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 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
radioactivity


Nuclei break apart
Emit particles or
waves (radiation)
Radiation
Radioactive Atom
Ionizing Radiation
Alpha Particle
Neutron Particle
Beta Particle
Gamma Ray (X Ray)
Average Annual Dose
Cosmic
28 mrem
Terrestrial
28 mrem
Radon
200 mrem
Internal
40 mrem
Medical X-Rays
40 mrem
Nuclear Medicine
14 mrem
Natural Sources
Man-Made Sources
Other
3 mrem
Consumer Products
10 mrem
Comparison of Radiation Dose
1 10
100
1,000
10,000
100,000
1,000,000
Lethal Dose
Radiation
Worker Limit
Argonne
Control Limit
Natural
Background
Average ANL
Radiation Worker
General
Employee Limit
Chest X-Ray
Doses shown in bar graph are in units of mrem
Half-Life
The time required for the
amount of radioactive material
to decrease by one-half
1200
1000
800
Activity
600
400
200
0
New
1 HalfLife
2 HalfLives
3 HalfLives
4 HalfLives
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
;8g
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?