= Solution
An energetic neutron can collide with a lattice atom and create a <primary knock-on atom>. That recoil transfers energy to other atoms in a <displacement cascade>. Some atoms leave their normal lattice sites, creating <Frenkel pairs>: a <vacancy> and an extra atom at an interstitial position.
Many nearby defects recombine during the cascade and its thermal relaxation. The surviving <point defects> are therefore fewer than a simple count of all displacement events. Some defects are already clustered within the cascade; others migrate and aggregate later.
A collection of <vacancies> can collapse into a platelet bounded by a vacancy-type <dislocation loop>. An aggregation of <self-interstitial> atoms can create an extra atomic platelet bounded by an interstitial-type <dislocation loop>. The boundary is a line defect because a circuit around it has a nonzero <Burgers vector>. The allowed Burgers vectors and habit planes depend on the material's <crystal structure> and fault energies.
Loops can grow by absorbing mobile <point defects>, shrink by emitting or absorbing the opposite defect type, and interact with existing <dislocations>. Some remain faulted, while suitable loops can unfault or become part of a larger <dislocation> network. Repeated cascades and subsequent defect transport therefore create a population of extended defects even in material with a low initial <dislocation density>.
This describes the main displacement-damage mechanism. Ionizing energy deposited in a material does not automatically create the same defect population as a neutron recoil cascade; recoil spectrum, displacement thresholds and material response matter.
\Image[/past-exam-of-the-mphil-in-nuclear-energy-course-of-the-university-of-cambridge/2021/paper-ne3-defect-pathways.png]
{title=Original schematic of a displaced atom, a <Frenkel pair>, and the competing pathways to recombination, sinks, loops and voids}
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