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10.2 Rates of
Nuclear Decay
• Artifacts found at the archeological site in Cactus Hill, Virginia
• In the 1990s, archaeologists found stone tools, charcoal, and animal bones that were at least 15,000-17,000 yrs old.
• The age suggests that the first Americans settled here earlier than previously thought.
• Scientists can determine how old the artifacts are based on the rates of nuclear decay.
Review of Nuclear Decay
atoms of one element can change into atoms of a
different element altogether.
Rates of Nuclear Decay
Describes how fast nuclear
changes take place in a
radioactive substance.
Half Life… The time required for one half of a sample of a radioisotope to decay.
• After one half-life, half of the atoms in a radioactive sample have decayed, while the other half remains unchanged.
• After two half-lives, half of the remaining radioisotope decays.
• After three half-lives, the remaining fraction is one eighth.
• Every radioisotope decays at a specific rate.
• Half-lives can vary from fractions of a second to billions of years.
Radioactive Decay of Uranium
All uranium atoms are mildly radioactive and decay through a number of steps on
the way to becoming stable lead. Each step has a different half life, and a
characteristic type of radiation. The shorter-lived each kind of radioisotope in
the decay series, the more radiation it emits per unit mass. Much of the natural radioactivity in rocks and soil comes from
the uranium-238 (U-238) decay chain (but not from the uranium itself).
•Nuclear decay rates are
CONSTANT. –Ex: regardless of the temperature, pressure,
or surface area of a uranium-238 sample, its
half life is still 4.5 billion years.
The half-life for the beta decay of iodine-131 is 8.07 days.
– A half-life is the amount of time it takes for half of the
isotope to decay.
• Radiometric dating uses decay of unstable isotopes.
– Isotopes are atoms of an element that differ in their
number of neutrons.
Radiometric dating provides an accurate way to estimate the age of fossils.
Carbon-14 is a radioactive isotope that is
naturally incorporated from carbon
dioxide into living organisms, the amount
remains relatively constant during the life
of the organism
When the living organisms dies the carbon 14 is
no longer being replaced in the organism and will start to decay. The amount of loss from the that compared to living organisms can be used
to determine when the organism died.
22,920 years ago
17,190 years ago
11,460 years ago
5730 years ago
Present
Calculate Age
Problem:
The carbon-14 radioactivity in the
bones of a body was measured to
be 1/8 of that compared to a living
person
How long ago did the person live?
Calculation of Age:
The carbon-14 has decreased
by 1/8 which is three half
lives (1/2 times 1/2 times 1/2
= 1/8)
Carbon-14 half life = 5730
years
3 times 5730 = 17,190 years
Present
One Half-Life
5730 years ago
Two Half-Lives
11,460 years ago
Three Half-Lives
17,190 years ago
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