameters of primary particles were calculated. Comparison of calculations with experimental data gives a satisfactory agreement.

Effect of recoil atoms on radiation-defect formation in semiconductors under 1–10-MeV proton irradiation

The formation of radiation defects in Si under 1–10-MeV proton bombardment is analyzed. Numerical simulation is carried out, and histograms of the distribution of the energy transferred to recoil atoms are obtained. Two energy ranges are considered when analyzing the histograms. Single Frenkel pairs with closely located components are produced in the first range (small energies). Recoil atoms of the second range have an energy sufficient for the production of a displacement cascade. As a result, nanoscale regions with high densities of vacancies and different types of their complexes appear. In addition, as the energy of the primary knocked-out atoms increases, the average distance between genetically related Frenkel pairs increases, and, as a consequence, the fraction of pairs that are not recombined under bombardment increases.

Formuvannya stiykykh poverkhnevykh struktur pry ionnomu rozporoshenni v ramkakh anizotropnoyi modeli Kuramoto–Sivashyns'koho

The processes of change of a surface morphology and formation of a stationary pattern at the ion sputtering are considered. A linear stability analysis was carried out, and the range of parameters, at which the patterning is possible, is determined. Assuming the existence of a stabilization parameter that involves the redistribution of knocked-out atoms, all evolution scenarios for the surface are obtained numerically. The dynamics of defects is numerically analyzed for every structure type, and the corresponding time dependences are plotted.

Computer simulation radiation damages in condensed matters

As part of the cascade-probability method were calculated the energy spectra of primary knocked-out atoms and the concentration of radiation-induced defects in a number of metals irradiated by electrons. As follows from the formulas, the number of Frenkel pairs at a given depth depends on three variables having certain physical meaning: firstly, Cd (Ea h) is proportional to the average energy of the considered depth of the PKA (if it is higher, than the greater number of atoms it will displace); secondly is inversely proportional to the path length λ2 for the formation of the PKA (if λ1 is higher than is the smaller the probability of interaction) and thirdly is inversely proportional to Ed. In this case calculations are in satisfactory agreement with the experimental data (for example, copper and aluminum).

Role of the recoil atom energy in the formation of radiation-induced defects in semiconductors under electron bombardment

A comparative analysis of the formation of radiation-induced defects in semiconductors (silicon and silicon carbide are used as examples) under bombardment with electrons with energies of 0.9 MeV or higher is carried out. Experimental values of the rate of charge-carrier removal at electron energies of 0.9 MeV are less by an order of magnitude than at higher electron energies (6–9 MeV). The formation cross section for primary radiation defects (Frenkel pairs) in this range is almost energy-independent. It is suggested that the reason for this difference is the influence of the energy of primary knocked-out atoms. As the energy of these atoms increases, first, the average distance between genetically related Frenkel pairs increases, and, as a consequence, the fraction of pairs that are not recombined under bombardment increases. Second, the possibility of forming new, more complex, secondary radiation effects appears as the energy of the primary knocked-out atoms increases.

Uniformity of Properties of 4H-SiC CVD Films under Exposure to Radiation

Non-uniformities of electrical properties of 4H-SiC CVD films have been revealed using physico-chemical reactions occurring upon introduction of radiation-induced structural defects. Primary knocked-out atoms and vacancies actively interact with defects of the starting material and thereby form the final system of radiation centers. The samples were irradiated with 900 keV electrons and 8 MeV protons at doses not leading to conductivity compensation ( 7.5  1012 cm–2) and a dose of 6  1014 cm–2 causing deep compensation. Capacitance methods demonstrated that characteristics of samples ~3 mm in size are not identical. The nuclear spectrometry technique, which enables microprobing of samples, demonstrated individual behavior of separate parts of a film with areas of tens of square micrometers (the dimension of α-particles track cross-section).

Radiation hardness of SiC subjected to alternating irradiation and annealing

Effect of the cycle “introduction of defects, annealing, and repeated introduction of defects” on the SiC properties has been studied by means of nuclear spectrometry for an example of degradation of characteristics of a p-n nuclear radiation detector. The defects were introduced by irradiation with 8-MeV protons in two equal fluences of 3 × 10 14 cm−2. The total fluence of 6 × 1014 cm−2 corresponded to an introduction of 2.4 × 10 17 cm−3 primary knocked-out atoms. The annealing was made in two stages, each 1 h long, at temperatures of 600 and 700 °C. The detectors were tested with 5.4-MeV α particles, with the charge collection efficiency and specific features of the amplitude spectrum determined. The measurements were performed in the temperature range of 20–250 °C. It was shown that the effect of the first irradiation and the subsequent annealing does not significantly change the radiation hardness of SiC. The effective concentration of centers introduced in the course of the second irradiation (at the same fluence) is higher by a factor of 1.2. The nonequivalence of the fluences can also be attributed to the effect of the high total proton fluence of 6 × 1014 cm−2.

