Nuclear Physics

1. 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 = 1.6605402 x 10-27 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 =931.50 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

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