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