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