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