Transcript Modern Physics

Nuclear Physics
20th Century Discoveries
Historical Developments
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1895: Roentgen discovers X-rays
1896: Becquerel discovers radioactivity
1897: Thomson discovers electron
1900: Planck “energy is quantized”
1905: Einstein’s theory of relativity
1911: Rutherford discovers the nucleus
1913: Millikan measures electron charge
Historical Developments
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1925: Pauli’s exclusion principle
1927: Heisenberg’s uncertainty principle
1928: Dirac predicts existence of antimatter
1932: Chadwick discovers neutron
1942: Fermi first controlled fusion reaction
1964: Gell-Mann proposes 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 27 Al
13
aluminum, Z = 13, A = 27
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
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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 radiation
Particle Symbol Composition Charge Effect
alpha
a
2 protons
2 neutrons
+2
mass loss
new element
beta
bb+
electron
positron
-1
+1
same mass
new element
photon
0
energy
loss
gamma g
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
• 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
1
0
4
• alpha: 2 He beta: -1 e neutron: 0 n
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