1. Half-Lives

2. Three Common Types of Radioactive Emissions
Alpha
Beta
Gamma

3. Positron emission – a way to convert protons to electrons by giving off a positron (electron anti-particle)

4. Radioactive Isotopes
Radioactive elements are unstable. They decay, and change into different elements over time.
Not all elements are radioactive. Those that are listed below are the most useful for geologic dating of fossils are:
U-238 Half-life = 4.5 Billion Years
K-40 Half-life = 1.25 Billion Years
C-14 Half-life = 5, 730 Years

5. Half Life of Radioactive Atoms
The half-life of an element is the time it takes for half of the material you started with to decay.
2. Each element has it’s own half-life
Each element decays into a new element
4. The half-life of each element is constant. It’s like a clock keeping perfect time.

7. Complete Half-Life Graphing Activity

11. Type of Problem Starting Amount % 100% Fraction 1 Amount Given amount
Can ask for
Gives
Need to
How many half-lives?
Starting/ending amounts
Determine number of arrows (half lifes to get from A B)
How long (time)?
Starting/ending, length of half-life
Multiply number of half lives by how long each half life is
How long is one half-life
How long total time, number of half lives
Divide total amount of time by number of half lives to figure out how long just one is

12. Practice Problems
The half life of iodine-131 is 8.1 days. How long will it take for the sample to reduce to ¼ its original amount?
How long would it take iodine to reduce to less than 1% its original amount?
Radon-222 is a radioactive gas. How long is its half-life if it takes 65 days to reduce to 1/16th its original amount?
Uranium-238 decays very slowly, with a half-life of 4.47 billion years. What percentage of a sample of U-238 would remain after 13.4 billion years?

13. Practice Problems
The half life of iodine-131 is 8.1 days. How long will it take for the sample to reduce to ¼ its original amount?
1  ½  ¼ 2 half lives
2 x 8.1 = 16.2 days

14. Practice Problems
2. How long would it take iodine to reduce to less than 1% its original amount?
100%  50%  25%  12.5%  6.25%  3.125%  1.56%  0.781%
7 half lives 7 x 8.1 days = 56.7 days

15. Practice Problems
3. Radon-222 is a radioactive gas. How long is its half-life if it takes 65 days to reduce to 1/16th its original amount?
1  ½  ¼  1/8  1/16
4 half lives 65 days/4 = days

16. Practice Problems
4. Uranium-238 decays very slowly, with a half-life of 4.47 billion years. What percentage of a sample of U-238 would remain after 13.4 billion years?
13.4 / 4.47 = 3 half lives
100%  50%  25%  12.5%

17. Problem A sample of 3x107 Radon atoms are trapped
in a basement that is sealed. The half-life of
Radon is 3.83 days. How many radon atoms
are left after 31 days?
answer:1.2x105 atoms