Gamma rays consist of pure energy. Beta particles consist of one electron that is released at high speeds from an unstable nucleus and an alpha particle is a helium nucleus.
We know that Gamma rays consist of pure energy because, during Gamma decay, the atomic and mass numbers remain unchanged. Therefore, only energy is emitted.
Gamma rays are weakly ionising, however, they are strongly penetrating and thick concrete or a few centimetres of aluminium is needed to stop them.
