Gamma rays - need refreshing for exam
I'm having physics exam tomorrow and need refreshing on the topic.
I've tried looking through the wikipedia ( Gamma ray - Wikipedia, the free encyclopedia ) and my school papers, yet it isn't fully clear.
What is the reason of a gamma ray's existance? Like, how does a gamma ray get unleashed? Where does it come from? As for now I understand it to be the cracking of an atom, but I ain't fully sure...
So please refresh my mind on this topic and thanks in advance.