Very high frequency (and therefore very high energy) electromagnetic radiation emitted as a consequence of radioactivity.
Doc Bruce Banner, belted by gamma rays, Turns into the Hulk – Ain’t he unglamour-rays [=unglamorous]!1966, “The Incredible Hulk”, Jacques Urbont (lyrics) (theme song of The Marvel Super Heroes):
There’s a tongue-in-cheek book titled “Learn Greek in 25 years,” but you’ll be surprised at how straightforward it is to learn the alphabet: those alpha males, beta releases and gamma rays have seen to that.2021 April 25, John Malathronas, “Which languages are easiest – and most difficult – for native English speakers to learn?”, in CNN, archived from the original on 22 Mar 2022: