The amount of material required to attenuate a beam of radiation to one-half of its original level. Units are given in lengths, such as centimeters.
The amount of lead, copper, cement, or other material that would dissipate a beam of radiation by 50%. The number of half value layers required for safety in blocking the area on a patient is five, because that represents 50% of 50% and 50% of that, and so forth. For example, 50% + 25% + 12.5% + 6.23% + 3.12% = 96.9%. Thus the patient would be shielded from all but about 3% of the radiation. (Examples of the thickness of material required to protect from radiation are 2 in [5 cm] of lead or 2 ft [61 cm] of cement.)
The thickness of a material (usually metal) when introduced in the path of radiation, will reduce the intensity of radiation to half of the initial value. Also called half-value thickness.