 # Determination of the Age of the Earth (Second Method)

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Determination of the Age of the Earth (Second Method)

Radioactivity in the Earth’s Crust One can find the age of a rock by measuring the amount of radioactivity In particular one is interested in the daughter decay series K 40 half life (T 1/2 1277 million years) Isotopic Abundance 0.118 % Age of earth about 4 billion years

Second Method Uranium and Actinium decay series U 238 T 1/2 = 4468 million years U 235 T 1/2 = 703.8 million years Both of these isotopes are not made in stars. They are “Transuranic Elements” Probably made during big bang and in Supernovae explosions Assume original abundance of these two isotopes were the same

Radioactive Decay Radioactive Decay is a logarithmic decay N is the number of atoms after a time t N 0 is the original number of atoms at time 0 is the decay constant t is the elapsed time = ln e 2 /T 1/2 = 1/(1.44 T 1/2 ) (Can be either years or seconds)