= Solution
Energetic <neutron> collisions create <displacement cascades> and <Frenkel pairs>: displaced atoms become <self-interstitials>, leaving <vacancies>. During migration, some opposite defects recombine, while others cluster or reach sinks. Interstitials can condense into extra atomic planes bounded by <dislocation loops>; <vacancy> clustering can instead give missing-plane loops. Their subsequent growth is driven by defect absorption and <dislocation climb>.
Randomly oriented loops do not select a macroscopic shear direction in an initially <isotropic> polycrystal. That does \b[not] prove that every external dimension remains unchanged: interstitial and <vacancy> loops have opposite relaxation-volume contributions, and an imbalance can change the volume. Macroscopic <void swelling> requires the additional vacancy-cavity mechanism; it must not be identified automatically with the mere presence of loops. Texture, stress-biased absorption and unequal loop populations can also give anisotropic dimensional change.
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