Conceptual Physics Nuclear Physics

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Percival Fleming
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1 Conceptual Physics Nuclear Physics Lana Sheridan De Anza College Aug 9, 2017

2 Overview strong nuclear force binding energy and mass defect types of nuclear decay nuclear fission

3 Atomic Structure Atoms have a compact nucleus composed of protons and neutrons. The nucleus therefore has an overall positive charge. A cloud of electrons surrounds the nucleus, attracted by the electrostatic force. 1 Figure from Wikipedia.

4 The Nucleus Why is there an atomic nucleus? Why does it stick together?

5 The Nucleus Why is there an atomic nucleus? Why does it stick together? (Positive charges should repel each other.) Nuclear researchers suggested there should be a strong force (stronger than electric repulsion) that would hold the nuclear particles together.

6 The Fundamental Forces Force Rel. strength Range (m) Gravitational Electromagnetic 10 2 Weak Nuclear < Strong Nuclear 1 < The strong nuclear force holds the nucleons together, overcoming the electrical repulsion. It is sometimes called the residual strong force because the fundamental strong interaction actually holds quarks together to make up protons and neutrons.

7 Neutrons and Protons Proton Neutron The fundamental strong interaction holds quarks together to make up protons and neutrons. 1 Figures by Arpad Horvath, Wikipedia.

8 Binding Energy The binding energy is the amount of energy needed to separate bound particles into individual free particles.

9 Mass Defect Does the inertia of a body depend upon its energy content? (Einstein 1905)

10 Mass Defect Does the inertia of a body depend upon its energy content? (Einstein 1905) Yes. E = mc 2 Bound systems have less inertia (relativistic mass) than the sum of the masses of their parts. m is the mass defect. E bind = ( m)c 2

11 Mass Defect The atomic mass unit (u) is 1/12 of the mass of a carbon-12 atom.

12 Binding Energy H Atom: m = kg (negligible) He Nucleus: m = kg

13 Binding Energy Per Nucleon 1 Figure from Wikipedia, user Bdushaw.

14 Nuclear Stability and Decay Modes 1 Figure from Wikipedia, user Bdushaw.

15 e of the Ra decay described in Figure If the parent nucleus is at r ore the decay, the total kinetic energy of the products is 4.87 MeV. (See Examp Types of Radioactive Decay: α decay.) Most of this kinetic energy is associated with the alpha particle because th ticle is Amuch nucleus less loses massive 2 protons than the and daughter 2 neutrons, nucleus forming 222 arn. newthat lighter is, because t em is element also isolated and ejecting terms and of momentum, α particle (He the nucleus). lighter alpha particle recoils wi uch higher speed than does the daughter nucleus. Generally, less massive p es carry Example, off most Radium of the energy transmutes in nuclear to Radon: decays. xperimental observations of alpha-particle energies show a number of discre 226 rgies rather than a single energy 88Rabecause Rn the + 4 2He daughter nucleus may be left in 226 Ra 88 Before decay K Ra 0 S p Ra 0 K Rn S p Rn 222 Rn 86 After decay K α α S p α Alpha-emitters are used in smoke detectors.

16 Types of Radioactive Decay: β decay Neutrons are a little unstable. A neutron decays to a proton and an electron (β particle) within a nucleus. The resulting nucleus has increased its atomic number by one. Example, Carbon-14 transmutes to Nitrogen: Before decay 14 6 C 14 7N + e + ν Used for carbon dating of organic samples. S pn K N 14 N 7 14 C 6 After decay K C 0 S pc 0 K e S p e Electron Antineutrino K S p The final products of the beta decay of the carbon-14 nucleus S p The final decay of

17 Types of Radioactive Decay: γ decay Immediately after another radioactive process, for example alpha or beta decay, a nucleus can be left in an excited (high energy) state. When that nucleus decays to a low energy state, it emits a high energy photon: a gamma ray C 12 6 C + γ Used for medical imaging. Also to kill bacteria on food or medical equipment.

18 Nuclear Fission An large nucleus captures a free neutron. It splits into two more stable nuclei releasing a lot of energy. Typical example: 1 0n U Ba Kr + 3 ( 1 0 n )

19 Fission Chain Reaction U Ba Sr Kr Xe Y I Nb Sb ons sion U Figure 45.3 A reaction initiate of a neutron. Ur shown in tan, ne and daughter nu

20 Fission Chain Reaction 1 Photo, TVA Watts Bar Nuclear Power Plant, 2

21 Summary strong nuclear force binding energy and mass defect types of nuclear decay nuclear fission Final Thursday, Aug 10.

16.5 Coulomb s Law Types of Forces in Nature. 6.1 Newton s Law of Gravitation Coulomb s Law

16.5 Coulomb s Law Types of Forces in Nature. 6.1 Newton s Law of Gravitation Coulomb s Law 5-10 Types of Forces in Nature Modern physics now recognizes four fundamental forces: 1. Gravity 2. Electromagnetism 3. Weak nuclear force (responsible for some types of radioactive decay) 4. Strong nuclear