570 Nuclear StructureQ44.28Carbon-14 is produced when carbon-12 is bombarded by cosmic rays. Both carbon-12 and carbon-14combine with oxygen to form the atmospheric CO2that plants absorb in respiration. When theplant dies, the carbon-14 is no longer replenished and decays at a known rate. Since carbon-14 is abeta-emitter, one only needs to compare the activity of a living plant to the activity of the sample todetermine its age, since the activity of a radioactive sample exponentially decreases in time.Q44.29The samples would have started with more carbon-14 than we first thought. We would increase ourestimates of their ages.Q44.30There are two factors that determine the uncertainty on dating an old sample. The first is the factthat the activity level decreases exponentially in time. After a long enough period of time, theactivity will approach background radiation levels, making precise dating difficult. Secondly, the
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