Radiometric Dating “clocks in rocks”

Absolute Dating Gives a numerical age Works best with igneous rocks & fossils Uses isotopes

Isotopes- different number of neutrons Carbon 12 (C) Atomic mass is on periodic table stable Carbon 14 (14C) 2 “extra” neutrons radioactive

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

Radiation Radioactive Atom Ionizing Radiation Alpha Particle Neutron Particle Beta Particle Gamma Ray (X Ray)

radioactivity Nuclei break apart Emit particles or waves (radiation)

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

Average Annual Dose Cosmic 28 mrem Terrestrial Radon 200 mrem Internal Medical X-Rays Nuclear Medicine 14 mrem Consumer Products 10 mrem Other 3 mrem Radon 200 mrem Natural Sources Man-Made Sources

Comparison of Radiation Dose Doses shown in bar graph are in units of mrem

Material Can Enter the Body Four Ways Radioactive Material Can Enter the Body Inhalation • Breathing • Smoking Wound or Cut Ingestion • Eating • Drinking • Chewing Absorbtion

Half-Life The time required for the amount of radioactive material 200 400 600 800 1000 1200 New 1 Half- Life 2 Half- Lives 3 Half- 4 Half- Activity The time required for the amount of radioactive material to decrease by one-half

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

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

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

Mass spectrometer measures D/P ratio

Preparing a rock for mass spectrometer

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

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