Chapter 30: Nuclear Energy and Elementary Particles

1. Chapter 30: Nuclear Energy and Elementary Particles Homework : Read and understand the lecture note. • What is nuclear fission? • Nuclear fission occurs when a heavy nucleus, such as , splits, • or fissions, into two smaller nuclei. • The fission of by slow (low-energy) neutron can be represented • symbolically by : • : an intermediate excited and short-lived state • X, Y : fission fragments that satisfy conservation of energy and charge • A typical reaction of this type is : NuclearFission

2. Nuclear Fission • Nuclear fission (in some detail) • Sequence of events in the nuclear fission • The nucleus captures a thermal (slow-moving) neutron. • An excited state is formed and the excess energy cause oscillation • of the nucleus. • - The nucleus highly elongated, and the repulsive force among • protons enhances the deformation. • - The nucleus splits into to fragments, emitting several neutrons.

3. Nuclear Fission • Nuclear fission (cont’d) • Energy released in the nuclear fission • The binding energy per nucleon for heavy nuclei (mass~240) :~7.2 MeV • The binding energy per nucleon of intermediate mass :~8.2 MeV • Nuclei of intermediate mass are more tightly bound than heavy nuclei. • For a total of 240 nucleons, the energy released (Q-value) in a fission: • Q=240 nucleons/(8.2 MeV/nucleon – 7.2 MeV/nucleon) = 240 MeV

4. Nuclear Fission • Example 30.1 : Fission of uranium • How many neutrons are produced in the fission process • ? • Example 30.2 : A fission-powered world • Calculate the total energy released if 1.00 kg of undergoes fission, • taking the disintegration energy per event to be Q = 208 MeV? Number of nuclei in 1.0 kg of uranium Total energy released • How many kilograms would provide for world’s annual energy needs • (4x1020 J)? Total energy released from Nkg kg of uranium

5. Nuclear Reactors • Nuclear chain reaction • Neutrons emitted when undergoes fission can in turn trigger • other nuclei to undergoes with the possibility of a chain reaction. • Calculations show that without control the chain reaction goes out of • control and results in the sudden release of enormous amount of • energy (1 kg of would produce energy equivalent to 20 ktons of TNT).

6. Nuclear Reactors • Nuclear reactors • A nuclear reactor is designed to control nuclear reactions and maintain • a self-sustained chain reaction. • Moderator slows down neutrons so that they can be absorbed by • uranium more easily. • Control rods absorb very efficiently neutrons to control the reaction rate. • The reproduction constant K, defined as the average number of neutrons • from each fission event that will cause another event. A self-sustained • chain reaction is achieved when K=1. cadmium D2O

7. Nuclear Fusion • Nuclear fusion • The binding energy of light nuclei (mass number <20) is much smaller • than that of heavier nuclei. • When two light nuclei combine to form a heavier nucleus, the process is • called nuclear fusion. • Because the mass of the final nucleus is less than the masses of the • original nuclei, there is extra energy released. • Nuclear fusion in Sun (thermal nuclear fusion reactions) • proton-proton chain: to sustain the nuclear fusion • - the temperature needs to be high enough to overcome the • repulsive Coulomb force between protons • - the density of nuclei must be high enough to ensure a high rate • of collisions The liberated energy is carried by gamma rays, positrons and neutrinos.

8. Nuclear Fusion • Fusion reactors • Scientists and engineers have been trying to create similar conditions • to those in the interior of Sun to achieve self-sustained nuclear fusion • reactions on Earth. • Most promising reactions as fusion reactors are: • Deuterium is abundant on Earth but tritium is radio active with T1/2= • 12.3 yr and undergoes beta decay to 3He. So tritium is rare on Earth. • One of the major problems to achieve fusion reactors is to give to the • nuclei enough kinetic energy to overcome the repulsive Coulomb force.

