The characteristic time for the activity of a radionuclide to fall by half. It ranges from fractions of a second to billions of years: about 9 hours for xenon-135, 8 days for iodine-131, 5.3 years for cobalt-60 and 30 years for caesium-137. After ten half-lives about a thousandth of the original activity remains. Biological and effective half-lives, which add removal from the body, are used for internal dose.
Iodine half-life is eight days, so in a month that milk sample will be down to a sixteenth.
Seven half-lives cut activity to under 1 percent and ten to about a thousandth, the quick mental arithmetic every tech carries.
Also heard as
- half life
- T1/2
- radioactive half-life
Related terms
Becquerel
The SI unit of radioactivity, equal to one atomic decay per second.
Activation
The process by which neutrons make stable materials radioactive, creating isotopes such as cobalt-60.
Xenon poisoning
The loss of reactivity caused by xenon-135, a fission product that absorbs neutrons with enormous efficiency.
Committed dose
The total dose a person will receive over time from radioactive material taken into the body.