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Absolute Time:

Objectives:

    1.    Explain the process of radioactive decay.

    2.    Define half-life.

    3.    Describe how radiometric dating is used to determine absolute time.

 

Notes: (29.3)

Before modern science, scientists only had methods of estimating the absolute time of events.  They were generally limited to aging recent occurrences, counting varves (annual sediment deposits, and counting tree rings.  Since then the 20th century, the term absolute dating has become almost exclusively tied to the measurement of the radioactive decay of atoms in a sample.

Methods of Dating:

Relative dating is a technique used by  scientists to compare the strata where the fossil was found to others above and below it.

Not very accurate

Absolute dating

Radioactive elements have a very predictable decay cycle and make excellent dating materials.  Called radioactive dating or radiometric dating.

The amount of decay is measured in half-lives

A half-life is the time it takes a ½ of the radio active material to decay

Each radioactive element has a unique ½ life

C14 – 5770 years 

useful in dating living organisms

U238 – 4.5 billion years

useful in dating Earth's oldest rocks

K40 - 1.3 billion years

useful in dating rocks too young for U238

Rb87 - 47 billion years

useful in dating Earth's oldest rocks to confirm U238 findings

Types of radioactive decay

1)    Alpha
Consists of 2 protons & 2 Neutrons - identical to Helium
Low penetrating power - easily stopped by paper or clothing
Can travel only a few centimeters through the air
Do not pose a health hazard

(http://www.hpwt.de/Starte.htm)

2)    Beta
Consists of a high speed electron
The electron is formed in the nucleus when a neutron is converted into a proton and a high speed electron
Has essentially no mass
About 100 times more penetrating than the alpha particle
Can pass through clothing and damage skin

3)    Gamma
Energetic form of light similar to X-ray
Have no mass
Accompany alpha and beta radiation
Can travel through tissue and solid material - stopped by concrete and lead

 

 

 

 

 

 

 

Last modified: March 26, 2004