Radiometric Dating

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Radiometric Datingclocks in rocks
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Absolute Dating

• Gives a numerical age
• Works best with igneous rocks fossils
• Uses isotopes

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Isotopes- different number of neutrons

• Carbon 12 (C)
• Atomic mass is on periodic table
• stable
• Carbon 14 (14C)
• 2 extra neutrons
• radioactive

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Common dating isotopes
Parent Daughter Half life
Potassium 40 Argon 40 1.3 by
Rubidium 87 Strontium 87 48.8 by
Uranium 235 Lead 207 .7 by
Carbon 14 Nitrogen 14 6,000 years
Sodium 22 Aluminum 27 15 hours
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Radiation
Ionizing Radiation
Radioactive Atom
Alpha Particle Neutron Particle Beta Particle
Gamma Ray (X Ray)
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radioactivity

• Nuclei break apart
• Emit particles or waves (radiation)

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The average annual dose to the general
population from natural background and man-made
sources is 360 mrem.
Terrestrial Sources
Cosmic Radiation
Radon
Radon
Internal Sources
Other
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Average Annual Dose
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Comparison of Radiation Dose
Doses shown in bar graph are in units of mrem
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Four Ways Radioactive Material Can Enter the Body
Inhalation Breathing Smoking
Wound or Cut
Ingestion Eating Drinking Chewing
Absorbtion
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Half-Life
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Half-lives
Parent Daughter Half life
Potassium 40 Argon 40 1.3 by
Rubidium 87 Strontium 87 48.8 by
Uranium 235 Lead 207 .7 by
Carbon 14 Nitrogen 14 6,000 years
Sodium 22 Aluminum 27 15 hours
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Half life problem

• If you begin with 80 grams of 14C after a time,
  20 grams are left. How old is the sample?

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14 Carbon

• Only accurate for ages less than 100,000 years
• Parent gets too small to accurately measure

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Mass spectrometermeasures D/P ratio
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Preparing a rock for mass spectrometer
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Using daughter/parent ratio

• 1) D/P ratio
• 2) figure out number of half-lives
• Use graph
• 3) multiply number of half-lives by the time of
  one half-live
• Example 240 g 14N 8 g 14C

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Another half-life problem

• 1,000 grams of radioactive element is in a rock
  when it is formed. The elements half-life is 2
  million years. After a time, 125 grams of the
  original element remain. How old is the rock?