Cerenkov radiation is also spelled as Cherenkov is an electromagnetic radiation that comes for particles as they travel at speeds greater that the speed of light. The radiation if seen is often blue and is not harmful.

J.V Jelley has written:

'Cerenkov radiation, and its applications' -- subject(s): Radiation

Cherenkov radiation is seem by the naked eye is a bright blue it is not considered to be harmful. The Cherenkov radiation is generating from electromagnetic radiation that comes from the speedo of particles traveling.

Cerenkov radiation.

No. It's not green either. Radioactivity is invisible.

There's a blueish-green glow (more blue than green) due to something called Cerenkov radiation that's sometimes associated with radioactive objects. But that's just ordinary blue(ish) light, not "radioactivity". (It's still "radiation", of course, as is all other light.)