Chapter-29-Nuclear-Physics.ppt

Chapter 29Nuclear PhysicsnRutherford showed the radiation had three typesnAlpha (He nucleus)nBeta(electrons)nGamma(high-energy photons)Some Properties of NucleinAll nuclei are composed of protons and neutronsnException is ordinary hydrogen with just a protonnThe atomic number,Z,equals the number of protons in the nucleusnThe neutron number,N,is the number of neutrons in the nucleusnThe mass number,A,is the number of nucleons in the nucleusnA=Z+NnNucleon is a generic term used to refer to either a proton or a neutronnThe mass number is not the same as the massSymbolismnSymbol:nX is the chemical symbol of the elementnExample:nMass number is 27nAtomic number is 13nContains 13 protonsnContains 14(27 13)neutronsnThe Z may be omitted since the element can be used to determine ZMore PropertiesnThe nuclei of all atoms of a particular element must contain the same number of protonsnThey may contain varying numbers of neutronsnIsotopes of an element have the same Z but differing N and A valuesnExample:ChargenThe proton has a single positive charge,+enThe electron has a single negative charge,-enThe neutron has no chargenMakes it difficult to detectne=1.60217733 x 10-19 CMassnIt is convenient to use unified mass units,u,to express massesn1 u=1.660 559 x 10-27 kgnMass can also be expressed in MeV/c2n1 u=931.494 MeV/c2Summary of MassesMassesParticlekguMeV/c2Proton1.6726 x 10-271.007276938.28Neutron1.6750 x 10-271.008665939.57Electron9.109 x 10-315.486x10-40.511Nuclear StabilitynThere are very large repulsive electrostatic forces between protonsnThese forces should cause the nucleus to fly apartnThe nuclei are stable because of the presence of another,short-range force,called the nuclear forcenThis is an attractive force that acts between all nuclear particlesnThe nuclear attractive force is stronger than the Coulomb repulsive force at the short ranges within the nucleusNuclear Stability,contnLight nuclei are most stable if N=Z(P=N)nHeavy nuclei are most stable when N Z(NP)nAs the number of protons increase,the Coulomb force increases and so more nucleons are needed to keep the nucleus stablenNo nuclei are stable when Z 83(N83)Binding EnergynThe total energy of the bound system(the nucleus)is less than the combined energy of the separated nucleonsnThis difference in energy is called the binding energy of the nucleusnIt can be thought of as the amount of energy you need to add to the nucleus to break it apart into separated protons and neutronsBinding Energy per NucleonBinding Energy NotesnExcept for light nuclei,the binding energy is about 8 MeV per nucleonRadioactivitynRadioactivity is the spontaneous emission of radiation nExperiments suggested that radioactivity was the result of the decay,or disintegration,of unstable nucleiRadioactivity Types nThree types of radiation can be emittednAlpha particlesnThe particles are 4He nucleinBeta particlesnThe particles are either electrons or positronsnA positron is the antiparticle of the electronnIt is similar to the electron except its charge is+enGamma raysnThe“rays”are high energy photonsPenetrating Ability of ParticlesnAlpha particlesnBarely penetrate a piece of papernBeta particlesnCan penetrate a few mm of aluminumnGamma raysnCan penetrate several cm of leadThe Decay ConstantnThe number of particles that decay in a given time is proportional to the total number of particles in a radioactive samplenN=-N tn is called the decay constantnThe decay rate or activity,R,of a sample is defined as the number of decays per second(dps)Decay CurvenThe decay curve follows the equationnN=No e-t nThe half-life is also a useful parameternThe half-life is defined as the time it takes for half of any given number of radioactive nuclei to decayUnitsnThe unit of activity,R,is the Curie,Cin1 Ci=3.7 x 1010 decays/second(dps)nThe most commonly used units of activity are the mCi and the CiAlpha DecaynWhen a nucleus emits an alpha particle it loses two protons and two neutronsnN decreases by 2nZ decreases by 2nA decreases by 4nSymbolicallynX is called the parent nucleusnY is called the daughter nucleusAlpha Decay ExamplenDecay of 226 RanHalf life for this decay is 1600 yearsnExcess mass is converted into kinetic energynMomentum of the two particles is equal and oppositeDecay General RulesnWhen one element changes into another element,the process is called spontaneous decay or transmutation.nThe sum of the mass numbers,A,must be the same on both sides of the equationnThe sum of the atomic numbers,Z,must