One sievert of radiation produces a constant biological effect regardless of the type of radiation. The gray results from direct measurements made on tissue or cells, whereas the sievert is a radiological protection unit based on the best available evidence that includes quality and weighting factors. The ability to penetrate matter differs greatly among the various types of nuclear radiation. A sheet of paper, a layer of clothing, or an inch of air can stop relatively slow moving, heavy alpha particles. Thus, it is easy to shield against alpha radiation, unless the alpha-emitting substance enters the body.
Beta particles are lighter and travel faster than alpha particles.
They can penetrate a fraction of an inch in solids and liquids and several feet in air. Gamma rays and neutrons are electrically neutral and thus not slowed by collisions with the target materials. They do not interact strongly as the charged alpha and beta particles do and are therefore highly penetrating. Their ability to penetrate the target material depends upon their energy. High-energy gamma rays may require several feet of material for adequate shielding.