Inside the body, because they give up their energy over a relatively short distance, alpha particles can inflict more biological damage than other radiations. Beta particles are fast-moving electrons ejected from the nuclei of atoms. These particles are much smaller than alpha particles and can penetrate up to 1 to 2 centimetres of water or human flesh. They can be stopped by a sheet of aluminium a few millimetres thick. Beta particles are emitted from many radioactive elements.
Cosmic radiations consist of very energetic particles which bombard the earth from outer space.
They are more intense at higher altitudes than at sea level where the earth's atmosphere is most dense and gives the greatest protection. Neutrons are particles which are also very penetrating. They come from outer space, from collisions with atoms in the atmosphere, and from the splitting, or fissioning, of certain atoms inside a nuclear reactor. Water and concrete are the most commonly used shields against neutron radiation from the core of the nuclear reactor. It is important to understand that X-rays, gamma rays and alpha and beta particles and neutrons do not cause the body to become radioactive.