Nuclear Physics …or ≠ 4

Nuclear Basics Mass Number = number of nucleons (p + + n 0 ) 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 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 ( 12 C 6 ) and Carbon 13 ( 13 C 6 ) Identification Chemical symbol and atomic number (subscript) must agree! Neutron: 1 n 0 Proton: 1 p 1 Electron: 0 e -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 w/ Becquerel) Marie 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:3 types Alpha – positive, low penetration, high damage Beta – negative, medium penetration Gamma – neutral, deep penetration Some effects of radiation on the human body Some effects Positive uses of radiation Positive uses

Decay Types Alpha Unstable nucleus releases α particle ( 4 He 2 nucleus) α particle is massive New element is formed with 2 less protons and 2 less neutrons e.g. 232 U 92 -> 228 Th He 2 Beta Unstable nucleus releases β particle (electron) Neutron decays into proton Electron is created by decay in nucleus (didn’t exist before) e.g. 14 C 6 -> 14 N e -1

Decay TypesDecay 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. 12 C 6 * -> 12 C 6 + γ Atoms that remain in excited state for long time are called “metastable”

Decay Rates and Half-Life Decay Rate: N = N o e -λt N o = number of nuclei at t = 0 N = number of nuclei remaining at time t λ = decay constant (isotope specific: s to yrs ) Half Life Time for half of the original amount to decay N = N o /2 T 1/2 = (ln 2) / λ T 1/2 = (ln 2) / λ Radioactive Dating Dating based on the known decay rate of the omnipresent 14 C 6 molecule 14 C 6 molecule

2 + 2 ≠ 4 Hydrogen Nucleus m p = u m n = u m p + m n = u Mass of H nucleus (p + n) = u Δ m = u !!! Atomic nuclei always have less mass than the combined masses of their constituent particles “Mass Defect” ( Δ m) = Nuclear Binding EnergyNuclear Binding Energy = Δmc 2 Energy required to break apart nucleus: MeV/u Hydrogen: ( 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 ( 235 U) 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 graphNuclear Binding Energy graph

Fission - Chain Reaction Ejected neutrons collide with more 235 U 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 reactornuclear reactor Increase: atomic bombatomic bomb “Critical Mass” needed to sustain reaction based on geometryCritical Mass

Fission Bomb Manhattan Project (formed 1941) Manhattan Project Einstein’s letter to Roosevelt (1939) Einstein’s letter Led by Robert Oppenheimer First controlled nuclear fission reaction Enrico Fermi (N.P – radioactivity/neutrons) Enrico Fermi Chicago – underneath U. of Chicago football field Most difficult obstacle: Needed enriched 235 U separated from the much more common 238 U Oak Ridge, TN –Gaseous diffusion * –Thermal diffusion Electromagnetic (cyclotron – Ernest Lawrence)cyclotron 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) 2 H H 1 = 4 He 2 + Energy High temperature needed to overcome repulsion Sun Fusion equilibrium All elements in universe created by Hydrogen raw material

H Bomb 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