Nuclear forces and Radioactivity

1. Nuclear forces and Radioactivity The nucleus is a competition between opposing forces Radioactivity is a result of imbalance between the forces

2. Learning objectives • Describe the basic forces and particles involved in nuclear structure • Describe principles behind nuclear decay and radioactivity • Describe the particles emitted in nuclear decay • Define half-life and apply the concept to simple problems • Describe the relationship between energy and matter • Identify the differences between nuclear fission and fusion and their importance in generation of nuclear power

3.     Forces act in opposing directions • Electrostatic repulsion: pushes protons apart • Strong nuclear force: pulls protons together • Nuclear force is much shorter range: protons must be close together

4.   Neutrons only experience the strong nuclear force • Proton pair experiences both forces • Neutrons experience only the strong nuclear force • But: neutrons alone are unstable

5.     Neutrons act like nuclear glue • Helium nucleus contains 2 protons and 2 neutrons – increase attractive forces • Overall nucleus is stable  

6. As nuclear size increases, electrostatic repulsion builds up • There are electrostatic repulsions between protons that don’t have attractive forces • More neutrons required Long range repulsive force with no compensation from attraction      

7. Neutron to proton ratio increases with atomic number Upper limit of stability

8. Upper limit to nuclear stability • Beyond atomic number 83, all nuclei are unstable and decay via radioactivity • Radioactive decay (Transmutation) – formation of new element Alpha particle emitted Mass number Atomic number Atomic number decreases

9. Stability is not achieved in one step: products also decay • Atomic number increases • Neutron:proton ratio decreases • Beta particle emission occurs with neutron-excess nuclei • Alpha particle emission occurs with proton-heavy nuclei Beta particle emitted

10. Summary of types of radiation

11. Radioactive series are complex The decay series from uranium-238 to lead-206 Each nuclide is radioactive and undergoes nuclear decay Left-pointing longer arrows (red) are alpha emissions M and Z decrease Right-pointing shorter arrows (blue) are beta emissions M constant, Z increases

12. Half-life measures rate of decay • Concentration of nuclide is halved after the same time interval regardless of the initial amount – Half-life • Can range from fractions of a second to millions of years

13. Fission and fusion: Radical nuclear engineering • Attempts to grow larger nuclei by bombardment with neutrons yielded smaller atoms instead. • Distorting the nucleus causes the repulsive forces to overwhelm the attractive • The foundation of nuclear energy and the atomic bomb

14. Nuclear fission • Nuclear fission produces nuclei with lower nucleon mass • One neutron produces three: the basis for a chain reaction – explosive potential • Neutrons must be obtained from other nuclear processes such as bombardment of aluminum with alpha particles

15. Chain reactions require rapid multiplication of species

16. Chain reactions have the potential for nuclear explosions • Bomb requires creation of high rate of collisions in small volume • How to achieve this at the desired time in a controlled manner?

17. The importance of U-235 • U-235 is less than 1 % of naturally occurring uranium, but undergoes fission with much greater efficiency than U-238 • Fission can follow many paths: over 200 different isotopes have been observed • Each process produces more neutrons than it consumes

18. Enrichment of uranium • Weapons grade uranium requires a high concentration of U-235 • This is achieved by isotope separation • The lighter U-235 diffuses more rapidly than the heavier U-238 in the gas phase as UF6

19. Total of mass and energy is conserved but are inter-changeable • Fission: combined mass of smaller nuclei is less than the original nucleus A  B + C MA > MB + MC • Loss in mass equals energy released: E = mc2 (Einstein’s relation) • Smaller nuclei are more stable • Fission of U-235: 0.08 % of mass is converted into energy

20. Comparison of nuclear and chemical energy sources • Conversion of tiny amount of matter into energy produces masses: 1 gram  1014 J • Chemical process: • 1 gram fuel produces 103 J • Nuclear process: • 1 gram uranium at 0.08 % produces 1011 J

21. Nuclear fusion: opposite of fission • Small nuclei fuse to yield larger ones • Nuclear mass is lost • Example is the deuterium – tritium reaction • About 0.7 % of the mass is converted into energy +E

22. The sun is a helium factory • The sun’s energy derives from the fusion of hydrogen atoms to give helium

23. Fusion would be the holy grail if... • The benefits: • High energy output (10 x more output than fission) • Clean products – no long-lived radioactive waste or toxic heavy metals • The challenge: • Providing enough energy to start the process – positive charges repel • Reproduce the center of the sun in the lab • Fusion is demonstrated but currently consumes rather than produces energy