General Physics (PHY 2140) Lecture 38 Modern Physics Nuclear and Particle Physics Radioactivity Nuclear reactions Nuclear Energy Elementary particles http://www.physics.wayne.edu/~apetrov/PHY2140/ Chapter 29-30 9/21/2018

Lightning Review Last lecture: Nuclear physics Radiation Review Problem: Isotopes of a given element have many different properties, such as mass, but the same chemical properties. Why is this? 9/21/2018

Alpha Decay When a nucleus emits an alpha particle it loses two protons and two neutrons N decreases by 2 Z decreases by 2 A decreases by 4 Symbolically X is called the parent nucleus Y is called the daughter nucleus 9/21/2018

Beta Decay During beta decay, the daughter nucleus has the same number of nucleons as the parent, but the atomic number is one less In addition, an electron (positron) was observed The emission of the electron is from the nucleus The nucleus contains protons and neutrons The process occurs when a neutron is transformed into a proton and an electron Energy must be conserved 9/21/2018

Beta Decay – Electron Energy The energy released in the decay process should almost all go to kinetic energy of the electron Experiments showed that few electrons had this amount of kinetic energy To account for this “missing” energy, in 1930 Pauli proposed the existence of another particle Enrico Fermi later named this particle the neutrino Properties of the neutrino Zero electrical charge Mass much smaller than the electron, probably not zero Spin of ½ Very weak interaction with matter 9/21/2018

Beta Decay Symbolically  is the symbol for the neutrino is the symbol for the antineutrino To summarize, in beta decay, the following pairs of particles are emitted An electron and an antineutrino A positron and a neutrino 9/21/2018

Gamma Decay Gamma rays are given off when an excited nucleus “falls” to a lower energy state Similar to the process of electron “jumps” to lower energy states and giving off photons The excited nuclear states result from “jumps” made by a proton or neutron The excited nuclear states may be the result of violent collision or more likely of an alpha or beta emission Example of a decay sequence The first decay is a beta emission The second step is a gamma emission The C* indicates the Carbon nucleus is in an excited state Gamma emission doesn’t change either A or Z 9/21/2018

Uses of Radioactivity Carbon Dating Smoke detectors Radon pollution Beta decay of 14C is used to date organic samples The ratio of 14C to 12C is used Smoke detectors Ionization type smoke detectors use a radioactive source to ionize the air in a chamber A voltage and current are maintained When smoke enters the chamber, the current is decreased and the alarm sounds Radon pollution Radon is an inert, gaseous element associated with the decay of radium It is present in uranium mines and in certain types of rocks, bricks, etc that may be used in home building May also come from the ground itself 9/21/2018

29.5 Natural Radioactivity Classification of nuclei Unstable nuclei found in nature Give rise to natural radioactivity Nuclei produced in the laboratory through nuclear reactions Exhibit artificial radioactivity Three series of natural radioactivity exist Uranium Actinium Thorium 9/21/2018

Decay Series of 232Th Series starts with 232Th Processes through a series of alpha and beta decays Ends with a stable isotope of lead, 208Pb 9/21/2018

29.6 Nuclear Reactions Structure of nuclei can be changed by bombarding them with energetic particles The changes are called nuclear reactions As with nuclear decays, the atomic numbers and mass numbers must balance on both sides of the equation 9/21/2018

Which of the following are possible reactions? Problem Which of the following are possible reactions? (a) and (b). Reactions (a) and (b) both conserve total charge and total mass number as required. Reaction (c) violates conservation of mass number with the sum of the mass numbers being 240 before reaction and being only 223 after reaction. 9/21/2018

Q Values Energy must also be conserved in nuclear reactions The energy required to balance a nuclear reaction is called the Q value of the reaction An exothermic reaction There is a mass “loss” in the reaction There is a release of energy Q is positive An endothermic reaction There is a “gain” of mass in the reaction Energy is needed, in the form of kinetic energy of the incoming particles Q is negative 9/21/2018

Problem: nuclear reactions Determine the product of the reaction What is the Q value of the reaction? 9/21/2018

Determine the product of the reaction What is the Q value of the reaction? In order to balance the reaction, the total amount of nucleons (sum of A-numbers) must be the same on both sides. Same for the Z-number. Given: reaction Find: Q = ? Number of nucleons (A): Number of protons (Z): Thus, it is B, i.e. The Q-value is then 9/21/2018

Threshold Energy To conserve both momentum and energy, incoming particles must have a minimum amount of kinetic energy, called the threshold energy m is the mass of the incoming particle M is the mass of the target particle If the energy is less than this amount, the reaction cannot occur 9/21/2018

