PKA (irradiation)
Primary Knock-on Atom (PKA) (Irradiation)
A Primary Knock-on Atom (PKA) is an atom within a material that is directly displaced from its lattice site by an incident energetic particle, such as an ion, neutron, or electron during irradiation. This initial displacement event initiates a cascade of further atomic collisions, leading to a variety of material defects.
Process:
When an energetic particle interacts with a material, it transfers some of its kinetic energy to the target atoms. If the energy transferred to an atom exceeds its displacement threshold energy (Ed), the atom will be ejected from its original lattice position, becoming a PKA. The displacement threshold energy varies depending on the material and crystallographic direction, typically ranging from 10 to 50 eV.
The PKA then collides with other atoms in the lattice, transferring its kinetic energy and potentially displacing them as well. This process creates a collision cascade, which can involve numerous atoms being displaced. The overall number of displaced atoms depends on the energy of the initial PKA and the properties of the material.
Significance:
PKAs are central to understanding radiation damage in materials. The energy and number of PKAs directly influence the type and extent of damage produced during irradiation. The subsequent collision cascade leads to the formation of point defects (vacancies and interstitials), dislocations, and even amorphization in some materials.
Understanding the PKA energy spectrum (the distribution of energies of the primary knock-on atoms) is crucial for predicting and mitigating radiation damage in various applications, including nuclear reactors, fusion devices, and spacecraft. Computational modeling, such as Molecular Dynamics (MD) simulations and Binary Collision Approximation (BCA) methods, are frequently used to study PKA generation and the resulting collision cascades. Factors such as the type and energy of the irradiating particle, the material composition, and the temperature of the material all affect the PKA energy spectrum and the resulting damage.
Further Considerations:
- The energy of the incident particle and the displacement threshold energy of the target material dictate the number of PKAs generated.
- The energy transfer process is governed by collision cross-sections, which depend on the type of interaction between the incident particle and the target atom.
- The spatial distribution of PKAs influences the spatial distribution of subsequent damage.
- The thermal conductivity of the material impacts how quickly the energy from the collision cascade dissipates, affecting defect recombination and annealing processes.