be the same on both sides of the equationnConservation of mass-energy and conservation of momentum must holdBeta DecaynDuring beta decay,the daughter nucleus has the same number of nucleons as the parent,but the atomic number is changed by onenSymbolically Beta Decay,contnThe emission of the electron is from the nucleusnThe nucleus contains protons and neutronsnThe process occurs when a neutron is transformed into a proton and an electronBeta Decay Electron EnergynThe energy released in the decay process should almost all go to kinetic energy of the electron(KEmax)nExperiments showed that few electrons had this amount of kinetic energyNeutrinonTo account for this“missing”energy,in 1930 Pauli proposed the existence of another particlenEnrico Fermi later named this particle the neutrinonProperties of the neutrinonZero electrical chargenMass much smaller than the electron,probably not zeronVery weak interaction with matterBeta Decay Completed nSymbolicallyn is the symbol for the neutrinon is the symbol for the antineutrinonTo summarize,in beta decay,the following pairs of particles are emittednAn electron and an antineutrinonA positron and a neutrinoGamma DecaynGamma rays are given off when an excited nucleus“falls”to a lower energy statenSimilar to the process of electron“jumps”to lower energy states and giving off photonsnThe photons are called gamma rays,very high energy relative to lightnThe excited nuclear states result from“jumps”made by a proton or neutronnThe excited nuclear states may be the result of violent collision or more likely of an alpha or beta emissionGamma Decay ExamplenExample of a decay sequencenThe first decay is a beta emissionnThe second step is a gamma emissionnThe C*indicates the Carbon nucleus is in an excited statenGamma emission doesnt change either A or ZNatural RadioactivitynClassification of nucleinUnstable nuclei found in naturenGive rise to natural radioactivitynNuclei produced in the laboratory through nuclear reactionsnExhibit artificial radioactivitynThree series of natural radioactivity existnUraniumnActiniumnThoriumNuclear ReactionsnStructure of nuclei can be changed by bombarding them with energetic particlesnThe changes are called nuclear reactionsnAs with nuclear decays,the atomic numbers and mass numbers must balance on both sides of the equationNuclear Reactions ExamplenAlpha particle colliding with nitrogen:nBalancing the equation allows for the identification of XnSo the reaction isQ ValuesnEnergy must also be conserved in nuclear reactionsnThe energy required to balance a nuclear reaction is called the Q value of the reactionnAn exothermic reactionnThere is a mass“loss”in the reactionnThere is a release of energynQ is positivenAn endothermic reactionnThere is a“gain”of mass in the reactionnEnergy is needed,in the form of kinetic energy of the incoming particlesnQ is negative Radiation Damage in MatternRadiation absorbed by matter can cause damagenThe degree and type of damage depend on many factorsnType and energy of the radiationnProperties of the absorbing matternRadiation damage in biological organisms is primarily due to ionization effects in cellsnIonization disrupts the normal functioning of the cellTypes of DamagenSomatic damage is radiation damage to any cells except reproductive onesnCan lead to cancer at high radiation levelsnCan seriously alter the characteristics of specific organismsnGenetic damage affects only reproductive cellsnCan lead to defective offspringUnits of Radiation ExposurenRoentgen RnThat amount of ionizing radiation that will produce 2.08 x 109 ion pairs in 1 cm3 of air under standard conditionsnThat amount of radiation that deposits 8.76 x 10-3 J of energy into 1 kg of airnRad(Radiation Absorbed Dose)nThat amount of radiation that deposits 10-2 J of energy into 1 kg of absorbing materialMore UnitsnRBE(Relative Biological Effectiveness)nThe number of rad of x-radiation or gamma radiation that produces the same biological damage as 1 rad of the radiation being usednAccounts for type of particle which the rad itself does notnRem(Roentgen Equivalent in Man)nDefined as the product of the dose in rad and the RBE factornDose in rem=dose in rad X RBERadiation LevelsnNatural sources rocks and soil,cosmic raysnBackground radiationnAbout 0.13 rem/yrnUpper limit suggested by US governmentn0.50 rem/yrnExcludes background and medical exposuresnOccupationaln5 rem/yr for whole-body radiationnCertain body parts can withstand higher levelsnIngestion or inhalation is most dangerous