Half-Life, Effective (Teff): The time required for the amount of a radioactive nuclide deposited in
a living organism to be diminished 50 percent as a result of the combined action of radioactive
decay T and biological elimination Tbio.
Teff = Tbio x T / Tbio + T
Half-Life, Physical (T): The time in which half the atoms of a particular radioactive substance
disintegrate to another nuclear form. Measured half-lives vary from millionths of a second to
billions of years.
Half-Value Layer (HVL): Thickness of a specified substance which, when introduced into the
path of a given beam of radiation, reduces the exposure rate by one half.
Health Physicist: A person trained to advise on operating procedures for minimizing radiation
exposures, perform radiation surveys, oversee radiation monitoring, and estimate the degree of
Health Physics: The science concerned with recognition, evaluation, and control of health hazards
from ionizing radiation.
Health, Radiological: The art and science of protecting human beings from injury by radiation, as
well as promoting better health through beneficial applications of radiation.
High Efficiency Particulate Air (HEPA): An air filter generally rated as being capable of
removing at least 99.97 percent of the particulate material in an air stream.
Indirect Bioassay: The assessment of radioactive material deposited in the body by detection of
radioactivity in material excreted or removed from the body.
Intake: The quantity of material (activity or mass) initially taken into the body. (For example, in
the case of inhalation, the intake includes the quantity of material immediately exhaled.)
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