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