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Determination of the Age of the Earth (Second Method)

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)

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Determination of the Age of the Earth (Second Method)

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  1. Determination of the Age of the Earth (Second Method)

  2. 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 • K40 half life (T1/2 1277 million years) • Isotopic Abundance 0.118 % • Age of earth about 4 billion years

  3. Second Method Uranium and Actinium decay series • U238 T1/2 = 4468 million years • U235 T1/2 = 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

  4. Radioactive Decay • Radioactive Decay is a logarithmic decay • N is the number of atoms after a time t • N0 is the original number of atoms at time 0 • is the decay constant t is the elapsed time  = lne2 /T1/2 = 1/(1.44 T1/2) (Can be either years or seconds)

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