1. Radiometric Dating “clocks in rocks”

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

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

4. 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

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

6. radioactivity
Nuclei break apart
Emit particles or waves (radiation)

7. 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

8. 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

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

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

11. 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

14. 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

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

16. 14 Carbon Only accurate for ages less than 100,000 years
Parent gets too small to accurately measure

18. Mass spectrometer measures D/P ratio

19. Preparing a rock for mass spectrometer

20. 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

21. 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?