Lecture 35 Chapter, Sections 4-6 Nuclear Reactions Fission Reactions Fusion Reactions Stellar Radiation Radiation Damage
Induced Nuclear Reactions Reactions in which a nuclear projectile collides and reacts with another nucleus Neutron-Capture Reactions usually exothermic, the produced nuclide usually decays by proton or γ emission. 5 4 7 N 0n 7N 6C 4 m 5 7 N 0n 7N 6C 4 m 3 Binuclear reactions collision of two nuclei at very high energy. 4 8 7 7 N α 9F 8O 4 m p p
Elements Z=43, 6, 85 and Z > 9 are not naturally occurring on Earth. They can be made from nuclear reactions. Cyclotron a positive particle accelerator used to fabricate some of the unnatural elements. Doesn t work well for heavy particles Linear accelerator a heavy-ion accelerator. Accelerators mainly used to study what happens during nuclear reactions. Making Synthetic Elements
Nuclear Fission Fission splits a nucleus into two fragments, which are themselves usually unstable The reaction also releases neutrons and loads of energy. 35 38 95 9 U 0 n 54 Xe 38 Sr 3 The total energy released could be determined by the same kind of mass difference calculation we used earlier. 0 n
Nuclear Fission Critical Mass the amount of material that is just large enough to recapture one neutron for every fission reaction, thus causing another fission, which causes another, which Thus, fission becomes self-sustaining. Reaction growing exponentially = bomb
Nuclear Reactors If number of neutrons absorbed can be carefully controlled, then the rate of reaction can be kept constant Constant release of energy = useful The rate of fission is controlled by adjusting the number of recaptured neutrons with Cd Moderator (water or graphite) slows neutrons smaller mass needed to become critical
Nuclear Fusion Fusion is easiest for lightest nuclides (smallest charge) Generates a small amount of radioactive by-products Fusion can be induced from a particle accelerator Small scale does not release useful amounts of energy Fusion begins when temperature is high enough to overcome Coulombic repulsion Not dependent on mass of reactants (i.e. no critical mass) So, just get some hot fusion! Easy! Critical temperature is 0 7 K The whole trick is to contain the fusion reaction 6 3 Li 3 0 n e e Fusion not contained == bomb Fusion bomb can be BIG because of lack of critical mass 5m 7m 3 Li 4 4 e 3 0 n
Tokamak One solution is a toroid of magnets a tokamak reactor eat provided by electrical resistance, neutron beams and RF
Inertial Fusion Second possibility is to confine the plasma with the same beams that supply heat (laser or particle) Momentum of incoming beams holds fuel in place
Stellar Nuclear Reactions First-generation star Initial fuel almost entirely hydrogen Eventually creates elements up to iron (Z=6) Supernovae the explosion of a star that ejects its nuclides into space. Second-generation stars create still-heavier elements Our sun is probably a third-generation star e Be e e e e m m 4 6 4 3 0 0 3 0 γ β γ β
Effects of Radiation Radiation Damage Radiation that passes through matter rips off electrons ionization One α particle can generate more than 0 5 cations The cations are often chemically reactive. Immediate health effects The formation of ions destroys living cells. Cells that divide most rapidly tend to be the most easily damaged bone marrow, white blood cells, blood platelets, the lining of the GI tract, the cells in the gonads Long-term effects Alteration of DNA Genetic mutations of offspring
Tissue Penetration
Effects of Radiation on DNA Oxidation Breakage of DNA strand Chemical modification of bases Methylation of bases ydrolysis of bases γ Damage can be induced by gamma rays, X- rays, UV-light and reactive oxygen species generated by normal metabolic processes If one strand is damaged, this can usually be repaired Double strand damage is harder to repair
Radiation Shielding Because alpha particles are massive easily stopped ~ mm of solid material (paper, cloth, skin) Beta particles penetrate much farther 0-00 mm of dense solid (plastic, metal, concrete) Gamma rays are difficult to shield Several meters of concrete/metal Several cm of lead
Today Finish CAPA #0 A little review anyone? Death by Chocolate seminar by oward Peters Schaap 000 7:30 pm Thursday Death by Chocolate seminar by oward Peters Wichers Auditorium 3:00 pm Friday Chem seminar Mary Rodgers (Wayne St.) Transition Metal Complexes Schaap 000 4:00 pm Don t forget CAPA # due Saturday! (Last seminar of semester coming up next Wednesday)