Which type of radiation has the greatest penetrating power?

Explanation

Gamma rays are uncharged electromagnetic radiation and need several centimetres of lead or a metre of concrete to attenuate them appreciably, whereas beta particles are stopped by a few millimetres of aluminium and alpha by paper. Penetrating power and ionising power run in opposite directions, so alpha ionises most heavily but travels least far. That is why an alpha emitter is most dangerous when it is inhaled or swallowed.