QUICK QUIZ If the Q value of an endothermic reaction is -2.17 MeV, the minimum kinetic energy needed in the reactant nuclei if the reaction is to occur must be (a) equal to 2.17 MeV, (b) greater than 2.17 MeV, (c) less than 2.17 MeV, or (d) precisely half of 2.17 MeV. (b). In an endothermic reaction, the threshold energy exceeds the magnitude of the Q value by a factor of (1+ m/M), where m is the mass of the incident particle and M is the mass of the target nucleus. 9/21/2018

Nuclear Energy

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 9/21/2018

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 235U by a slow (low energy) neutron 236U* 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 9/21/2018

Sequence of Events in Fission The 235U nucleus captures a thermal (slow-moving) neutron This capture results in the formation of 236U*, and the excess energy of this nucleus causes it to undergo violent oscillations The 236U* 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 9/21/2018

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 9/21/2018

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 4.0 × 109 J. The amount of mass converted into energy in this event is nearest to: (a) 1 g, (b) 1 mg, (c) 1 g, (d) 1 kg, (e) 20 kilotons (c). The total energy released was E = (30 ×103 ton)(4.0 × 109 J/ton) = 1.2 × 1014 J. The mass equivalent of this quantity of energy is: 9/21/2018

Chain Reaction Neutrons are emitted when 235U 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 20000 tons of TNT 9/21/2018

Nuclear Reactor A nuclear reactor is a system designed to maintain a self-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 In practice, K is less than this A self-sustained reaction has K = 1 9/21/2018

Basic Reactor Design Fuel elements consist of enriched uranium 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 9/21/2018

Elementary Particles

1. The Big Question of Particle Physics… How did we get from here… … to here? And what does it have to do with heavy quarks?

Time Seems like…

Just after the Big Bang: symmetric Universe equal number of particles and antiparticles Now: asymmetric Universe planets, stars, galaxies, Wayne State, … Note: macroscopic laws of Nature do not distinguish matter and antimatter

A 10,000,000.00 Swedish Kronor question: Where did all the antimatter go? The “Onion paradigm”: identify degrees of freedom see if the problem has a solution if not, dig deeper…

What are the right degrees of freedom? Fire Water Earth Air … that is, according to the Greeks!

What would be the modern picture? Imagine that we have a very powerful microscope…

Modern understanding: the ``onion’’ picture Atom Let’s see what’s inside!

Modern understanding: the ``onion’’ picture Nucleus Let’s see what’s inside!

Modern understanding: the ``onion’’ picture Protons and neutrons Let’s see what’s inside!

Modern understanding: the ``onion’’ picture Collective name for particles containing 3 quarks Mesons and baryons Collective name for particles containing quark and antiquark Let’s see what’s inside!

Modern understanding: the ``onion’’ picture Collective name for particles containing 3 quarks (such as proton and neutron) Mesons and baryons Collective name for particles containing quark and antiquark Let’s see what’s inside! Note: apparent excess of matter over antimatter can be traced to excess of the number of baryons over antibaryons. Thus our Big Problem is called Problem of Baryon Asymmetry of the Universe.

Modern understanding: the ``onion’’ picture Quarks and gluons Let’s see what’s inside!

Modern understanding: the ``onion’’ picture ? … so the answer depends on the energy scale!

… same thing about the interactions

Unification of forces

The Standard Model of particle physics

The Standard Model of Elementary Particle Physics ``Periodic table’’ of matter Interactions: electromagnetic, weak, strong, (gravity)… Contains 26 parameters: needs experimental input + Higgs particle

Conditions for baryon asymmetry Matter-antimatter imbalance in the Universe A.D. Sakharov Baryon (and lepton) number - violating processes to generate asymmetry Universe that evolves out of thermal equilibrium to keep asymmetry from being washed out Matter interactions differ from antimatter interactions (“Microscopic CP-violation”) to keep asymmetry from being compensated in the “anti-world”

Can Standard Model explain baryon asymmetry? does it have “the right stuff”? what are the conditions for the baryon asymmetry? does it have enough of “the right stuff”?

Experimental methods video

Experimental methods

Experimental Facilities I Cornell University SLAC

Experimental Facilities II KEK (Japan) Fermilab (Batavia, IL)

What do physics PhD’s do? Science route Research in physics (national lab, research university) Teaching and research (college) Industry route Computing/engineering jobs in companies Finance industry (problem solving) Scientific Publishing route