Nuclear Physics Notes CP Physics Ms. Morrison

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

Alpha Decay Helium nuclei Positive charge Slow Easily stopped by paper

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

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

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

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

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: 238 U  4 He + 234 Th 92 2 90 Example: beta decay: 234 Th  0 e + 234 Pa 90 -1 91

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

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

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

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