Half-Life and Nuclear Reactions

1. Half-Life and Nuclear Reactions

2. Review • We learned that all radioactive atoms eventually decay into stable isotopes. • We did not talk about how long this takes.

3. Half-Life • Cannot predict when a radioisotope (RI) will decay. • Can only give probability. • Half-life – Amount of time during which one-half of a radioactive substance will decay. • During a RI’s half-life, every atom has a 50% chance of decaying. • Half-lives can be as short as a fraction of a second or as long as billions of years.

4. Half-Lifes • N-13 is radioactive. • It decays into C-13 via a process called electron capture. • The half-life of 13N is 10 minutes. • If you start with 1000 atoms of N-13, approx. how many will remain after 10 minutes? • 10 minutes = 1 half-life, so there would be about 500 atoms left. • Approx. how many will remain after 20 minutes? • 20 minutes = 2 half-lives, so there would be about 250 atoms left. • Approx. how many will remain after 30 minutes? • 30 minutes = 3 half-lives, so there would be about 125 atoms left.

5. Half-Lifes

6. Half-Lifes • The half-life of 26Al is 710,000 years. • If you start with 28.0 grams of 26Al, how much will remain after 1,420,000 years? • At start, 28.0 grams remain. • At 710,000 years, 14.0 grams remain. • At 1,420,000 years, 7.0 grams remain.

7. Half-Lifes • The half-life of 61Fe is 6.0 minutes. How much time must pass before a 600-mg sample decays to 75 mg? • At start, 600 mg remain. • At 6.0 minutes, 300 mg remain. • At 12.0 minutes, 150 mg remain. • At 18.0 minutes, 75 mg remain.

8. Half-Lifes • In 48 minutes, 12 mg of 212Rn will decay to 3 mg. What is the half-life of 212Rn? • At start, 12 mg remain. • At (1st half-life), 6 mg remain. • At (2nd half-life), 3 mg remain. • It takes 2 half-lives for 12 mg to decay to 3 mg. • 2 half-lives = 48 minutes • 1 half-life = 24 minutes

9. Nuclear Fission • A heavy nucleus is struck by a neutron. • It becomes unstable and splits into smaller fragments, releasing energy. • In some cases extra neutrons are also released. • The neutrons can strike other nuclei and cause them to split. • Chain reaction.

10. 90Rb no no no no 235U Energy 143Cs Nuclear Fission

11. Uses for Nuclear Fission • Nuclear reactors use fission to produce energy.

12. Uses for Nuclear Fission • Weapons: the bombs that destroyed Hiroshima and Nagasaki, Japan, were fission bombs.

13. Nuclear Fusion • Two light nuclei are joined to produce a larger nucleus. • Energy is released.

14. 2H 3H 4He no Nuclear Fusion Energy

15. Uses for Nuclear Fusion • Nuclear fusion can be performed at low temperatures, but it does not produce much useable energy. • Fusion only produces significant energy at extremely high temperature and pressure. • The Sun produces energy via nuclear fusion.

16. Fusion in the Sun • The Sun is mostly made of hydrogen. • Intense heat and pressure at its center causes nuclear fusion to occur. • This releases energy that supports the mass of the Sun and prevents it from collapsing.

17. The Sun

18. Uses for Nuclear Fusion • Weapons: Thermonuclear (hydrogen) bombs. • Use a conventional fission bomb to trigger nuclear fusion in a lithium hydride shell. • Much more destructive than fission bombs. • Never used in war.