The voltage between anode and cathode is set sufficiently high that the counter is extremely sensitive to radiation, but on the other hand the output of the counter gives no further information about the radiation except that it is present. See proportional counter . The time taken for a given amount of a radioactive isotope to decay to half its original value. For each isotope the half life is constant and is not affected by changes in the environment. As an ideal case, consider a freshwater lake in a limestone basin.
Limestone is insoluble in pure water but dissolves in acid.
As the lake absorbs CO 2 from the atmosphere the water becomes slightly acidic. If the CO 2 can be regarded as distributed uniformly through the water then in theory one atom of carbon will be released from the limestone for each molecule of absorbed CO 2 . Since the limestone contains no 14 C, the effect is to dilute the concentration of the 14 C in the incoming CO 2 by half. The effect on the measured CRA of the carbon in the water is as if the carbon had decayed by one half life , so the dissolved carbon will give an age about 5,700 years older than the incoming CO 2 . I Isotope Isotopes are atoms of an element having identical chemical properties but different masses.