nuclear physics …or 2 + 2 ≠ 4. nuclear basics mass number = number of nucleons (p + + n 0 )...
TRANSCRIPT
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 TypesAlpha
• 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