1 Nuclear and Particle Physics

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 m e = 9.1 x kg m N = x kg m P = x kg James Chadwick nucleons

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 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 Masses Mass spectrometer 1 atomic mass unit (u.) = x kg = MeV Fixed so that carbon = u m N = x kg = u m P = x kg = u

6 Binding Energy Total mass of a nucleus < sum of masses Example: Mass of helium nucleus = x kg Contains 2 protons and 2 neutrons Mass = 2 x ( x x ) kg = x kg Difference = ( – ) x = x kg Energy = mc 2 = x x c 2 = 4.53 x J = (4.53 x ) / (1.6 x ) = 2.83 x 10 7 eV = 28.3 MeV

7 Atomic Mass Units 1 u = MeV m N = x kg = u m P = x kg = u Mass of helium nucleus = u

8 Atomic Mass Units Same calculation Mass of 2p + 2n = 2 x ( ) = u Difference = u Binding energy = x = 28.3 MeV u

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

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

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

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 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 Example u  u u total = u Lost mass = – = u u x MeV/u = 5.4 MeV (some recoil)

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 Example Keep track of electrons! Carbon 14 has m = u 6 electrons Nitrogen 14 has m = 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 = u = MeV = 156 keV

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

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

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

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

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 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 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