Nuclear Physics
20th Century Discoveries
Historical Developments
• 1895: Roentgen discovered X-rays• 1896: Becquerel discovered radioactivity• 1897: Thomson discovered electron• 1900: Planck “energy is quantized”• 1905: Einstein’s theory of relativity• 1911: Rutherford discovered the nucleus• 1913: Millikan measured electron charge
Historical Developments
• 1925: Pauli’s exclusion principle• 1927: Heisenberg’s uncertainty principle• 1928: Dirac predicts existence of antimatter• 1932: Chadwick discovered neutron• 1942: Fermi first controlled fission reaction• 1964: Gell-Mann proposed quarks
The Nucleus
• Mass number (A) is number of nucleons (protons + neutrons)
• Atomic number (Z) is number of protons• Neutron number (N) number of neutrons• Often, mass number and atomic number are
combined with chemical symbol
aluminum, Z = 13, A = 27Al27
13
Isotopes
• Atoms of the same element have same atomic number but can have different mass numbers
• These are called isotopes: atoms of the same element with different number of neutrons
• Chemical properties are the same but nuclear properties are different
Nuclear Mass
• Nuclei are extremely dense, about 2.3 x 1014 g/cm3
• Nuclear mass usually measured with atomic mass unit (u)
• Based on mass of carbon-12 atom whose mass is defined as 12 u
• 1 u = 1.6605402 x 10-27 kg
Mass-Energy
• Nuclear mass can also be expressed in terms of rest energy by using Einstein’s famous equation E = mc2
• Mass is often converted to energy in nuclear interactions
• Substituting values for mass of 1u and converting to eV, we find 1u =931.50 MeV
Nuclear Stability
• Since protons have positive charge, they will repel each other with electric force
• Must be a stronger, attractive force holding them together in nucleus
• This force usually called the strong force• Strong force acts only over extremely small
distances• All nucleons contribute to strong force
Nuclear Stability
• Neutrons add to strong force without adding to repelling electrical force, so they help stabilize nucleus
• For Z > 83, repulsive forces can’t be overcome by more neutrons and these nuclei are unstable
Binding Energy
• Binding energy is difference between energy of free, unbound nucleons and nucleons in nucleus
• Mass of nucleus is less than mass of component parts
• Difference in mass is mass defect and makes up binding energy (E = mc2)
Nuclear Decay
• Unstable nuclei spontaneously break apart and emit radiation in the form of particles, photons, or both
• Process is called radioactivity• Can be induced artificially• Parent nucleus decays into daughter
nucleus
Types of radiationParticle Symbo
lComposition Charge Effect
alpha a 2 protons2 neutrons
+2 mass lossnew element
beta b-
b+electronpositron
-1+1
same massnew element
gamma g photon 0 energyloss
Alpha radiation
• Least penetrating, can be stopped by sheet of paper
• Decreases atomic number by 2, mass number by 4
• Is actually a He nucleus, will quickly attract 2 electrons and become helium
Beta radiation
• Usually a neutron decays into a proton and an electron
• Missing mass becomes kinetic energy of electron
• Atomic number increases by 1, neutron number decreases by 1, mass number is the same
Beta Radiation
• Inverse beta decay proton emits positron and becomes neutron, decreasing atomic number
• Betas can be stopped by sheet of aluminum• Involves emission of antineutrinos (with e-)
or neutrinos (with e+) also
Gamma radiation
• Most penetrating, will penetrate several centimeters of lead
• High energy photon emitted when nucleons move into lower energy state
• Often occurs as a result of alpha or beta emission
Nuclear Decay
• In many cases decay of parent nucleus produces unstable daughter nucleus
• Decay process continues until stable daughter nucleus is produced
• Often involves many steps called a decay series
Writing Nuclear Reactions
• Write chemical symbol with mass number and atomic number of parent nucleus
• On right side of arrow, leave a space for the daughter element and write the symbol for the type of emission occurring
• alpha: beta: neutron:He42 e0
1 n10
Writing Nuclear Reactions
• Mass and charge are conserved quantities so totals on left side of equation must equal totals on right of equation for both the mass numbers and the atomic numbers
• Calculate atomic number of daughter and look up its symbol on periodic table
• Calculate mass number of daughter
Half-Life
• Decay constant for a material indicates rate of decay
• Half-life is the time for ½ of a sample to decay; after 2 half-lives, ¼ of sample remains; after 3, 1/8 remains
• Half-lives range from less than a second to billions of years
Nuclear Fission
• Heavy nucleus splits into two smaller nuclei• Energy is released due to higher binding
energy per nucleon (and so less mass) in smaller nuclei
• Often started by absorption of a neutron by large nucleus making it unstable
• U-235 and Pu-239 are usual fission fuels for reactors and atomic bombs
Nuclear Fission
• Fission products include two smaller elements, high energy photons, and 2 or 3 more neutrons
• Neutrons then can be absorbed by other nuclei creating chain reaction
• Need a minimum amount of fuel for sustained reaction called critical mass
Nuclear Fusion
• Two light nuclei combine to form heavier nucleus
• Product has higher binding energy (less mass) so energy is released
• Fusion occurs in stars and hydrogen bombs (thermonuclear)
• Stars fuse protons (hydrogen) and helium atoms
Nuclear Fusion
• Fusion fuel on earth usually deuterium (heavy hydrogen)
• For fusion to occur, electrostatic repulsion forces must be overcome so nuclei can collide
• Extremely high temperatures and pressures needed
Nuclear Fusion
• Sustained, cost-effective fusion reaction has not been achieved
• Would be better then fission because:• products are not radioactive• fuel is cheap and plentiful• no danger from critical mass
Quarks and Antimatter• Protons and neutrons are composed of
smaller particles called quarks, considered fundamental particles
• 6 types of quarks exist but only two in common matter: up and down
• Proton = uud; neutron = udd• Each fundamental particle has a
corresponding antimatter particle with opposite charge