An electron ejected from matter by ionizing radiation WordNet 3.0 & Open English WordNet

A delta ray is a secondary electron with enough energy to escape a significant distance away from the primary radiation beam and produce further ionization,: 25  and is sometimes used to describe any recoil particle caused by secondary ionization. Wikipedia

secondary electron with enough energy to escape a significant distance away from the primary radiation beam and produce further ionization Wikidata

A secondary ionizing particle ejected by recoil when a primary particle passes through matter. Wiktionary