nuclear forces and radioactivity

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Nuclear forces and Radioactivity. The nucleus is a competition between opposing forces Radioactivity is a result of imbalance between the forces. Learning objectives. Describe the basic forces and particles involved in nuclear structure - PowerPoint PPT Presentation


  • Nuclear forces and RadioactivityThe nucleus is a competition between opposing forcesRadioactivity is a result of imbalance between the forces

  • Learning objectivesDescribe the basic forces and particles involved in nuclear structureDescribe principles behind nuclear decay and radioactivityDescribe the particles emitted in nuclear decayDefine half-life and apply the concept to simple problemsDescribe the relationship between energy and matterIdentify the differences between nuclear fission and fusion and their importance in generation of nuclear power

  • Forces act in opposing directionsElectrostatic repulsion: pushes protons apart

    Strong nuclear force: pulls protons together

    Nuclear force is much shorter range: protons must be close together

  • Neutrons only experience the strong nuclear forceProton pair experiences both forces

    Neutrons experience only the strong nuclear force

    But: neutrons alone are unstable

  • Neutrons act like nuclear glueHelium nucleus contains 2 protons and 2 neutrons increase attractive forcesOverall nucleus is stable

  • As nuclear size increases, electrostatic repulsion builds upThere are electrostatic repulsions between protons that dont have attractive forces

    More neutrons requiredLong range repulsive force with no compensation from attraction

  • Neutron to proton ratio increases with atomic numberUpper limit of stability

  • Upper limit to nuclear stabilityBeyond atomic number 83, all nuclei are unstable and decay via radioactivityRadioactive decay (Transmutation) formation of new element

    Atomic number decreasesAlpha particle emittedMass numberAtomic number

  • Stability is not achieved in one step: products also decayAtomic number increasesNeutron:proton ratio decreases

    Beta particle emission occurs with neutron-excess nucleiAlpha particle emission occurs with proton-heavy nucleiBeta particle emitted

  • Summary of types of radiation

  • Radioactive series are complexThe decay series from uranium-238 to lead-206Each nuclide is radioactive and undergoes nuclear decayLeft-pointing longer arrows (red) are alpha emissionsM and Z decreaseRight-pointing shorter arrows (blue) are beta emissionsM constant, Z increases

  • Half-life measures rate of decayConcentration of nuclide is halved after the same time interval regardless of the initial amount Half-lifeCan range from fractions of a second to millions of years

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

  • Nuclear fissionNuclear fission produces nuclei with lower nucleon mass

    One neutron produces three: the basis for a chain reaction explosive potentialNeutrons must be obtained from other nuclear processes such as bombardment of aluminum with alpha particles

  • Chain reactions require rapid multiplication of species

  • Chain reactions have the potential for nuclear explosionsBomb requires creation of high rate of collisions in small volumeHow to achieve this at the desired time in a controlled manner?

  • The importance of U-235U-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 observedEach process produces more neutrons than it consumes

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

  • Total of mass and energy is conserved but are inter-changeableFission: combined mass of smaller nuclei is less than the original nucleusA B + C MA > MB + MCLoss in mass equals energy released:E = mc2 (Einsteins relation)Smaller nuclei are more stableFission of U-235: 0.08 % of mass is converted into energy

  • Comparison of nuclear and chemical energy sourcesConversion of tiny amount of matter into energy produces masses:1 gram 1014 JChemical process: 1 gram fuel produces 103 J Nuclear process: 1 gram uranium at 0.08 % produces 1011 J

  • Nuclear fusion: opposite of fissionSmall nuclei fuse to yield larger onesNuclear mass is lost

    Example is the deuterium tritium reactionAbout 0.7 % of the mass is converted into energy+ E

  • The sun is a helium factoryThe suns energy derives from the fusion of hydrogen atoms to give helium

  • 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 repelReproduce the center of the sun in the labFusion is demonstrated but currently consumes rather than produces energy