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the time needed for half of a sample of radioactive substance to undergo radioactive decay
an atom that has the same number of protons as other atoms of the same element do but that has a different number of neutron
the process by which the age of a rock is determined by measuring the amount of radioactive isotopes present in the rock or rock sample
the process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element
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