From the Last TimePhysics of the NucleusQuestionNeutrons and ProtonsCarbonIsotopesNuclear ForceThe Strong Nuclear ForceEstimating the strong forceA strong nuclear forceNuclear Binding EnergyNuclear binding energyNuclear fusionNucleus bound very tightlyNucleons are not fundamentalThe ‘new’ nuclear forceVisualizing a nucleusParticles in the nucleusWhat makes a nucleus stable?Quantum states in the nucleusProton and Neutron statesPopulating nucleon statesOther helium isotopesNuclear spinMagnetic resonance imagingSlide 26Energy of nucleusRadioactivityStability of nucleiRadioactive nucleiRadioactive decayDiscovery of radioactivityDetecting radiationA random processRadioactive half-lifeRadioactive decay questionAnother examplePowerPoint PresentationPhy107 Fall 20061•Superconductor = zero-resistance material–Critical temperature–Critical current–Critical magnetic field - –no superconductivity outside of critical ranges•Superconductor types–Type I - superconductivity at low temperature only–High T superconductors–Type II - superconductivity in high magnetic fields•Meissner effect = exclusion of magnetic fieldFrom the Last TimeToday: The NucleusPhy107 Fall 20062Physics of the Nucleus•Nucleus consists of protons and neutrons densely combined in a small space (~10-14 m)–Protons have a positive electrical charge–Neutrons have zero electrical charge (are neutral)•Spacing between these nucleons is ~ 10-15 m•Size of electron orbit is 5x10-11 m•Nucleus is 5,000 times smaller than the atom!NeutronProtonPhy107 Fall 20063QuestionHydrogen is the element with one electron. Which of the following is NOT the nucleus of an isotope of hydrogen?A. One protonB. One proton and one neutronC. Two protons and one neutronHydrogenDeuteriumTrituiumOne protonOne protonone neutronOne protontwo neutronsAll with one proton and one electronPhy107 Fall 20064Neutrons and Protons•The number of protons in a nucleus is the same as the number of electrons since the atom has a net zero charge.•The number of electrons determines which element it is.–1 electron Hydrogen–2 electrons Helium–6 electrons Carbon•How many neutrons?Neutron: zero charge (neutral)Proton: positive charge (equal and opposite to electron)Phy107 Fall 20065CarbonAnother form of Carbon has 6 protons, 8 neutrons in the nucleus. This is 14C.•Example: carbon •Carbon has 6 electrons (Z=6), this is what makes it carbon.•Zero net charge so there are 6 protons in the nucleus.•Most common form of carbon has 6 neutrons in the nucleus. Called 12CC126Phy107 Fall 20066Isotopes•Both 12C and 14C have same chemical properties.•This is why they are both called carbon. Same # electrons and same # protons in nucleus.•But the nuclei are different. They have different number of neutrons. These are called isotopes.•Difference is most easily seen in the binding energy.•Nuclei that are bound more tightly are less likely to ‘fall apart’.•In fact 14C is radioactive or unstable.Phy107 Fall 20067Nuclear Force•So what holds the nucleus together?•Coulomb force? Gravity?•Coulomb force only acts on charged particles–Repulsive between protons, and doesn’t affect neutrons at all.•Gravitational force is much too weak. Showed before that gravitational force is much weaker than Coulomb force.Phy107 Fall 20068The Strong Nuclear Force•New force.•Dramatically stronger than Coulomb force.•But not noticeable at large distances.–I.e. Atoms do not attract each other.•Must be qualitatively different than Coulomb force.•How can we characterize this force?–Range is on the order of the size of nucleus.–Stronger than Coulomb force at short distances.Phy107 Fall 20069Estimating the strong forceThe Coulomb attraction energy (~10 eV) binds the hydrogen atom together. Protons in nucleus are 50,000 times closer together than electron and proton in hydrogen atom.The Coulomb energy is inversely proportional to the separation.Attractive energy must be larger than the Coulomb repulsion, so nuclear binding energies are greater than.A. 5000 eVB. 500,000 eVC. 5,000,000 eVPhy107 Fall 200610A strong nuclear force•Electron is bound in atom by Coulomb attraction. Strength ~10 eV.•Protons in nucleus are 50,000 times closer together.Coulomb repulsion ~500,000 eV = 0.5 MeV•Nuclear force must be much stronger than this.•Experimentally, the strong nuclear force is~ 100 times stronger than Coulomb force•Nucleons are bound in nucleus by ~ 8 MeV / nucleon(8,000,000 eV / nucleon)Phy107 Fall 2006112 protons & 2 neutronsNuclear Binding Energy•Mass of nucleus is less than mass of isolated constituents.•The difference is the binding energy.Arises from E=mc2Equivalence of mass and energy.Helium nucleusPhy107 Fall 200612Nuclear binding energy•Helium nucleus has less mass than sum of two neutrons & two protons•Why is this?•The ‘missing mass’ makes up the ‘binding energy’ 12C has a mass of 12.00000 u (1 u = 1.661x10-27 kg)‘Missing mass’ in He case is € 4.0320 u− 4.0015 u 0.0305 u= 5.06x10-29 kgPhy107 Fall 200613Nuclear fusion5.06x10-29 kg of mass released as energy when protons & neutrons combined to form Helium nucleus.This is the ‘binding’ energy of the nucleus.E = mc2 = (5.06x10-29 kg)x(3x108 m/s)2 = 4.55x10-12 J= 28 MeV = 28 million electron volts!Binding energy/nucleon = 28 MeV / 4 = 7 MeVPrinciple of nuclear fusion: Energy released when ‘manufacturing’ light elements.Phy107 Fall 200614Nucleus bound very tightly•To change properties of nucleus, need much larger energies than to change electronic states.•Properties of nucleus that might change are –Exciting nucleus to higher internal energy state–Breaking nuclei apart–Fusing nuclei together.•Required high energies provided by impact of high-energy……protons, electrons, photons, other nuclei•High energies produced in an accelerator facilityPhy107 Fall 200615Nucleons are not fundamental•We now know that protons and neutrons are not fundamental particles.•They are composed of quarks, which interact by exchanging gluons.Phy107 Fall 200616The ‘new’ nuclear force•Strong force is actually between quarks in the nucleons.•Quarks exchange gluons.•Most of the strong force glues quarks into protons and neutrons. •But a fraction of this force leaks out, binding protons and neutrons into atomic nucleiPhy107 Fall 200617Visualizing a nucleusA nucleon made up of interacting quarks.A nucleus of several
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