General Physics (PHY 2140)

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General Physics (PHY 2140) Lecture 20 Modern Physics Nuclear Energy and Elementary Particles Fission, Fusion and Reactors Elementary Particles

1 General Physics (PHY 2140) Lecture 20 Modern Physics Nuclear Energy and Elementary Particles Fission, Fusion and Reactors Elementary Particles Fundamental Forces Classification of Particles Conservation Laws Chapter 30 Chapter 30

2 Nuclear Physics Nuclear Reactions Medical Applications Radiation Detectors Previously Review Problem: A beam of particles passes undeflected through crossed electric and magnetic fields. When the electric field is switched off, the beam splits up in several beams. This splitting is due to the particles in the beam having different A. masses. B. velocities. C. charges. D. some combination of the above E. none of the above v = E/B r=mv/qb

3 Fission Processes of Nuclear Energy A nucleus of large mass number splits into two smaller nuclei Fusion Two light nuclei fuse to form a heavier nucleus Large amounts of energy are released in either case

4 Processes of Nuclear Energy Fission A nucleus of large mass number splits into two smaller nuclei Fusion Two light nuclei fuse to form a heavier nucleus Large amounts of energy are released in either case Fusion Fission

5 Nuclear Fission A heavy nucleus splits into two smaller nuclei The total mass of the products is less than the original mass of the heavy nucleus First observed in 1939 by Otto Hahn and Fritz Strassman following basic studies by Fermi Lisa Meitner and Otto Frisch soon explained what had happened Fission of 235 U by a slow (low energy) neutron n+ 92U 92U* X + Y + neutrons 236 U* is an intermediate, short-lived state X and Y are called fission fragments Many combinations of X and Y satisfy the requirements of conservation of energy and charge

6 Sequence of Events in Fission The 235 U nucleus captures a thermal (slow-moving) neutron This capture results in the formation of 236 U*, and the excess energy of this nucleus causes it to undergo violent oscillations The 236 U* nucleus becomes highly elongated, and the force of repulsion between the protons tends to increase the distortion The nucleus splits into two fragments, emitting several neutrons in the process

7 Energy in a Fission Process Binding energy for heavy nuclei is about 7.2 MeV per nucleon Binding energy for intermediate nuclei is about 8.2 MeV per nucleon Therefore, the fission fragments have less mass than the nucleons in the original nuclei This decrease in mass per nucleon appears as released energy in the fission event An estimate of the energy released Assume a total of 240 nucleons Releases about 1 MeV per nucleon 8.2 MeV 7.2 MeV Total energy released is about 240 MeV This is very large compared to the amount of energy released in chemical processes

8 QUICK QUIZ In the first atomic bomb, the energy released was equivalent to about 30 kilotons of TNT, where a ton of TNT releases an energy of J. The amount of mass converted into energy in this event is nearest to: (a) 1 μg, (b) 1 mg, (c) 1 g, (d) 1 kg, (e) 20 kilotons (c). The total energy released was E = ( ton)( J/ton) = J. The mass equivalent of this quantity of energy is: m = c E 2 = J 8 ( m/s) 1 3 = kg ~1g Note: 1 gram TNT = 4184 J (exactly)

Chain Reaction Neutrons are emitted when 235 U undergoes fission These neutrons are then available to trigger fission in other nuclei This process is called a

9 Chain Reaction Neutrons are emitted when 235 U undergoes fission These neutrons are then available to trigger fission in other nuclei This process is called a chain reaction If uncontrolled, a violent explosion can occur The principle behind the nuclear bomb, where 1 g of U can release energy equal to about tons of TNT

10 11 Mt H-bombH

11 Nuclear Reactor A nuclear reactor is a system designed to maintain a self-sustained sustained chain reaction The reproduction constant,, K, is defined as the average number of neutrons from each fission event that will cause another fission event The maximum value of K from uranium fission is 2.5 Two 235 U reactions, one yields 3 the other 2 neutrons In practice, K is less than this A self-sustained sustained reaction has K = 1

Basic Reactor Design Fuel elements consist of enriched uranium (a few % 235 U rest 238 U) The moderator material helps to slow down the neutrons The control rods absorb neutrons When K = 1, the

12 Basic Reactor Design Fuel elements consist of enriched uranium (a few % 235 U rest 238 U) The moderator material helps to slow down the neutrons The control rods absorb neutrons When K = 1, the reactor is said to be critical The chain reaction is self- sustaining When K < 1, the reactor is said to be subcritical The reaction dies out When K > 1, the reactor is said to be supercritical A run-away chain reaction occurs SCRAM = Safety Control Rod Axe Man Cadmium D 2 O, graphite

