Nuclear Physics

1. Nuclear Physics Paul J. Thomas Department of Physics and Astronomy UW - Eau Claire

2. Early Atomic Ideas • Greek atomos = “cannot be cut”. • Early atomistic theories of Democritus and Leucippus. • Dalton: elements combine in constant ratios of mass. • Brownian motion

3. The Structure of the Atom • Static electricity implies that atoms contain separate charges. • “Plum pudding” model of Thomson. • Rutherford’s experiment.

8. Particles Inside Atoms • Inside the nucleus: • Protons, charge +1, mass 1. • Neutrons, charge 0, mass 1. • Surrounding the nucleus: • Electrons, charge -1, mass 1/2000.

9. Atomic and Mass Number • The type of element is determined by the number of protons. This is the atomic number (Z). • The number of protons + neutrons is the mass number (A). • For light elements, there are roughly as many protons and neutrons. Heavier elements have more neutrons than protons.

11. Isotopes • Atoms of the same element, with different numbers of neutrons in the nucleus. • They are chemically identical, and thus cannot be separated by any chemical process.

12. Isotopes • Have the same chemical behavior. • Can be very different in nuclear behavior (e.g. radioactivity). • Example: 12C is stable, but 14C is radioactive, with a half-life of 5730 y. • Half-life: time required for half of the initial sample to decay.

13. Half-lives 26Al 1.6 × 106 y 3H 12.33 y 14C 5,730 y 90Sr 28.1 y 235U 7.04 × 108 y 239Pu 24,000 y • After N half-lives, ½N of original remains.

15. Marie and Pierre Curie • Deduced that the radiation emitted by uranium and thorium was identical and must be a property of the interior of the atom. • Discovered the radioactive elements Polonium and Radium. • First known victims of radiation poisoning. • Awarded the 1903 Nobel Prize in Physics; Marie won the 1911 Nobel Prize in Chemistry.

16. Radioactive Decay • Alpha Decay 212Bi 208Tl + 4He 238U 234Th + 4He • Beta Decay 14C 14N + e- +  12N 12C + e+ +  • Gamma Decay 4He*4He + 

18. Alpha Decay

19. Beta Decay

20.  decay and the neutrino

21. Neutrinos • Probably most common fundamental particle. • Neutral. • Small mass (recently discovered): 0.14 millionth of electron mass. • Extremely nonreactive with other particles. • Three types: electron neutrino ne, muon neutrino nm and tau neutrino nt.

22. How are Neutrinos Made? • Nuclear reactions, particularly at very high energies. • During the Big Bang. • In the centers of stars. • In supernovas. • When cosmic rays hit the Earth’s atmosphere. • Radioactive beta decay.

23. Neutrinos are nonreactive! • 100 billion solar neutrinos pass through each square inch of our bodies every second, day or night! • Big bang models predict there should be 50 billion neutrinos per electron. • Neutrino mass has to be very small, or the universe would have already collapsed.

24. Super-Kamiokande • In Kamioka, Gifu-ken, Honshu, Japan. • 12.5 million gallons of pure water in a stainless steel lined cavity, with 13,000 photomultiplier tubes. • Data taken during 1996-98 indicates neutrinos undergo “oscillations”. • This means that they possess mass: (0.14 millionth of an electron mass).

25. Forces inside the Atom • Typical size of a nucleus: 10-15 m. Strong nuclear force holds protons and neutrons together. • Typical size of an atom: 10-10 m. Electromagnetic force holds protons and electrons together.

26. Particle Accelerator Fermilab Accelerator (Proton Synchrotron), Batavia, Illinois

27. Particle Accelerator Main accelerator ring, Fermilab

28. Four Forces

29. Hundreds of New Particles • “The muon, who ordered that?” I.I. Rabi • “If I wanted to remember the names of all of these particles, I would have been a biologist.” Leon Lederman • “As the number of particles increases, all that increases is our ignorance.” Martinus Veltman

30. Leptons and Hadrons • Leptons • e, , , e, , , • no internal structure • conserved • do not interact via the strong force • Hadrons • p, n + hundreds of particles • internal structure (quarks) • baryons are conserved, mesons are not • interact via the strong force

32. Quark Properties

34. Cloud Chamber • First developed in 1911 by Charles Wilson. • Uses a supercritical vapor that is triggered to condense when a charged particle passes through.

35. Cosmic Rays • Energetic particles from space. • When particles interact with our atmosphere, they release showers of high energy particles.

36. Where do Cosmic Rays come from? • Most (lower energy) cosmic rays come from the Sun. • The amount of solar cosmic rays correlates with low numbers of sunspots. • This is because the Sun’s magnetic field partially shields the Earth.

37. Galactic Cosmic Rays • Other cosmic rays come from outside the solar system. (And perhaps the Galaxy). • Possible origins: • Supernovas • Pulsars • Black holes at centers of active galaxies

38. Nuclear Energy and Nuclear Weapons Paul J. Thomas Department of Physics and Astronomy UW-Eau Claire

39. Nuclear Fission

42. Making Plutonium from Uranium • 238U is 99.3% of naturally occurring uranium. It is not fissionable. • 235U is 0.7% of naturally occurring uranium. It is fissionable. • 239Pu is also fissionable, and can be made from 238U in nuclear reactors.

44. How to make a nuclear weapon

45. The Trinity Test: July 16, 1945 Plutonium implosion bomb 18.6 kT TNT equivalent

46. Hiroshima: August 6, 1945 Uranium gun bomb 15 kT TNT equivalent 150,000 dead by end of 1945

47. Nuclear Fusion: pp chain Mass of 4 protons + 2 electrons: 6.694 × 10-24 g Mass of 4He nucleus: 6.644 × 10-24 g Difference: 0.050 × 10-24 g Energy released (E = mc2): 4.4 × 10-12 J 6 × 1014 g of H  He per second!