Half-Life

The time taken for half of a given amount of a radioactive substance to decay into a non-radioactive substance. It is also defined as the time taken for half the quantity of a radioactive isotope in a sample to decay and form a stable element. It is the basis of radiocarbon and other radiometric dating methods. This decay rate, expressed as a statistical constant, is different for each isotope. If a sample, such as a piece of wood, has half of the original amount of radiocarbon remaining, then a time equivalent to the half-life has passed since it died. The half-life of radiocarbon is 5730 ? 40 years, while the half-life of radioactive potassium, used in potassium-argon dating is 1.3 billion years. The half-life in effect determines the general age range over which a radiometric dating method is potentially useful.

The time taken for half a radioactive isotope to decay. Thus after a time of one halflife has elapsed, one half of the isotope will be left. After two half-lives, one quarter is left; after three half-lives, one eighth — and so on. The most recent determination of half-life for 14C is 5730 years. The original Libby half-life was 5568 years and this has caused some slight complexity. Radiocarbon laboratories have agreed (for consistency) to continue to use the 5568 half-life as a standard, even though it is now known to be incorrect (see radiocarbon dating). Therefore dates as quoted by the laboratory are in terms of the 5568 half-life. Calibration tables are so arranged that they automatically correct for this — so the archaeologist usually does not need to worry about it. If, for any reason, it is desired to correct a 5568 to the 5730 half-life, this is easily accomplished by adding the 5568 date to 3 per cent of itself.