An absorber is any material that is efficient at stopping (absorb) ionizing radiation from passing through. Absorbers can be made of a variety of materials, depending on the purpose; lead, tungsten and liquid hydrogen are common choices. Most absorbers are used as part of a particle detector, particle accelerators use absorbers to reduce the radiation damage on accelerator components. Alpha particles are totally absorbed by a sheet of paper. Beta particles are absorbed by a few centimeters of plastic material or 1 cm of aluminum. Materials with a high atomic number and high density (e.g., lead, steel, and concrete) are used as absorbers of gamma rays. Neutron absorbers such as boron, hafnium, and cadmium are used in control rods for reactors.
Gas absorbers are used to remove contaminants from gas streams. Multiple designs are used to accomplish this objective. A packed bed column uses gas and liquid streams running counter to each other in a column packed with loose packing materials, such as ceramics, metals, and plastics, or structured packing. The packed bed uses surface area created by the packing to create a maximum amount of efficient contact between the two phases. The systems are low maintenance and can handle corrosive materials with high mass transfer rates. Spray columns are another type of absorber, which uses constant direct contact between the two phases, with gas moving up and liquid being sprayed down into the gas flow. This system only has one stage and poor mass transfer rates, but is very effective for solutes with high liquid solubility.