Primary Recoil Energy Research Articles

Simulation of neutron irradiation damage in lead lanthanum zirconate titanate by Monte Carlo method

Lead lanthanum zirconate titanate (PLZT) has a broad application prospect for energy storage devices with high energy density, since it possesses excellent dielectric and energy storage properties. To investigate the irradiation damage to the PLZT induced by neutrons with different energy, the primary energetic recoil spectra of each kind of element are first extracted from the transportation simulations of neutrons with energy ranging from 1 to 14 MeV, respectively. Then, the displacement damages (including vacancies and interstitial atoms) induced by each type of recoil with different energy are simulated based on the binary collision approximation method. Finally the number of defects in PLZT produced by neutrons with an energy range from 1 to 14 MeV is calculated based on the recoil energy spectra and the defect number produced by the recoils. The results show that the number of defects produced in the PLZT material with a thickness of 3 cm is approximately independent of the neutron energy for the fast neutrons with energy in a range from 1 to 14 MeV, even though the primary recoil energy spectra from neutrons with different energy are completely different. The average number of defects produced in 3-cm-thick PLZT is about 460 ± 120 vacancies/neutrons. For neutrons with energy ranging from 1 to 14 MeV, the produced defect concentration in PLZT decreases slightly with the depth increasing within a thickness of 3 cm. The difference in defect concentration in this 3 cm is in a range of 50%. This decrease is caused mainly by the fact that some of neutrons are back-scattered during transport. The average defect concentration produced by neutron irradiation in the PLZT with a thickness of 3 cm is slightly(～20%) higher than that in the PLZT with a thickness of 1 mm. The reason for the higher defect concentration in a thicker (3 cm) PLZT can be attributed to the following facts: (i) the (n, 2n) reactions between neutron and material can make the number of neutrons increase during transport; (ii) the scattering can make the path of neutron longer; (iii) the inelastic scattering can lead to a smallnumber of moderated neutrons, which have a slightly larger interaction cross section with materials. This indicates the damage produced in thick PLZT is quite complicated and closely related to the process of neutron transport. This work presents a method of calculating the displacement damage of neutrons in materials, and the simulation results can provide guidance for studying the neutron irradiation effects of PLZT-based electronic devices.

Deuterium retention in tungsten irradiated by different ions

Tungsten was irradiated with different ion species (H, D, He, Si, Fe, Cu, W) at energies between 0.3 and 20.3 MeV to two different calculated damage levels of 0.04 dpa and 0.5 dpa. Samples were exposed to a low-temperature deuterium (D) plasma at 370 K to decorate the radiation defects. D retention was studied by nuclear reaction analysis using the D(3He, p)α reaction and by thermal desorption spectroscopy. For tungsten irradiated by light ions (H, D, He) the depth profiles as well as D desorption spectra show clear differences. On the other hand, tungsten irradiated by medium- to high-mass ions (Si, Cu, Fe, W) to identical dpa values shows similar D depth profiles and nearly identical D desorption spectra, i.e. the D retention is comparable. Since the large differences in the primary recoil energy distribution due to different incident energies of the different ions (Si, Cu, Fe, W) do not result in a different D retention, we assume that neutron irradiation of tungsten in a future fusion reactor will result in a similar D retention as in tungsten damaged by medium- to high-mass ions. Irradiation with ions with a mass of 28 amu (Si) or higher seems to be a suitable proxy for investigating the influence of displacement damage by neutrons on D retention.

Damage production in atomic displacement cascades in beryllium

The paper presents the results of a molecular dynamics simulation of cascade damage production in beryllium caused by self-ion recoils in the energy range of 0.5–3keV. It is demonstrated that point defects are produced in Be preferentially in well-separated subcascades generated by secondary and higher order recoils. A linear dependence of the point defect number on the primary recoil energy is obtained with the slope that corresponds to formal atom displacement energy of ∼21eV. Most of the damage is created as single defects and the relatively high part of created point defects (∼50%) survives the correlated recombination following the ballistic cascade stage and becomes freely-migrating.

Atomic collision cascades on void evolution in vanadium

Molecular dynamics simulations were performed to study void evolution subject to unidirectional self-bombardment and radiation-induced variation of mechanical properties in single crystalline vanadium. 3D simulation cells of perfect body-centered cubic (BCC) vanadium, as well as those with one, two, four, and six voids, were investigated. For the no void case, the maximum number of defects, maximum volumetric swelling, and the number of defects left in bulk after a sufficiently long recovery period increased with higher primary recoil energy. For the cases containing voids, a primary recoil energy was carefully assigned to an atom so as to initiate a dense collision spike in the voids center, where some self-interstitial atoms gained kinetic energy by secondary replacement collision sequence traveling along the ⟨ 111⟩ direction. It is found that the larger or the greater the number of voids contained initially in the box, the larger the normalized void volume, and the smaller the volumetric swelling after sufficient recovery of systems. In the single void case, the void became elongated along the bombarding direction; in the multiple void cases, the voids coalesced only when the intervoid ligament distance was short. After sufficient relaxation of the irradiated specimen, a hydrostatic tension was exerted on the box, where the voids were treated as dislocation sources. It is shown that with higher primary recoil energy, the yield stress dropped in cases with smaller or fewer voids but rose in those with larger or greater number of voids. This radiation-induced softening to hardening transition with increasing dislocation density can be attributed to the combined effects of the defect-induced dislocation nucleation and the resistance of defects to dislocation motion. Moreover, as the primary recoil energy increased, the ductility of vanadium in the no void case decreased, but was only slightly changed in the cases containing void.

