The biological half-life is the time required for the concentration of a substance in a biological system to be reduced by one half. A biological half-life includes usual processes for eliminating substances without radioactive decay.

See also Biological Radioactive Half-Life.

Time in which 50% of a radioactive substance through biological (physiological) processes and decay is removed.
The effective radioactive half-life is shorter or equal than the physical or the biological radioactive half-life alone.

Effective Radioactive Half-Life (t_eff) =
(Biological Radioactive Half-Life x Physical Radioactive Half-Life) /
(Biological Radioactive Half-Life + Physical Radioactive Half-Life)

See also Half-Life, Biological Half-Life, Physical Radioactive Half-Life.

Radiotoxicity refers to radioactive materials that are toxic to living cells or tissues. Radiotoxicity results from the type of radiation, the radioactive half-life of the used radionuclide, the biological half-life in the tissue and the radioactivity absorbed in the organ. Radiotoxic substances can be collected following ingestion, inhalation and absorption.

Time in which 50% of an administered dosage of a radioactive substance through biological (physiological) processes is removed.

See also Half-Life, Physical Radioactive Half-Life and Effective Radioactive Half-Life.

(NAA) Neutron activation analysis is a very sensitive analytical technique to determine even very low concentration of chemical elements, trace elements for example, in small biological samples.
NAA becomes commercial available in the USA in 1960.
In the activation process stable nuclides in the sample, which is placed in a neutron beam (neutron flux, 90-95% are thermal neutron with low energy levels under 0.5 eV), will change to radioactive nuclides through neutron capture (artificial radioactivity). These radioactive nuclides decay by emitting alpha-, beta-particles and gamma-rays with a unique half-life. Qualitative and quantitative analysis of the sample is done with a high-resolution gamma-ray spectrometer.
NAA is subdivided into the following techniques: