1. 20th Century Discoveries
Nuclear Physics
20th Century Discoveries

2. 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

3. 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

4. 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

5. 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

6. 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 = x kg

7. 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 = MeV

8. 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

9. 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

10. 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)

11. 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

12. Types of radiation Particle Symbol Composition Charge Effect alpha a
2 protons
2 neutrons +2
mass loss
new element
beta b- b+
electron
positron -1 +1
same mass
gamma g
photon
energy
loss

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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:

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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