19.2 Nuclear PhysicsProperties of nucleiBinding EnergyRadioactive decayNatural radioactivityNuclear PhysicsThe nucleus is the small + charged object at the centerof the atom.It is composed of protons and neutrons bound together by an enormously strong nuclear force.Nuclei can be stable or unstable Unstable nuclei decay to smaller particles with the release of energy, and radiation.Nuclei can also be changed by fusion to form larger particles. Properties of the nucleusConsists of protons and neutronsZ = no. of protons (Atomic number)N = no. of neutrons (Neutron number)A = Z+N (Mass number)Notation :For element X with mass no. Aand Atomic no. ZAZXIsotopesIsotopes are nuclei that have the same no. of protonsbut different no. of neutrons.The chemical properties are the same but the nuclearproperties are different. i.e. some isotopes may be unstable and are radioactive.eg.11H21H21HHydrogen - stableDeuterium - stableTritium - radioactiveStable NucleiNZPlot of N vs Z forstable nuclei •Outline shows unstable nucleiMax Z=83Unstable NucleiSome radioactivenuclei146C3215P13153I••23892U•Size of the nucleusRadius varies as the cube root of Ar= roA1/3where ro= 1.2x10-15mexampleFor Uranium 238, A=23815 1/ 3 15r 1.2x10 (238 ) 7.4x10 m−−==23892U2Forces in the nucleiCoulomb forcesThe protons repel each otherwith Coulomb forces.These are enormously large dueto the small size.Nuclear forcesThe nucleus is held together by the nuclear force. This force acts only at short range (~10-15m) and is independent of charge (i.e. acts between proton-proton,proton-neutron and neutron-neutron). Equivalence of mass and energyA famous result from Einstein’s Special Relativity TheoryE= mc2mass can be converted into energyEnergy equivalent of an electron massE=mc2= (9.1x10-31kg)(3x108m/s)2= 8.2x10-14J=5.1x105 eV= 0.51MeVAn electron can be annihilated (converted completely toenergy). A 0.51 MeV photon is produced.Mass changes when energy is lost or gainedGasoline + O2-> CO2+water + energyThe energy released is equal to the change in massin the reaction.E=mc2CO2+ water is lighter. Burning 1 kg of gasoline releases 44x106J of energy.The change in mass is610282E44x10Jm5x10kgc(3x10m/s)−== =Σmc2+ ΣEnergy =constantsmall changein massBinding energyThe binding energy of the nucleus can be determinedby measuring the mass of the components andthe final product.For the deuterium nucleusformed from a proton and neutron21Dp + n -> 21D∆m = mass (hydrogen atom) + mass (neutron) –mass (deuterium atom)∆m = 1.007825u +1.008665u -2.014102u= 2.39x10-3u+ energyu=1.660559 x10-27 kgE=∆mc2= (2.39x10-3u)(1.66x10-27kg/u)(3x108 m/s)2=3.6x10-13JE=2.2x106eV =2.2 MeV Binding energy of the deuteronE=∆mc2(atomic mass unit)Binding energy per nucleon (E/A)Goes through a maxima at 56Fenuclear contacts saturated.Electrostatic forces favor lower Anuclear contacts favor higher ABinding energy per nucleon (E/A)Goes through a maxima at 56FefusionfissionFusion (combine small nuclei)– increases binding energyFission (break large nuclei)– increases binding energy3RadioactivityUnstable nuclei decay releasing energy and radiation.Three types of radiationalpha (α) particles -42Henucleibeta (β) particles - electrons gamma (γ) particles - high frequency electromagneticradiation.Increasingpenetration(+ charge)(- charge) (uncharged)Radiationalpha particles – Stopped by a sheet of paperbeta particles - Stopped by a mm of aluminumgamma particles - Stopped by a few cm of leadPenetration depthMeasuring radiationGeiger Counterradiationionizationof gascauses current flow.Natural radioactivityMany elements found in nature are unstable and decayemitting radioactivity. These include Uranium, 238U , Radon 224Ra and Potassium 40K. Carbon 14C, Natural radioactive decayThorium decaygives a variety ofunstable products.Cosmic raysCosmic rays –High energy particles (protons, alpha particles, atomic nuclei) from distant stars collide with atoms in the atmosphere and break them apart to sub atomic particles. Some particles e.g. muons rain down to the earth’s surface and are a source of background radiation.4Cloud chamberA cloud chamber can be used to visualize radioactive particles. The chamber contains a supersaturated vaporradioactivesourceThe radioactive particlecauses ionization of atomsThe charge initiates the condensation of the vapor, Leaving a visible