1 Radioactivity and Half-Life

2 Radioactivity An unstable atomic nucleus emits a form of radiation (alpha, beta, or gamma) to become stable. In other words, the nucleus decays into a different atom.

3 Radioactivity Alpha Particle – Helium nucleus Beta Particle – electron Gamma Ray – high-energy photon

4 Half-Life Amount of time it takes for one half of a sample of radioactive atoms to decay

Daughter isotope Decay curve Half-life Parent isotope Radiocarbon dating

It can be difficult to determine the ages of objects by sight alone. –Radioactivity provides a method to determine age by measuring relative amounts of remaining radioactive material to stable products formed. See pages

Carbon dating measures the ratio of carbon-12 and carbon-14. – Stable carbon-12 and radioactive carbon-14 exist naturally in a constant ratio. – When an organism dies, carbon-14 stops being created and slowly decays. Carbon dating only works for organisms less than years old. See pages Using carbon dating, these cave paintings of horses, from France, were drawn years ago.

Half-life measures the rate of radioactive decay. –Half-life = time required for half of the radioactive sample to decay. –The half-life for a radioactive element is a constant rate of decay. –Strontium-90 has a half-life of 29 years. If you have 10 g of strontium-90 today, there will be 5.0 g remaining in 29 years. See pages

Decay curves show the rate of decay for radioactive elements. –The curve shows the relationship between half-life and percentage of original substance remaining. See pages The decay curve for strontium-90

10 Medical Applications of Half-Life NuclideHalf-LifeArea of Body I– daysThyroid Fe– daysRed Blood Cells Sr–872.8 hoursBones Tc–996.0 hoursHeart Na– hoursCirculatory System

11 Half-Life Calculation #1 You have 400 mg of a radioisotope with a half-life of 5 minutes. How much will be left after 30 minutes?

12 Half-Life Calculation #2 Suppose you have a 100 mg sample of Au-191, which has a half-life of 3.4 hours. How much will remain after 10.2 hours?

13 Half-Life Calculation # 3 Cobalt-60 is a radioactive isotope used in cancer treatment. Co-60 has a half-life of 5 years. If a hospital starts with a 1000 mg supply, how many mg will need to be purchased after 10 years to replenish the original supply?

14 Half-Life Calculation # 4 A radioisotope has a half-life of 1 hour. If you began with a 100 g sample of the element at noon, how much remains at 3 PM? At 6 PM? At 10 PM?

15 Half-Life Calculation # 5 How many half-lives have passed if 255 g of Co-60 remain from a sample of 8160 g?

16 Half-Life Calculation # 6 Suppose you have a sample containing 400 nuclei of a radioisotope. If only 25 nuclei remain after one hour, what is the half-life of the isotope?

17 Half-Life Calculation # 7 If a radioactive element has diminished by 7/8 of its original amount in 30 seconds, what is its half-life?

18 Answers to Half-Life Calculations Half-Life Calculation #1 – 6.25 mg Half-Life Calculation #2 – 12.5 mg Half-Life Calculation #3 – 750 mg

19 Answers to Half-Life Calculations Half-Life Calculation #4 – 12.5 g, g, g Half-Life Calculation #5 – 5 half-lives

20 Answers to Half-Life Calculations Half-Life Calculation #6 – 15 minutes Half-Life Calculation #7 – 10 seconds