Try This
Complete "Practice Problem" 2 on page 813 and "Practice Problems" 1 and 2 on page 814 of the textbook.

Radioactive Dating

Using nuclear decay to determine age is only possible because radioactive decay is a predictable process.  Decay can be used to determine the age of rocks, fossils, and artifacts and is called radiometric dating.

The isotopes that are useful for measuring the age of rocks and fossils have very long half-lives.  As previously mentioned, the carbon-14 used to date organic material has a half-life of 5730 years, while uranium-235, used to date rocks, has a half-life of 704 million years.

 
Read
Read "Radioactive Decay Rates" on pages 811 to 816 of your physics textbook.

Self-Check

Answer the following self-check questions then click the "Check your work" bar to assess your responses.

SC 1. The half-life of strontium-90 is 28 years.  If 60 g of strontium-90 is currently in a sample of soil, how much will be in the soil in 84 years?

 

SC 2. The half-life of strontium-90 is 28 years. If 100 g of strontium-90 is currently in a sample of soil, how much will be in the soil in 65 years?

 

SC 3. Tritium (hydrogen-3), a by-product of the CANDU nuclear power reactor, has a half-life of 12.3 years.  How much time is required for its radioactivity to reach 1/4 its original level?

 

Self-Check

Contact your teacher if your answers vary significantly from the answers provided here.

SC 1.

Given

Note: a or y are acceptable units for years.

 

Required

the remaining amount of strontium-90 in 84 years

 

Analysis and Solution

Determine the number of half-lives in 84 years.

Determine the remaining amount of strontium-90.

 

Paraphrase

The amount of strontium-90 remaining in the soil in 84 years is 7.5 g.

 

SC 2.

Given

 

Required 

the remaining amount of strontium-90 in 65 years

 

Analysis and Solution

Determine the number of elapsed half-lives in 65 years.


Determine the amount of remaining strontium-90.

 

Paraphrase

The amount of strontium-90 remaining in the soil in 65 years is 20 g.

 

SC 3.

There are two ways of solving this question.

Method 1: How many ½ are there in ¼?

 

Therefore, two half-lives have passed.

The elapsed time is 24.6 years.

 

Method 2: Logarithms

This method is optional. If you have seen logarithms in math class, you can use them here. In Physics 30 all questions like this should have whole-number answers for the number of half-lives.

 

Determine the time for two half-lives.

 

Paraphrase

The elapsed time is 24.6 years.