1. Nuclear reactions Micro-world Macro-world Lecture 17 Using the strong nuclear force to produce useful energy

2. Strong Nuclear Force It is very strong –It overcomes the electrical repulsion between positively charged protons that are only 10 -15 m apart. It acts over a very short range –It is not felt by nucleons when they are more than 10 -15 m apart. It is selective –It is felt by neutrons & protons, but not by electrons

3. Nuclear “bullets” Protons are repelled by electrical the repulsion force of the positively nucleus. Only protons with KE of a few MeV or more can get within the range of the strong nuclear force & produce “nuclear reactions” + + + + + + + + + + + + + + + + + + + + v F Producing nuclear reactions with protons (or any other charged nuclei) is a challenge

4. Neutron induced nuclear reactions Neutrons don’t feel the electrical force so even very slow, low-energy neutrons can strike the nucleus & produce “nuclear reactions” + + + + + + + + + + + + + + + + + + + v Low energy neutrons are effective nuclear “bullets”

5. Nuclear fission n + 92 U  56 Ba + 36 Kr + 2n 235 142 92

6. Energy balance in a fission reaction 141 Ba + 92 Kr + 2n 200 MeV  KE  heat 235 U + n

7. Chain reaction Use the neutrons produced by one fission to initiate another fission Enrico Fermi

8. Requirements for A-bomb Fissionable material: 235 U or 239 Pu Critical mass Mechanism

9. Critical Mass Enriched 235 U50kg 239 Pu10kg M crit

10. Fissionable Material Fortunately, only certain nuclear isotopes undergo the fission process: 235 U only 0.7% of naturally occurring U (99.3% is 238 U, which doesn’t fission) 239 Pu doesn’t occur naturally, but is produced in nuclear reactors …. There are other fissionable isotopes, e.g. 233 U & 232 Th, but they are very rare

11. Little boy ( 235 U) (doughnut-like)

12. Fat man ( 239 Pu)

13. Devastation Hiroshima Aug 6 1945 8:15AM 80,000 people killed immediately; ~100,000 people were exposed to lethal radiation & died painful slow deaths

14. Hiroshima aftermath

15. Devastation Nagasaki Aug 9 1945 10:45AM 39,000 people killed immediately; ~70,000 people were exposed to lethal radiation & died painful slow deaths

16. Nagasaki aftermath

17. Nuclear fusion 2 H + 3 H  4 He + n Two light nuclei fuse together to form a heavier one Here the nuclei have to start out with large energy in order to overcome the electrical repulsion

18. Energy balance in a fusion reaction 4 He+n 12.3 MeV  KE  heat 2 H + 3 H

19. Need to overcome electric repulsion + + Protons need ~2MeV energy to get within 10 -15 m of each other (where strong nuclear force can be felt) This requires super-high temperatures (several Million degrees). Such high temperatures exist in the core of the Sun or in an Atomic-Bomb explosion

20. H-bomb: powered by nuclear fusion Nuclear fission bomb “detonator” produces the high temperature required to initiate fusion processes Nuclear fusion bomb

21. Brighter than 1000 suns 1000 times the power of an A-bomb!!

22. Dangers of teaching nuclear physics Oh, and I suppose it was me who said ‘what harm could it be to give the chickens a book on nuclear physics?’

23. Fusion in the Sun The core temperature is ~14 million degrees Here a tiny fraction of the protons have enough thermal energy to undergo fusion

24. Solar fusion processes + 5.5 MeV + 1.4 MeV + 12.9 MeV

25. pp-cycle 6 protons  4 He + 2 protons + 2 “positrons” + 2neutrinos

26. Energy balance in the pp-cycle 4 He 25 MeV  KE  heat 4 protons + 2 neutrinos

27. How do we know what goes on inside the Sun?

28. Superkamiokande

30. Direction of neutrinos detected in Superkamiokande

31. Sun as seen by a neutrino detector

32. Neutrinos come directly from solar core

33. Neutrinos are everywhere T est test