The four types of radiations have very different penetrative abilities. Alpha and beta radiation can be stopped by a few mm of aluminium – but they have a high ionising power over their short path through matter. Gamma rays, on the contrary, do not ionize until they interact with an atom électron or nucleus. The relative matter transparency with respect to gamma is widely used in radiodiagnostics but make them difficult to absorb in radioprotection. The flux of neutrons are intense near the core of nuclear reactors. There are effective techniques to slow them down and capture them.