Energy Spectrum of Primary Knock-on Atoms and Atomic Displacement Calculations in Metallic Alloys Under Neutron Irradiation

Abstract

Materials subjected to neutron irradiation experience damage due to displacement cascades triggered by nuclear reactions. This paper presents a practical method to calculate primary atomic recoil events (PKAs), which lead to cascade damage, based on energy and recoiling species. We developed a custom code to identify PKAs and extract their properties using MCNPX and SRIM. This code determines the specifications of recoil atoms from the data provided by the PTRAC card in MCNPX. Consequently, the energy spectrum of PKAs generated through various reaction channels, including elastic/inelastic scattering and transmutations such as (n, α), (n, p), and (n, γ), is calculated. This PKA spectrum is then input into SRIM, which calculates the total number of atomic displacements using the binary collision approximation (BCA) and provides crucial information about the spatial distribution of defects within the irradiated material. Our results indicate that elastic scattering is the predominant reaction, producing most PKAs with energies in the range of several keV. In contrast, inelastic scattering becomes the dominant interaction for generating high-energy PKAs (~EPKA>1 MeV). Additionally, we observed that the number of Frenkel pairs versus ion energy curves for light particle ion implantation (such as H and He) is significantly smaller than for heavier ions.