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Nuclear Physics

Nuclear Physics. Micro-world Macro-world Lecture 16. Neutron. By 1930, the charge and masses of many different nuclei were known. The charges were found to be Q nucleus = Z x proton charge & the masses were M nucleus = A x proton mass Z is always an integer

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Nuclear Physics

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  1. Nuclear Physics Micro-world Macro-world Lecture 16

  2. Neutron By 1930, the charge and masses of many different nuclei were known. The charges were found to be Qnucleus = Z x proton charge & the masses were Mnucleus = A x proton mass Z is always an integer A is nearly an integer But, except for hydrogen where A=Z=1, A & Z are different.

  3. Electrons in the nucleus? One idea was that the nucleus contained A protons and A-Z electrons. But Heisenberg’s uncertainty principle forbids this: Suppose Dx= 1x10-14m typical size of a nucleus 6.6x10-34Js 1x10-14m Dpx > = 6.6x10-28kgm/s non-relativistic calculation is invalid Dpx me = 6.6x10-28Js 9x10-31kg Dvx > >speed of light Correct calculation gives Eelectron > 2x106 eV, such electrons would not stay inside a nucleus

  4. Discovery of the neutron source detector James Chadwick Po detector a Be n No electric charge; mass ≈ proton mass

  5. Nucleons(protons & neutrons) + proton neutron mass 1.672x10-27kg 1.675x10-27kg mc2 938.27 MeV 939.57 MeV Charge +1.6x10-19 C 0

  6. Isotopes Same chemistry; very different nuclear physics

  7. Light isotopes A=number of protons + neutrons AX Z Z=# of protons Chemical symbol

  8. All elements have isotopes 235 238 U U 92 92

  9. Nuclear force The very powerful electrical repulsion (100’s of Newtons) must be counteracted by some other very strong attractive force + + This new force is called the Strong Nuclear force

  10. Strong Nuclear force repulsive electrical force Force repulsion is stronger here attraction is stronger here Attractive nuclear force 1.0 2.0 3.0 4.0 5.0 Distance (x10-15m)

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

  12. Nuclei can’t be too large A proton feels electrical repulsion from every proton in the nucleus + It feels a Strong Nuclear attraction only to nearby nucleons + + + + + + + + + + In larger nuclei, the electrical force is bigger while the nuclear force stays the same. + + + + + + + + + + Eventually, the electrical repulsion overwhelms the nuclear attraction + + + There are no stable nuclei above Z=82 (Lead)

  13. Discovery of radioactivity

  14. Different types of radiation +charged Marie Curie a • -- neutral g +++ -charged b

  15. AA-44 adecay ZY Z-2X + 2a

  16. Conservation of energy in a-decay Energy of parent = Energy of daughters parent daughters AA-44 ZY  Z-2X + 2a

  17. Conserv. of energy in a-decay KE = MYc2 - MXc2 - Mac2 = MXc2 + Mac2+KE MYc2 parent daughters v AA-44 ZY  Z-2X + 2a

  18. Energy balance daughters the mass difference, times c2, becomes KE parent

  19. KE = Mparentc2 – (Mdaughters c2) Some mass is changed into Kinetic Energy All the a particles have the same Kinetic Energy a-particle Kinetic Energy

  20. Beta (b) decay 6C 7N 14 14 + + + + +  + e- + 6p 8n + 7p 7n + + + + + + n  p + e- AA ZY  Z+1X + e-

  21. Other beta decays 228228 88Ra  89Ac + e-

  22. Tritium Beta decay 3H 3He + + e- + + 33 1H  2He + e-

  23. Energy balance in beta decay 14N+e- 160 keV 14C 1 keV = 1000 eV

  24. Electron energy in 14C beta decay None of the electrons have 160 keV of kinetic energy they all have less than that amount KEe- What has happened to the “missing” energy?? KEe- 160 keV

  25. Pauli’s “desperate solution” Another “unseen” particle is emitted in b decay process neutrino!! no charge no mass n

  26. 14Cb decay revisited 6C 7N 14 14 e- n 1414 6C  7N + e- + n

  27. 3Hb decay revisited 3H 3He e- n 33 1H  2He + e- +n

  28. Energy balance in beta decay daughters KEe- + KEn parent

  29. NEUTRINOS, they are very small. They have no charge and have no mass And do not interact at all. The earth is just a silly ball To them, through which they simply pass, Like dustmaids down a drafty hall Or photons through a sheet of glass. They snub the most exquisite gas, Ignore the most substantial wall, Cold shoulder steel and sounding brass, Insult the stallion in his stall, And scorning barriers of class, Infiltrate you and me! Like tall and painless guillotines, they fall Down through our heads into the grass. At night, they enter at Nepal and pierce the lover and his lass From underneath the bed-you call It wonderful; I call it crass. John Updike Cosmic gall

  30. Radioactive half-life Initial number of atoms number of halflives N = N0 (½)n Number of remaining atoms

  31. Some half-lives T1/2 3H (3He + e- + n) 12.3 yrs 14C(14N + e- + n) 5730 yrs 238U(234Th + a) 4.5x109 yrs 235U(231Th + a) 7.1x108 yrs 226Ra(222Rn + a) 1600 yrs 28Mg(28Al + e- + n) 21 hrs 213Po(209Pb + a) 4x10-6 s

  32. Carbon-14 dating

  33. 14C content

  34. Shroud of Turin

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