Alpha and beta trajectories within the skin:
The alpha rays, like the ones emitted by plutonium, or the beta electrons of very low energy emitted by tritium have a very low penetrating power. If the radioactive source comes into contact with the skin, they are stopped in the upper layer of the epidermis. The more energetic beta electrons from carbon 14 can reach as far as the dermis, while the energetic electrons emitted by phosphorus 32 and strontium 90 can even penetrate the hypodermis.
Source K.G.Gerber/EURADOS