The types of radiation are Alpha, Beta and gamma radiation
Alpha radiation: is a radiation of particles like He elements. It is very heavy radiation. it can be stopped by a sheet of a paper.
Beta radiation: is a radiation of particles like as the electrons, with the same mass and positive or negative charge. It can be stopped by an aluminium sheet
Gamma radiation: is an electromagnetic radiation with out really mass, moves in the speed of the light and has very high energy, high frequency and very short wavelengths. It can be stopped only by a block of concrete, lead, or other heavy shielding.
Thus, the radiation has the lowest penetrating ability is Alpha radiation (particles)