A model study of displacement cascades distributions in zirconium

5 to 200keV displacement cascades in zirconium are studied in the binary collision approximation with the simulation code Marlowe. The cascades are analysed statistically by means of component analysis, fuzzy clustering and isodata analysis. As a consequence of the large recoil ranges and range straggling specific to open hcp lattices like Zr, a large dispersion of the frequencies of Frenkel pair distributions is found, as well as of the spatial extent and morphology of vacancy and interstitial distributions. In Zr, cascades are formed by a widespread distribution of displacement clusters that can be small. Remarkably, the size and morphology distributions of these clusters are found independent of the primary recoil energy in the energy range investigated.

Cascade and subcascade structure in fission neutron irradiated fcc metals and their correlation to fusion neutron irradiation

Thin foil specimens of Cu and Au were irradiated with fission neutrons at 285–295 K to neutron fluences of 7 × 10 21 n m −2 (>0.1 MeV), using the Kyoto University Reactor. The structure of cascades and subcascades was examined by fitting the experimentally observed distribution of vacancy-type defect clusters to the calculated primary recoil energy spectrum, and was compared directly with that for fusion neutron irradiation. In Cu, 70% of cascades consist of only one defect cluster and 2% of more than four clusters. This is in contrast to larger cascades produced by 14 MeV neutrons that contain more than 10 clusters and extend 60 nm in diameter. The subcascade energy was 18 and 15 keV for Cu and Au, respectively. These values are about 30–60% higher than those obtained for fusion neutron irradiation.

Heavy-ion cascade effects on radiation-induced segregation kinetics in Cu–1%Au alloys

Various dual ion irradiations were conducted to investigate the effect of heavy-ion cascades on the fluxes of freely migrating defects which drive radiation-induced segregation (RIS) in Cu–1at.%Au alloys. In situ Rutherford backscattering spectroscopy (RBS) was used to measure the RIS suppression effect of heavy-ion bombardment (with 300-keV Al +, 800-keV Cu +, and 1.2-MeV Ag +) on 1.5-MeV He +-RIS of Au in the near-surface region of the alloy during concurrent He + and heavy-ion irradiations at 400°C. Results demonstrated that the suppression of He +-RIS correlated well with the cascade volume produced by concurrent Al +, Cu +, and Ag + irradiation per second and was independent of the weighted average primary recoil energy. Model calculations of the kinetics of RIS during dual beam irradiation were also performed and compared with the measurements. Information regarding the energetics of freely migrating point defects and their relative production efficiencies was obtained from systematic fitting. Using the values previously reported for the energies of formation and migration for vacancies and interstitials in Cu, the binding and migration energies of Au-interstitial and Au-vacancy complexes in the alloy were found to be −0.14, 0.15, and 0.03 and 0.76 eV, respectively. The respective derived efficiencies of freely migrating defect production by energetic He +, Al +, Cu +, and Ag + ions were 0.25, 0.12, 0.09, and 0.08.

Destination of point defects and microstructural evolution under collision cascade damage

Microstructural evolution strongly depends on the annihilation sites of point defects and the time sequence for point defect reactions. Factors to determine the destination of point defects in collision cascade damage are categorized and the mechanism to control the destination is examined. Major factors that determine the destination are the initial distribution of point defects, the type and distribution of annihilation sites, and the style of migration of point defects and their mobility. The initial distribution of point defects varies with primary recoil energy, materials parameters and damage rate. Major annihilation sites for point defects are point defects, point defect clusters and permanent or fixed sinks such as dislocations, grain boundaries, surfaces, etc. Microstructural evolution strongly depends on the geometrical relationship between annihilation sites and cascades. The mobility and direction for migration of point defects are changed by the irradiation temperature and the strain fields from various sources, respectively. The consequence of the different mode of point defect migration is also discussed.