Specific features of use of wide-gap semi-insulating materials for recording of nuclear radiation

Operation of detectors in strong radiation fields or an imperfect fabrication technology gives rise to a considerable amount of structural defects in materials. Using a p +-v-n + detector structure in the unconventional forward-bias mode has been considered as a way to minimize the space-charge accumulation caused by long-term capture of carriers to deep levels. The efficiency of carrier transport and the signal amplitude spectrum were studied as functions of the applied voltage and time of signal shaping by the electronic circuit for the example of a CVD-grown 4H-SiC film containing up to 2.5 × 1017 cm−3 of primary knocked-out atoms. The detector was tested with 5.4 MeV α particles in the temperature range 20–140°C. It was demonstrated that, despite the transport of only one-fourth of the whole amount of charge, the main line of the spectrum has a high energy resolution (7%) for the given conditions.

Grazing scattering of fast rare-gas atoms by adsorbed monolayers as a method of investigating gas–surface interaction with Ångström spatial resolution

A new method is proposed for studying the dynamics of interaction between superthermal (0.1–10 eV) gas particles and a solid surface with Ångström spatial resolution. The method is based on the use of grazing scattering of fast rare-gas atoms ( θ=1–5°, E 1=1–10 keV ) by monolayers of physically adsorbed particles. Due to the impulsive character of interaction and the translational symmetry of the physisorbed monolayer, one can introduce the concept of a pseudobeam of knocked-out atoms directed toward a certain point of the surface unit cell. An example of grazing scattering of fast Ar atoms by the ordered argon monolayer physisorbed on the Pt (1 1 1) surface is considered in detail. It is shown that the energy transfer to the platinum surface from the superthermal argon atoms to different points of surface can be measured in this case.

A comparative study of induced damage after irradiation with swift heavy ions, neutrons and electrons in low doped silicon

A systematic study has been performed, investigating the radiation damage in high purity low doped silicon. During this study we irradiated bulk silicon samples and silicon detectors manufactured from the same bulk material. Identical series of samples (bulk and detectors) were irradiated with different projectiles: swift heavy ions, neutrons and electrons. The characterization of the samples has been performed with different experimental techniques: DLTS, Hall effect, photoluminescence, current–voltage I(V) and capacitance–voltage C(V) measurements. These measurements allow us to extract some quantitative informations about the radiation damage, in particular the introduction rates of vacancy-doping and divacancy complexes, and also the leakage current damage constant for irradiated detectors. In order to explain the macroscopic damage starting from the microscopic one, we calculate for each projectile a set of parameters: electronic energy loss, nuclear energy loss, total displacement cross-section, number of primary knocked-out atoms, displacement cascade size, density of energy deposited in a cascade by nuclear collisions. The experimental results suggest a way to explain the differences between the radiation damage effects of the different projectiles, taking into account the incidence of the above stated microscopic parameters. A comparison is made with the results obtained by other authors concerning the radiation damage of fullerene-irradiated silicon.

Radiation stability of corundum ceramic with pulsed irradiation by fast neutrons

Ceramic dielectrics are promising electrical insulating materials for thermonuclear power generation, which explains the interest towards changes of their microstructure and electrophysical properties in conditions of intense radiation action. The calculation of the number of displaced atoms was performed by taking account of elastic and inelastic scattering of neutrons with 14-MeV energy. The expression obtained for the energy distribution of the initially knocked-out atoms in the laboratory system of coordinates is presented. An experimental investigation of the process of defect formation was carried out by measuring the radiation conductivity by means of an electron microscope and the method of positron annihilation. The calculations and experiments performed confirm that changes of conductivity of ceramics based on A1/sub 2/O/sub 3/, observed in the case of the combined action of gamma-radiation and neutrons, are caused by the action not of the neutrons but of the gamma radiation.

The radiation annealing of frenkel pairs and the rate of the point defect accumulation in neutron irradiation

Abstract In neutron irradiation the primary knock-out atom, when producing secondary defects and slowing down, releases its energy in the cascade zone which, being heated up to a temperature higher than the ambient, provides the migration of the Frenkel pairs and their annihilation (radiation annealing). The system of equations which is presented describes the diffusion and annihilation of the defects and the cooling of the cascade zone resulting from the lattice and electron heat transfer. The solution of the system, using simplifying assumptions, brings one to the conclusion that the effectiveness of the Frenkel pair production, by the primary knocked-out atoms with an energy lower than a certain boundary energy, decreases because the atom dissipates its energy in a region of smaller dimensions, which is heated to higher temperature and, thus, the more incisive radiation annealing of the Frenkel pairs is provided. In accordance with this the number of secondary knocked-out atoms at energies lower than t...