# Half-Life

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## Half-Life

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1. Half-Life

2. Half-Life • Time required for ½ of the atoms in a sample of radioactive nuclei to decay or change into something else. • Each isotope has its own half-life. • Range from fractions of a second to billions of years.

3. Half-Life • Half-life of each isotope - always the same. • Not affected by the chemical or physical environment of the nuclide. Not affected by changes in T, P, or concentration. • Doesn’t depend on how much you have or how long it’s been sitting around.

4. Geiger Counter

5. Radioactive Decay Another animation Compare C-10: half-life = 20 sec C-15: half-life = 2.5 sec

6. Graphing Half-Life Data • Use Geiger counter to record decay events • Plot the data – always has the same shape. • Determine half-life from graph

7. Half-Life Map: Vertical

8. No. of half-lives Number of half-lives = time elapsed length of 1 half-life Look up the half-life in Table N.

9. Problem #1 • Cr-51 has a half-life of 28 days. • What fraction will remain after 168 days? • Find number of half-lives in 168 days. Time elapsed = 168 days = 6 HL’s Length of HL 28 days

10. Problem #1 • Go to half-life map. • Read fraction remaining after 6 HL

11. Problem #2 • If a sample of Cr-51 has an original mass of 52.0 g, how much will remain after 168 days? • Already calculated 168 days = 6 HL. • Go to half-life map. To calculate remaining mass, divide original by 2, SIX times! OR Multiply orignial by 1/64.

12. Problem 3 • After 62 hours, 1.0 g remains unchanged from a sample of K-42. How much K-42 was in the original sample? • Look up HL of K-42 in table N: 12.4 h • Calculate number of HL: 62 hours = 5 HL 12.4 h/HL

13. Problem 4 • 80 milligrams of a radioactive substance decays to 10 milligrams in 30 minutes. • Calculate the HL. • NOTICE: They gave BOTH masses. 80 mg  40 mg  20 mg  10 mg Took 3 HL to decay to 10 mg

14. Problem 4 • 3 HL = 30 minutes so 1 HL = 10 minutes