A form of hard radiation released by a ruptured baffle plate aboard an old class J training ship, exposure to which left Fleet Captain Christopher Pike paralysed and disfigured.
Delta rays are named in the Original Series two-part episode The Menagerie. Commodore Mendez tells Kirk that Pike was on an inspection tour of a cadet vessel, an ageing class J starship, when one of its baffle plates gave way.
Pike went into the affected compartment and pulled out the cadets who were still alive. The exposure destroyed his body, leaving him confined to a life-support chair and able to communicate only by a light that flashes once for yes and twice for no.
Strange New Worlds returns to the accident from the other side. Pike foresees it in a vision on Boreth, learns the names of the cadets he will save, and lives with the knowledge throughout the series.