1. 1 Nuclear and Particle Physics

2. 2 Nuclear Physics Back to Rutherford and his discovery of the nucleus Also coined the term “proton” in 1920, and described a “neutron” in 1921 Neutron discovered by Chadwick in 1932 Ernest Rutherford 1871-1937 m e = 9.1 x 10 -31 kg m N = 1.6749 x 10 -27 kg m P = 1.6726 x 10 -27 kg James Chadwick 1891-1974 nucleons

3. 3 Nuclides and Isotopes To specify a nuclide: Z is the atomic number = number of electrons or protons A is the mass number = number of neutrons + protons So number of neutrons = A-Z Number of protons = Z Isotopes – same atomic number, different mass number e.g. carbon: Many isotopes do not occur naturally, also elements > U

4. 4 Sizes We saw with the Bohr model that radius of the atom depended on atomic number Nucleus = protons + neutrons = mass number The volume of a nucleus is proportional to the mass number

5. 5 Masses Mass spectrometer 1 atomic mass unit (u.) = 1.6606 x 10 -27 kg = 931.5 MeV Fixed so that carbon = 12.00000 u m N = 1.6749 x 10 -27 kg = 1.0087 u m P = 1.6726 x 10 -27 kg = 1.0078 u

6. 6 Binding Energy Total mass of a nucleus < sum of masses Example: Mass of helium nucleus = 6.6447 x 10 -27 kg Contains 2 protons and 2 neutrons Mass = 2 x (1.6749 x 10 -27 + 1.6726 x 10 -27 ) kg = 6.6950 x 10 -27 kg Difference = (6.6950 – 6.6447) x 10 -27 = 0.0503 x 10 -27 kg Energy = mc 2 = 0.0503 x 10 -27 x c 2 = 4.53 x 10 -12 J = (4.53 x 10 -12 ) / (1.6 x 10 -19 ) = 2.83 x 10 7 eV = 28.3 MeV

7. 7 Atomic Mass Units 1 u = 931.5 MeV m N = 1.6749 x 10 -27 kg = 1.0087 u m P = 1.6726 x 10 -27 kg = 1.0078 u Mass of helium nucleus = 4.0026 u

8. 8 Atomic Mass Units Same calculation Mass of 2p + 2n = 2 x (1.0078 + 1.0087) = 4.0330 u Difference = 0.0304 u Binding energy = 0.0305 x 931.5 = 28.3 MeV 4.0330 u

9. 9 Average Binding Energy Graph He – 4 nucleons, 28.3 MeV total: average = 7.075MeV

10. 10 Attractive? How does nucleus stay together? Like charges repel! Force stronger than electric force Strong nuclear force Short range (~10 -15 m) Stable nuclides N = Z A > 30-40 – more neutrons Z > 82 – no stable nuclides Strong force can’t overcome repulsion

11. 11 Radioactivity Becquerel, 1896 Emission of radiation without external stimulus Curies – polonium (Po) and radium (Ra) Henri Becquerel 1852-1908 Marie Curie 1867 - 1934 Pierre Curie 1859 - 1906 1903 (Physics) 1911 (Chem) Radioactivity unaffected by heating, cooling, etc.

12. 12 Classification Rutherford classified 3 types of radioactivity according to penetration power Also different charge Important factor: Conservation of nucleon number (neutrons + protons) = (neutrons + protons) Video: “People Pretending to be Alpha Particles”People Pretending to be Alpha Particles

13. 13 Alpha Decay Least penetrating – nucleus of Radium 226 is an alpha emitter: ParentDaughter transmutation Mass of parent > mass of daughter + mass of alpha Difference = kinetic energy

14. 14 Example 232.03714 u  228.02873 u + 4.002603 u total = 232.03133 u Lost mass = 232.03714 – 232.03133 = 0.00581 u 0.00581u x 931.5 MeV/u = 5.4 MeV (some recoil)

15. 15 Beta decay  One electron What is lost is NOT an orbital electron Instead a neutron changes to a proton + electron So (6p + 8n) => (7p + 7n) + e -  - decay

16. 16 Example Keep track of electrons! Carbon 14 has m = 14.003242 u 6 electrons Nitrogen 14 has m = 14.003074 u normally 7 electrons But in the decay, the nitrogen would have 6 electrons However the total on the r.h.s. of the equation has 7 So difference = 0.000168 u = 0.156 MeV = 156 keV

17. 17 Conservation of energy Energy of decay = 156 keV = problem! ?

18. 18 A new particle Proposed by Pauli (1930) - neutrino Theory by Fermi Discovered 1956 Zero charge, ~0 rest mass Wolfgang Pauli 1900-1958 Enrico Fermi 1901-1954 antineutrino “Zero rest mass” – speed of light 1998 – Super Kamiokande – some mass Cosmic neutrino detection

19. 19 More on positrons Many isotopes have more neutrons than protons  Decay by emission of electron Other isotopes have more protons than neutrons  Decay by emission of positron Proton changes to a neutron + positron  + decay

20. 20 Annihilation Proton changes to a neutron + positron  + decay Positron annihilation Application – positron emission tomography

21. 21 Positron Emission Tomography PET – basis – use radio-labelled compounds, i.e. those containing a radionuclide. Positron emitters: As an example, oxygen-15 can be used to look at oxygen metabolism and blood flow. Fluorine-18 is commonly used to examine cancerous tumours.

22. 22 PET - method Annihilation produces two back-to-back 511 keV photons Simultaneous detection

23. 23 Electron capture Nucleus absorbs orbiting electron Proton changes to neutron Usually K electron X-ray emission as outer electron jumps down to K

24. 24 Gamma decay  Most penetrating  = photon. High energy *Excited nucleus  lower energy state Energy levels far apart = keV or MeV  - (13.4 MeV)  - (9.0 MeV)  (4.4 MeV)

25. 25 Homework... 1.p.902,#6; 2.p.908, Practice 25B; 3.p.912,Section Review 4.p.928, 30-37; 5.p. 930, 56,60; 6.Read through lab for next time; answer pre-lab questions