Effects of primary recoil (PKA) energy spectrum on radiation damage in fcc metals

Irradiation effects by different energetic particles such as electrons, various ions and neutrons are compared in fcc metals, particularly in Cu and Ni. It is discussed on the statistical consideration that the logarithm of the so-called PKA median energy, log T 1/2, is a good representative to characterize the primary recoil (i.e. PKA) energy spectrum with the resultant defect production. For the irradiations of electrons, various ions and neutrons to Cu and Ni, fundamental physical quantities such as the fraction of stage I recovery, the normalized defect production cross sections and the normalized radiation annealing cross sections can be well scaled as a function of log T 1/2, if the effects of the electron excitation caused by irradiating ions are excluded. (Here, the experimental cross sections are normalized with the calculated cross sections obtained from the NRT model.) Namely, all data of the respective physical quantity lie on a single continuous curve as a function of log T 1/2. This characteristic curve is utilized to predict the damage accumulation (i.e. defect concentration) as a function of dpa in Cu and Ni with the PKA median energy as a parameter.

Recoil energy spectrum effect in vanadium by electron and light- and heavy-ion irradiation

In order to understand defect reaction processes under cascade damage condition in vanadium, as one of the low activation materials for fusion reactor application, defect microstructures induced by light- and heavy-ion irradiations were investigated. The interest was focused to the difference in the primary recoil energy spectrum; collisions with small recoil energy are abundant in light-ion irradiation, whereas there are only large recoils in heavy ion irradiation. Electron irradiation with a high voltage electron microscope was utilized for the identification of the nature of ion irradiation induced point defect clusters, through the behavior of defect clusters under electron irradiation.

Cascade damage formation in gold under self-ion irradiation

To understand cascade damage formation as a function of primary recoil energy, thin foils of gold were irradiated with 20–400 keV self-ions to a fluence of 1.0 × 10 14 ions/m 2 at 300 K. The yield of groups of vacancy clusters per irradiated ion saturated at an ion energy higher than 100 keV. The number of clusters in a group was found to vary even for the same energy ions. The size distribution of the clusters was not strongly dependent on the number of clusters in a group and the ion energy. An attempt was made to predict the distribution of defect clusters in thin foils of gold irradiated with 14 MeV neutrons from the present data based on a primary recoil energy spectrum.

Binary collision calculation of subcascade structure and its correspondence to observed subcascade defects in 14 MeV neutron irradiated copper

A model calculation of subcascade structure in copper, based on the binary collision approximation using MARLOWE code, is made. The primary recoil energy spectrum by 14 MeV neutron irradiation and the threshold type subcascade energy of 10 keV are employed. The calculated results of the number of subcascades and the distance between subcascades show good agreement to reported experimental analysis. Distribution of the calculated cascade zone size extends towards larger size much more than that experimentally obtained for thin foil irradiation. This difference is understood as the under estimation in experimental measurement owing to the limitation by the thickness of the microscopically observable specimen foil (< 100 nm). Distribution of subcascade structure produced by monochromatic primary recoil energy is examined by the same calculation model. Primary recoil energy dependence of subcascade structure is also examined. The usefulness and the limitation of the present model calculation is discussed.

Influence of primary recoil energy spectrum on microstructural evolution

In order to decompose the effect on microstructure evolution from a wide recoil energy spectrum of neutron irradiation, self-ion irradiation with variable energy is performed. Binary collision simulation of subcascade formation is made in parallel with experiments. Microstructure formed by 14 MeV neutron irradiation is reconstructed both from self-ion irradiation data and from collision simulation data. The concept of subcascade formation is reconsidered from the viewpoint of microstructural evolution. Examination of the extremely small success probability of cascade collision induced interstitial cluster nuclei to grow into dislocation loops leads to a proposal of the important role of stochastic fluctuation of point defect reaction. Various sources which cause the unbalanced reaction of vacancies and interstitials are examined: interstitial predominance by vacancy cluster formation, vacancy predominance by interstitial cluster formation, and a particular emphasis on the cascade localisation induced bias effect.

Self-ion irradiation of fcc metals to elucidate primary recoil energy dependence of defect structure development during neutron irradiation

Irradiation effect with neutrons is generated from the superposition of recoils with a wide energy distribution, and knowledge of the effect from each recoil is essential. Self-ion irradiation is performed for fcc metals and induced defect structures are analyzed placing the prime interest in the variation of defect structures within the same ion energy and its variation with the ion energy. Mainly discussed are the data on silver as a typical material with closely spaced subcascades. Tendency for saturation of vacancy cluster formation is found to start at low irradiation dose, as low as 10 15 ions/m 2, and the intercascade reaction is discussed from this dose dependence. An appreciable percentage of incident ions is found not to contribute in vacancy cluster formation. A wide variation of the formation of vacancy cluster groups was found for the same ion energy. Intracascade/intersubcascade reactions is discussed from the variation of vacancy cluster size and the number of the clusters in groups. A method is proposed to construct neutron irradiation induced defect structures from ion irradiation data.

