Enrico Fermi Leo Szilard Al Wattenberg
Discovery of Radioactivity Radioactivity was first discovered in 1896 by Henri Becquerel (he left some pitchblende on an envelope of unexposed film in a drawer.) He won a Nobel Prize in 1903 for his discovery.
Marie Curie, the first person to win two Nobel Prizes and the only person to win one in physics and chemistry, discovered the radioactive elements radium and polonium. Element number 96, Curium, was named after her.
Atomic Nucleus • Atoms are usually electrically neutral • At the center of an atom is its nucleus • Extremely small1/100,000th of the atom’s diameter, measured in 10-15 meters or “femtometers” • Contains most of the atom’s mass and most of the atoms potential energy (E=mc2) • Contains protons and neutrons
Structure of Nucleus • Nucleus contains two kinds of nucleons • Protons (+) and Neutrons (neutral) • Two forces are active in a nucleus • Electrostatic repulsion between protons • Nuclear force attraction between nucleons Short distances: nuclear > electric Long distances: electric > nuclear
Heavy nuclei need more neutrons than protons to balance out electrostatic repulsion
New ideas for today • Radioactivity • Carbon dating • Fission • Nuclear weapons • Nuclear reactors
Radioactivity • Large nuclei have two problems: • Too many protons produce too much electrostatic potential • Too many neutrons, then neutrons are unstable • Delicate balance between protons and neutrons • Large nuclei tend to fall apart spontaneously; we call this fission protons+neutrons protons
Geiger counter Types of Radiation Cloud chamber • Alpha radiation Helium nucleus (2p+2n) • Beta Radiation high-energy electrons or positrons resulting from a nucleon decay • Gamma rays high-frequency light
i>clicker question 210Po decays by alpha emission. Which of the following is a product of the decay? • 206Po • 206At • 208Pb • 206Pb • 210At Phys 150 Lecture 7
Carbon Dating • 12C and 14C naturally occurring • Plants and animals ingest both until death • 14C decays, 12C stable
This animation is for a nucleus that has a half-life of 10 minutes • 14C has a half-life of 5730 years
i>clicker question You have 1g of pure 210Po. Twelve months later, your sample contains 0.125g (=1/8th g) of 210Po. What is the half life of 210Po? • 12 months • 6 months • 4 months • 3 months • 2 months Phys 150 Lecture 7
E = mc2 A little mass is converted into a lot of energy !!!
Induced Fission Large nuclei can breakwhen struck • Collision knocks nucleons outof stable equilibrium • Neutrons make ideal projectiles for inducing fission
Mouse traps Chain Reaction Dominoes • Neutrons can induce fission, which releases more neutrons • This cycle can repeat: Chain reaction! • Each fission releases energy • Many fissions release huge amounts of energy • Sudden energy release produces immense explosion
Requirements for an Atomic Bomb • Initial neutron source • Fissionable material (allowing induced fission) • Fissions must release additional neutrons • Material must use fissions efficiently (critical mass) Critical Mass:enough fissionable material Critical Assembly:put the critical mass together fast enough so you get a nuclear reaction and not a “dud”
Fissile Materials • 235U and 239Pu are the most common fissile materials. • “Fissile” means fission can be induced using low energy neutrons.
Fissile Materials: U-235 • Natural uranium contains mostly two isotopes: 235U (0.7 %) and 238U (99.3%) • Isotope: has same number of protons (same element) but different numbers of neutrons in nucleus • U-235 is needed to makereactors and warheads • Percentage of U-235 must beincreased by process calledenrichment • Uranium for commercialreactors is typically enrichedto a few percent • Uranium for weapons andNaval reactors are enriched to >90%
Little Boy • 235U hollow sphere below critical mass (60kg) • Cannon fired plug through sphere to exceed critical mass / critical assembly • Tungsten-carbide shell containedexplosion initially
Gadget & Fat Man • 239Pu sphere below critical mass (6 kg) • Crushed by explosives to above critical mass (and critical assembly) • Shell of 238U assisted implosion
Nuclear Reactors • Research • Power generation • Civilian • Aircraft carriers (US, France) • Submarines (US, UK, Russia, China, France)
Nuclear Accidents • Three Mile Island (US) • Cooling pump failed and core overheated (while off) • Chernobyl Reactor 4 (USSR) • Coolant boiled in over-moderated graphite reactor • Exceeded prompt critical • Combination of design flaws and procedural violations
For next class: Read Section 16.3 See you next class!