3. Half-Life Definition Half-life: the TIME it takes for half of a radioisotope sample to ‘decay’ and become another element. What are the three ways this can happen? –A–Alpha (  ) –B–Beta (  ) –G–Gamma (  )

4. Notice: it takes a very long time to completely decay! Left with at least a tiny amount for > 70 half lives! Example: Thorium-234  half-life is 24 days. Begin with a 1 kg of Th-234, after 24 days, you will have ½ kg of Th-234 (other ½ kg is something else). After 48 days? –¼–¼ kg (0.25 kg = ½ of ½ ) After 72 days? –1–1/8 kg (0.125 kg = ½ of ½ of ½ ) After 96 days? 1/16 kg (0.0625 kg = ½ of ½ of ½ of ½ )

5. Now you try! Start with 400 grams of Iodine- 131 Iodine- 131 has a half life of 8 days In 8 days you will have? In 16 days you will have? In 24 days? How long to get down to 25 grams?

6. The half-life for the beta decay of iodine-131 is 8 days. Half-life Graph Starting Sample of Iodine-131

7. Half-life Graph The half-life for the beta decay of iodine-131 is 8 days. Sample after 1 cycle

8. Half-life Graph The half-life for the beta decay of iodine-131 is 8 days. Sample after 2 cycles

9. Half-life Graph The half-life for the beta decay of iodine-131 is 8 days. Sample after 3 cycles

10. Iridium-182 undergoes beta decay to form osmium- 182. The half-life of iridium-182 is 15 minutes. After 45 minutes, how much iridium-182 will remain of an original 1-gram sample? Calculate how many half-lives will elapse during the total time of decay. Half-life Calculations

11. After three half-lives, the amount of iridium-182 has been reduced by half three times. Half-life Calculations After 45 minutes, 0.125 gram of iridium-182 remains. (1g x 1/8 = 0.125g) 0.875 gram of the sample has decayed into osmium-182.

12. What if we wanted to know the amount of iridium after 150 minutes? (10 Half-Life cycles) What are you going to do? After 45 minutes, the amount of iridium-182 was been reduced by half three times. Half-life Calculations # of Half-Life Cycles X Y-grams= Z-grams Half-Life Starting Amount of Isotope Amount of Isotope Left

13. Half-life Calculations

14. Half-life Calculations 2.23 x 10 -7 g 2.32 x 10 -7 g