Subcascade formation and defect cluster size scaling in high-energy collision events in metals

Abstract

Az -Theory and Models of Radiation Effects PACS 61.82.Bg -Metals and Alloys PACS 61.72.J--Point Defects and Defect clustersAbstract -Recently, a scaling law describing the formation of defect clusters under irradiation has been established [1,2]. A critical constraint associated with its application to phenomena occurring over a broad range of irradiation conditions is the limitation on the energy of incident particles. Incident neutrons or ions, with energies exceeding a certain energy threshold, produce a complex hierarchy of collision subcascade events, which impedes the use of the defect cluster size scaling law derived for an individual low-energy cascade. By analyzing the statistics of subcascade sizes and energies, we show that defect clustering above threshold energies can be described by a product of two scaling laws, one for the sizes of subcascades and the other for the sizes of defect clusters formed in subcascades. The statistics of subcascade sizes exhibits a transition at a threshold energy, where the subcascade morphology changes from a single molten domain below the energy threshold, to several or many molten sub-domains above the threshold. The number of sub-domains then increases in proportion to the primary knock-on atom energy. The model has been validated against direct molecular dynamics simulations and applied to W, Fe, Be, Zr and sixteen other metals, enabling the prediction of full statistics of defect cluster sizes with no limitation on the energy of cascade events. We find that populations of defect clusters produced by the fragmented high-energy cascades are dominated by individual Frenkel pairs and relatively small defect clusters, whereas the lower-energy non-fragmented cascades produce a greater proportion of large defect clusters.

Evolution of small defect clusters in ion-irradiated 3C-SiC: Combined cluster dynamics modeling and experimental study

“…. This relationship is qualitatively consistent with cluster size distribution in irradiated 3C-SiC [54] and with previously reported results for W, Fe, Be, Zr [55][56][57][58] and Mo [59]. Here, we vary B from 0.0 to 2.5, which covers the range of intra-cascade cluster production functions from cascade simulations in Refs.…”

Direct observation of the spatial distribution of primary cascade damage in tungsten

“…experimental cascades are similar in spatial extent. This is consistent with the subcascade splitting threshold being around 150keV for self-ions in tungsten, a value of 160keV has been suggested recently based on analysis of BCA cascades [30]. We emphasize that we are not claiming to see loops separated by λ = 0.75nm, but rather that the results are most consistent with the tails of a generating distribution with the characteristic size λ = 0.75nm.…”

“…produced in the same cascade. A single high-energy displacement cascade can be thought of as a branching tree of subcascades [28,29,30]. A full analysis of the cascade spatial structure should therefore have a rule for the production of defects within a single subcascade, a rule for the branching of a cascade into subcascades, and a rule for the separation of subcascades.…”

“…We can therefore continue by considering each cascade to be spherically symmetric. This is valid in the case of tungsten cascades, where the subcascade splitting threshold is high [31,30]. For high-energy iron cascades it may be necessary to use a full formulation including the treatment of splitting of a cascade into subcascades (see section 3.1).…”

Cascade fragmentation: deviation from power law in primary radiation damage

“…We find parameters for the distribution of subcascade sizes (Equation (4)) using a recently developed method, detailed elsewhere [18], based on the binary collision approximation (BCA) [19]. We have simulated 200 cascades with the BCA method for each PKA energy.…”