Any material “stopping” ionizing radiation. Alpha radiation can already be totally absorbed by a sheet of paper; beta radiation is absorbed by a few centimetres of plastic material or 1 cm of aluminium. Materials with a high atomic number and high density are used for gamma radiation absorbers (lead, steel, concrete, partially with special additions). Neutron absorbers such as boron, hafnium and cadmium are used in control rods for reactors.