Half-Lives

Three Common Types of Radioactive Emissions Alpha Beta Gamma

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

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

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.

Complete Half-Life Graphing Activity

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

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?

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

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

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 = 16.25 days

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%

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