13 Schematic of a Fission Reactor

14 Nuclear Fusion When two light nuclei combine to form a heavier nucleus Is exothermic for nuclei having a mass less than ~20 (Iron is the limit, Z=26, A=56) The sun is a large fusion reactor The sun balances gravity with fusion energy

15 First steps: Sun s s Proton Cycle Followed by H He or He He fusion: or H + H H + e + ν H + H He + γ H + He He + e + ν e He + He He + H + H Total energy released is 25 MeV e 2% of sun s energyis carried by neutrinos

16 Net Result 4 protons (hydrogen nuclei) combine to give An alpha particle (a helium nucleus) Two positrons One or two neutrinos (they easily escape) Some gamma ray photons (absorbed) The two positrons combine with electrons to form more gamma photons The photons are usually absorbed and so they heat the sun (blackbody spectrum)

17 Fusion Reactors Enormous energy in a small amount of fuel 0.06g of deuterium could be extracted from 1 gal of water This represents the equivalent energy of ~6x10 9 J Fusion reactor would most likely use deuterium and tritium H + H He + n, Q = 3.27 MeV H + H H + H, Q = 4.03 MeV H + H He + n, Q = MeV

18 Advantages of fusion power Fuel costs are relatively small Few radioactive by-products of fusion reaction (mostly helium-3 3 and helium-4) Disadvantages of fusion power Hard to force two charged nuclei together Reactor is complex and expensive Need high temperatures and pressures to achieve fusion (~10 8 K) need a plasma

19 Plasma confinement Plasma ion density, n Plasma confinement time, τ In order to achieve a fusion reaction need to satisfy Lawson s s criterion: nτ 10 s/cm 14 3 nτ 10 s/cm 16 3 Deuterium- tritium reactor Deuterium- deuterium reactor So need 10 8 K for 1 second

20 Fusion Reactors - 1 Inertial confinement Inject fuel pellets and hit them with a laser (lots of lasers) or ion beams to heat them Imploding pellet compresses fuel to fusion densities Doesn t t require plasma confinement via magnetic fields Requires large facility to house lasers and target chamber.

21 National Ignition Facility the facility is very large, the size of a sports stadium the target is very small, the size of a BB- gun pellet the laser system is very powerful, equal to 1,000 times the electric generating power of the United States each laser pulse is very short, a few billionths of a second

22 The beams are generated in the laser bay

23 and deliverd to the target bay.

24 The National Ignition Facility

25 The target chamber

26 Magnetic field confinement Tokamak design a toroidal magnetic field First proposed by Russian scientists Fusion Reactors - 2

27 Fusion Reactors cont. Tokamak Fusion Test Reactor ITER International Thermonuclear Experimental Reactor To be constructed in Cadarache in the South of France.

28 ITER s proposed site layout

29 30.4 Elementary Particles First we studied atoms Next, atoms had electrons and a nucleus The nucleus is composed of neutrons and protons What s s next?

30 Elementary particle interactions The scattering of two electrons via a coulomb force This virtual photon is said to mediate the electromagnetic force. The virtual photon can never be detected because it only lasts for a vanishing small time. An simple example of a Feynman diagram

31 Interactions continued Can have similar diagrams with other particles and other forces Strong force, weak force, gravity Basic idea of exchange of a virtual particle is the common theme.

32 More examples of Feynman diagrams

33 30.5 The Fundamental Forces in Nature Strong Force Short range ~ m (1 fermi) Responsible for binding of quarks into neutrons and protons Gluon Electromagnetic Force 10-2 as strong as strong force 1/r 2 force law Binding of atoms and molecules Photon Weak force ~ 10-6 times as strong as the strong force Responsible for beta decay, very short range ~10-18 W +, W - and Z 0 bosons Gravitational Force times as strong as the strong force Also 1/r 2 force law Graviton 18 m

34 30.6 Positrons and Antiparticles Dirac proposed the positron to solve a negative energy problem (Dirac sea) The general implication is that for every particle there is an antiparticle (symmetry) Other antiparticles: antiproton, antineutrino Usually denoted with a bar over symbol Some particles are their own antiparticles photon, neutral pion: γ, π 0

35 30.7 Mesons Part of an early theory to describe nuclear interactions Mass between a electron and a proton Flavors Charged π meson: π +, π,mass MeV/c 2 Netral π meson,, π 0,mass MeV/c 2 Lifetimes 2.6x10-8 s for π +, π 8.3x s for π 0

36 More Mesons Also have heavier mesons Kaons ~500 MeV/c 2 Eta s 548 and 958 MeV/c 2 (note, mass of η is greater than proton mass)

General Physics (PHY 2140)

General Physics (PHY 2140) Lecture 20 Modern Physics Nuclear Energy and Elementary Particles Fission, Fusion and Reactors Elementary Particles Fundamental Forces Classification of Particles Conservation