Nuclear Physics

20 th 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 = 27 27 13

Al

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 10 14 g/cm 3 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 = mc 2

Mass is often converted to energy in nuclear interactions Substituting values for mass of 1

u

and converting to eV, we find 1

u

=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 = mc 2

)

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 beta b b + gamma g 2 protons 2 neutrons electron positron photon +2 -1 +1 0 mass loss new element same mass new element energy loss

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 2

He

0 1

e

0 1

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

•

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