Influence of primary recoil energy spectrum on microstructural evolution

Influence of primary recoil energy spectrum on microstructural evolution

A comment on future intense high energy neutron sources from the variation of microstructural evolution by the difference in recoil energy spectrum

Abstract The recoil energy spectrum of the irradiation from presently proposed intense high energy neutron sources for the development of fusion reactor materials deviates appreciably from that in the fusion environment. A wide variation of representative elements of defect structure evolution are quoted from the experimental results and analyses of irradiation with various energetic particles; electrons, ions and neutrons in fission reactors and fusion neutrons. After all the discussion on the influence of primary recoil energy spectrum, a strong warning is sent concerning the establishment of a neutron source without serious consideration of the problems arising from the difference in the recoil energy spectrum from that in the fusion environment. The establishment of a medium sized neutron source with a nuetron energy spectrum closest to that in the fusion environment is strongly recommended.

Recoil energy spectrum effects and point defect processes characteristic of cascades

A procedure is proposed for the primary recoil energy spectrum analysis of the effects of high energy particle irradiation. Comparisons are made between fusion and fission neutrons. Examples of the analysis of cascades and subcascades by 14 MeV neutrons are quoted. Estimation of the deposited energy density and the concept of subcascade energy are among the major consequences. Point defect processes, including the point defect cluster formation from cascades and the reaction of freely migrating defects, are reviewed. The importance of two kinds of point defect processes characteristic of cascade collision damage is emphasized. These are the impact effect from a cascade to point defects produced by other cascades and the effect of the difference in the localization of vacancies and interstitials in a cascade (Cascade Localization Induced Bias (CLIB)).

Production of freely-migrating defects

The production rate of freely-migrating defects (those which migrate over distances large compared to typical cascade dimensions) decreases much more strongly with increasing primary recoil energy than does that for creating stable defects at liquid-helium temperatures. For fission or fusion neutron irradiations, the fraction of freely-migrating defects becomes only ~1% of the calculated dpa. This large decrease in the fraction of freely-migrating defects can be understood in terms of additional intracascade recombination, although recently it has been suggested that cascade remnants, e.g. vacancy dislocation loops, also contribute by enhancing intercascade defect annihilation. Measurements of low-temperature defect production as a function of primary recoil energy are briefly reviewed. The experimental evidence for the substantially stronger decrease in freely-migrating defect production with increasing recoil energy is summarized, and the role of vacancy loops is discussed. On the basis of the existing experimental information on vacancy-loop production as a function of irradiation temperature, it is argued that intracascade recombination remains the most probable explanation for the strong decrease in freely-migrating defect production with increasing recoil energy.

Fission-fusion correlation by fission reactor irradiation with improved control

The necessity for the elimination of exposure to neutrons at lower temperatures during start-up and shut-down of the reactor is confirmed by experiments which compare the result of irradiation of metals with conventional and improved temperature control in JMTR. Only several percent of exposure to neutrons at lower temperatures is found to result in a one hundred percent difference of radiation induced microstructures in some cases. All differences can be understood from the microstructural development mechanisms, i.e. from the temperature dependence of the stability of point defect clusters and from the relationship of the transient temperature to the temperature for nucleation and growth. Fission neutron irradiation data with improved control are compared with fusion neutron irradiation data from RTNS-II. The differences of vacancy and interstitial clusters formed directly from cascades, observed in samples irradiated as thin foils, are understood when the difference in the primary recoil energy spectrum and the thermal stability of the clusters are taken into consideration. The defect structures which are developed and/or modified by the reactions of freely migrating point defects, such as vacancy clusters interstitial type dislocation structures and voids, observed in samples irradiated as bulk, are remarkably different in the two cases. Factors to be applied to the fission neutron irradiation dose to introduce microstructures equivalent to those by fusion neutrons are found to range widely, depending on the kind of microstructure, materials and irradiation temperature, from a value less than one up to 30 in the scaling of damage energy per atom.

Recoil energy spectrum analysis and impact effect of cascade and subcascade in 14 MeV D-T fusion neutron irradiated fcc metals

Abstract An analysis of cascade and subcascade formation in fcc metals by D-T fusion neutron irradiation is made by fitting the experimentally observed distribution of cascade zone size and number of subcascades to the calculated primary recoil energy spectrum. The analysis is made by categorizing subcascades into closely space (Ag, Au) and widely separated (Cu, Ni). The energy subdivided in subcascades is estimated. The estimated energy density in a subcascade is as high as several tens of eV/atom, which is high enough to raise the local temperature far beyond the melting temperature. A discussion is made on the reactions during cascade cooling, suggesting the inadequacy of the conventional radiation damage parameter of DPA. The existence of impact effects from other cascades to reveal the cascade collision induced invisible vacancies as visible vacancy clusters is concluded. A kinetics type of analysis is made on the microstructure evolution by the impact effect, and the sphere of influence of the impact is estimated for each material.