time required for the body to eliminate 50 percent of a dose of any substance by the regular processes of elimination. This time is approximately the same for both stable isotopes and radionuclides of a particular element.
Half-Life, Effective
time required for a radioactive nuclide in a system to be diminished by 50 percent as a result of the combined action of radioactive decay and biological elimination.
Effective half-life =
Biological half-life x Radioactive half-life
Biological half-life + Radioactive half-life
Half-Life, Radioactive
time required for a radioactive substance to lose 50 percent of its activity by decay. Each radionuclide has a unique half-life.
Half Value Layer
the thickness of any specified material necessary to reduce the intensity of an x-ray or gamma ray beam to one-half its original value.
Health Physics
a term in common use for that branch of radiological science dealing with the protection of personnel from harmful effects of ionizing radiation. The science concerned with the recognition, evaluation and control of health hazards from ionizing and non ionizing radiation.
High Radiation Area
an area, accessible to individuals, in which radiation levels could result in an individual receiving a dose equivalent in excess of 0.1 rem (1 mSv) in one hour at thirty centimeters from the radiation source or from any surface that the radiation penetrates.
Hot Spot
the region in a radiation/contamination area in which the level of radiation/contamination is noticeably greater than in neighboring regions in the area.