Transcript Energy
Nuclear Physics …or 2+2≠4 Nuclear Basics • Mass Number = number of nucleons (p+ + n0) • Atomic Number = number of protons • Q: If like charges repel, how is a nucleus bound together? • A: Strong nuclear force • Stronger than gravitational or electrostatic forces but only over a very short range (± 3 x 10-15 m) • Puts upper limit on size of nucleus (protons at opposite ends of large nucleus repel each other and break nucleus apart Nuclear Basics (cont.) • Isotopes • Same number of protons but different number of neutrons • e.g. Carbon 12 (12C6) and Carbon 13 (13C6) • Identification • • • • Chemical symbol and atomic number (subscript) must agree! Neutron: 1n0 Proton: 1p1 Electron: 0e-1 Radioactivity • Discovered by Henri Becquerel – 1896 (by accident) • Found a certain mineral (turned out to be uranium!) darkened a photographic plate even when wrapped to exclude light • Unlike X-Rays, radiation occurred without external stimulus • Marie and Pierre Curie – 1898 (N.P. 1903 w/ Becquerel) • Unstable nucleus (polonium, radium) emits radiation as it decays or disintegrates • Radiation unaffected by physical (heating/cooling) or chemical treatments • Many more radioactive elements found soon after • Some were “naturally radioactive”, others could only be produced by nuclear reactions in a laboratory (“artificially radioactive”) Radioactivity • Ernest Rutherford (1898) • Emitted rays could be classified into 3 types: • Alpha – positive, low penetration, high damage • Beta – negative, medium penetration • Gamma – neutral, deep penetration • Some effects of radiation on the human body • Positive uses of radiation Decay Types Alpha • Unstable nucleus releases α particle (4He2 nucleus) • α particle is massive • New element is formed with 2 less protons and 2 less neutrons • e.g. 232U92 -> 228Th90 + 4He2 Beta • Unstable nucleus releases β particle (electron) • Neutron decays into proton • Electron is created by decay in nucleus (didn’t exist before) • e.g. 14C6 -> 14N7 + 0e-1 Decay Types (cont.) Gamma • Unstable nucleus of excited • • • • atom releases γ particle (photon) Photon is a very high energy EM wave (check spectrum) Dangerous! e.g. 12C6* -> 12C6 + γ Atoms that remain in excited state for long time are called “metastable” Decay Rates and Half-Life • Decay Rate: N = Noe-λt • No = number of nuclei at t = 0 • N = number of nuclei remaining at time t • λ = decay constant (isotope specific: 10-22 s to 1021 yrs) • Half Life • Time for half of the original amount to decay • N = No /2 • T1/2 = (ln 2) / λ • Radioactive Dating • Dating based on the known decay rate of the omnipresent 14C6 molecule 2+2≠4 • Hydrogen Nucleus • • • • mp = 1.0078 u mn = 1.0087 u mp + mn = 2.0165 u Mass of H nucleus (p + n) = 2.0141 u • Δm = 0.0024 u !!! • Atomic nuclei always have less mass than the combined masses of their constituent particles • “Mass Defect” (Δm) = Nuclear Binding Energy • = Δmc2 • Energy required to break apart nucleus: 931.5 MeV/u • Hydrogen: (0.0024 u)*(931.5 MeV/u) = 2.2 MeV Binding Energy Per Nucleon • Binding Energy is not energy that the nucleus has, it is energy debt • Greatest Binding Energy = Greatest mass defect (Fe) • Nuclei higher in the chart are more stable (most tightly bound together) Fission (or… Δm goes BANG!) • Slow moving neutron enters unstable nucleus (235U) • Nucleus oscillates and splits into: • 2 smaller nuclei (Ba/Kr) of less total mass than Uranium (mass defect) • 2 or 3 fast neutrons depending on split (2.4 avg.) • Δm -> ± 200 MeV Energy • Right to left on Nuclear Binding Energy graph Fission - Chain Reaction • Ejected neutrons collide with more 235U atoms to continue fission reactions • Neutrons need to be slowed in order to fission more Uranium • Fission rate can • Decrease: small explosion • Remain constant: nuclear reactor • Increase: atomic bomb • “Critical Mass” needed to sustain reaction based on geometry Fission Bomb • Manhattan Project (formed 1941) • Einstein’s letter to Roosevelt (1939) • Led by Robert Oppenheimer • First controlled nuclear fission reaction - 1942 • Enrico Fermi (N.P. 1938 – radioactivity/neutrons) • Chicago – underneath U. of Chicago football field • Most difficult obstacle: Needed enriched 235U separated from the much more common 238U • Oak Ridge, TN – Gaseous diffusion * – Thermal diffusion • Electromagnetic (cyclotron – Ernest Lawrence) • First nuclear bomb detonation • July 16, 1945 – New Mexico desert • Codename: “The Gadget” • 20 kT TNT yield • Oppenheimer: “I am become death, destroyer of worlds.” Destruction • Little Boy • • • • • Uranium – gun type detonator Enola Gay Dropped 8/6/45 over Hiroshima, Japan 18 kT yield 66,000 initial casualties / est. 144,000 total • Fat Man • • • • • • Plutonium – Implosion triggered Bock’s Car Dropped 8/9/45 over Nagasaki, Japan 20 kT yield Bomb missed target by over a mile 39,000 initial casualties / est. 70,000 total Fusion • Combining two smaller nuclei to form larger nucleus • Larger nucleus has greater binding energy (mass defect) • 2H1 + 2H1 = 4He2 + Energy • High temperature needed to overcome repulsion • Sun • Fusion equilibrium • All elements in universe created by Hydrogen raw material H Bomb – The Ultimate Destruction • “Thermonuclear” / “Superbomb” / “Atom Bomb” • Proposed by Edward Teller • "Inspiration for a hydrogen bomb came from the sun and the stars." • Many, including Oppenheimer, opposed • Fear of USSR possessing H bomb drove Truman to give the go-ahead in 1950 • First bomb attempt 1952 (Ivy Mike) • 10,000 kT yield • Less than 1 year later: USSR’s Joe 4