9. Particle Physics What is the world made of? nucleus Model of Atoms Old view proton electrons e- nucleus quarks Modern view Semi-modern view

10. Particle Physics What is matter made of? Building Blocks of Matter Discoveries of too many “elementary” particles and anti-particles lead to more fundamental model the Standard Model. Anti-particle has the same property as particle except that charge is opposite to that of particle. Hadrons electric charge Proton p : uud Neutron n : udd - Pion p+ : ud Particles made of quarks are called hadrons and among them they interact through strong force. +(2/3)e -(1/3)e Leptons 0 neutrinos n : feel only weak force charged lepton : feel electromagnetic e-,m-,t- and weak force +e

11. Particle Physics How many kinds of forces are there? Fundamental Forces There are four know fundamental forces: An example: Free neutron decay

12. Particle Physics Fundamental Forces Examples of weak interaction - Free neutron decay: n -> p + e- + ne - Muon decay: m- -> e- + ne + nm

13. Particle Physics What is our dream? Unification of Forces Grand Unified Theories (GUTs) Strong Electric Electromagnetic 19th c. Magnetic Electroweak GUTs 21st c.? 20th c. Weak GUTs predict: Neutrino mass/oscillation (found) hard Nucleon decays (not yet found) Gravitational

14. Particle Physics What is neutrino oscillation? Neutrino Oscillation There are three kinds of neutrinos: ne nm nt (flavours) If neutrinos have mass, they can change their identities (flavours) ne nm nt A simple example: nm nt nm n1 - sin q n2 cos q = nt sin q n1 + cos q n2 = n1,2 neutrinos with definite mass Probability nm nm Probability It depends on neutrino energy, masses, q and distance it travels nm nt 1-Probability ~Earth’s diameter 12,000 km Neutrino pathlength (km)

15. Atmospheric Neutrinos Source of atmospheric neutrinos Earth’s atmosphere is constantly bombarded by cosmic rays. Energetic cosmic rays (mostly protons) interact with atoms in the air. These interactions produce many particles-air showers. Neutrinos are produced in decays of pions and muons.

16. 50,000 tons of pure water equipped with 12,000 50 cm photomultipliers and 2,800 20 cm photomultipliers (PMTs). Physicists are having fun on a boat in Super-Kamiokande

17. Atmospheric Neutrinos How does a water Cherenkov detector work? Water Cherenkov Detector: Kamiokande,IMB,Super-Kamiokande,SNO Water is cheap and easy to handle! When the speed of a charged particle exceeds that of light IN WATER, electric shock waves in form of light are generated similar to sonic boom sound by super-sonic jet plane . These light waves form a cone and are detected as a ring by a plane equipped by photo- sensors.

18. Atmospheric Neutrinos How do we detect atmospheric muon and electron neutrinos ? muon-like ring Major interactions: ne + n -> p + e- nm + n -> p + m- Most of time invisible electron-like ring

19. Atmospheric Neutrinos How do we see neutrino oscillation in atmospheric neutrinos? a cos q = a/b q q b Neutrino pathlength downward-going upward-going cos (zenith angle) Probability (nm->nm) Actual probability for measured zenith angle due to measurement errors

20. Atmospheric Neutrinos Evidence of neutrino oscillation/mass with oscillation without oscillation low energy nm low energy ne high energy ne high energy nm First crack in the Standard Model!!!

21. Solar Neutrinos How does the Sun shine? Nuclear fusions generate: - energy/heat/light - neutrinos Kamiokande 1 MeV = 1x106 eV

22. Solar Neutrinos How does the neutral current confirm neutrino oscillation? n + e- -> n + e- Elastic scattering -This reaction is available only for ne . -Available for both water and heavy water.

23. Solar Neutrinos How do we see the Sun underground? Image of Sun by Super-Kamiokande Solar neutrinos background e e Seeing the Sun underground

24. Solar Neutrinos How do we see neutrino oscillation with solar neutrinos? Flux: measured/expected Neutrino deficit!!! Super-Kamiokande : 0.465+-0.005+0.016-0.015 nm is not visible to all experiments above

25. Supernova How do we know detected neutrinos are from a supernova? Birth of a supernova witnessed with neutrinos A few hours before optical observation Kamiokande Number of photomultipliers fired Taken by Hubble Telescope ( 1990) Background level