Nuclear Physics Notes

1. Nuclear Physics Notes CP Physics Ms. Morrison

2. Radioactive Decay • Elements with more than 83 protons – extra neutrons cannot stabilize nucleus • Lone neutron spontaneously decays into a proton plus an electron (radioactive) • Three types of Radioactive Decay • Alpha • Beta • Gamma

3. Alpha Decay • Helium nuclei • Positive charge • Slow • Easily stopped by paper

4. Beta Decay • Stream of high speed electrons • Negative charge • Results from neutron transforming into a proton • Faster moving • Stopped by sheet of aluminum

5. Gamma Decay • Massless energy • Photons of EM radiation • No charge • High energy • Stopped by thick layer of lead (or several feet of concrete)

6. Radioactive Isotopes • Isotope = atom of an element that has a different number of neutrons • All isotopes of an element are chemically identical • All elements have isotopes • Some isotopes are radioactive and unstable – they will undergo spontaneous radioactive decay

7. Radioactive Half-life • Isotopes decay at different rates • Half-life = time needed for half of radioactive atoms to decay • Decay rates are constant • To calculate amount of sample left: (1/2)n where n = number of half-lives

8. Nuclear Equations • Show alpha or beta decay • Transmutation: one element changes into another element through radioactive decay • Mass numbers and atomic numbers on each side of the equation must be equal • Example of alpha decay: 238U 4 He + 234Th 92290 • Example: beta decay: 234Th0 e + 234 Pa 90-191

9. Nuclear Fission • Splitting of atomic nuclei • Absorbing a neutron causes nucleus to elongate and deform and then split • Releases huge amounts of energy • When nucleus splits – releases neutrons which can be absorbed by other nuclei causing them to split  chain reaction • To sustain chain reaction need critical mass (amount of mass for which each fission event produces an average of one additional fission event) • Subcritical – chain reaction dies out • Supercritical – chain reaction builds up explosively

10. Nuclear Fission, pg 2 • Energy production – use heat from reaction to boil water to produce steam to turn a turbine and generate electricity • Reactor has 3 components: • Nuclear fuel (uranium) with moderator (graphite, water) to slow down neutrons • Control rods – usually cadmium or boron, absorb neutrons, move in and out of reactor to control reaction • Water to transfer heat from reactor to generator • Major drawback – radioactive waste products of fission, need special storage casks

11. Nuclear Fusion • Opposite of nuclear fission • Energy released by light nuclei fusing • Requires very high speeds for nuclei to collide and fuse • Associated with high temps – thermonuclear fusion (welding together of atomic nuclei by high temp) • Difficulties in using for energy production • Reaching high temps needed • Materials to confine fuel melt at high temps – need “nonmaterial” containers, ex. Magnetic field

12. Nuclear Fusion, pg 2 • Have achieved fusion in several devices but instabilities in plasma have prevented sustained reactions • Currently energy costs to achieve reaction greater than energy yield from the reaction • Ideal • Does not require critical mass so cannot get out of control • No air pollution – product is helium • By-products not radioactive • Fuel is plentiful element in universe – Hydrogen • Amount of energy released will be virtually unlimited