Effective half-life
Encyclopedia
Effective half-life denotes the halving of radioactive material in a living organism
by means of radioactive decay
and biological excretion
. A decay constant is needed to calculate the half-life
. It is the sum of the biological and physical decay constants, as in the formula:
With the decay constant it is possible to calculate the effective half-life using the formula:
The biological decay constant is often approximated as it is more difficult to accurately determine than the physical decay constant.
Alternatively, since the radioactive decay contributes to the "physical (i.e. radioactive)" half-life, while the metabolic elimination processes determines the "biological" half-life of the radionuclide, the two act as parallel paths for elimination of the radioactivity, and in analogy with parallel resistors, the effective half-life could also be calculated by the formula:
, or
.
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
by means of radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
and biological excretion
Excretion
Excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism. This is primarily carried out by the lungs, kidneys and skin. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell...
. A decay constant is needed to calculate the half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
. It is the sum of the biological and physical decay constants, as in the formula:
With the decay constant it is possible to calculate the effective half-life using the formula:
The biological decay constant is often approximated as it is more difficult to accurately determine than the physical decay constant.
Alternatively, since the radioactive decay contributes to the "physical (i.e. radioactive)" half-life, while the metabolic elimination processes determines the "biological" half-life of the radionuclide, the two act as parallel paths for elimination of the radioactivity, and in analogy with parallel resistors, the effective half-life could also be calculated by the formula:
, or.