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UW-Madison PHYSICS 107 - Physics 107 Lecture 31

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1Fri. Apr. 13, 2006 Phy107 Lect311From Last Time…• Nucleus is small, tightly boundsystem of protons & neutrons.• Proton number determines the element.• Different isotopes have different # neutrons.• Nucleus held together by the strong nuclear force– Stronger than coulomb force,– But much shorter range than coulomb force.• Some isotopes unstable, radioactively decayExam 3: Wed. Apr. 18 in classHW8 due Tue. Apr 17 midniteFri. Apr. 13, 2006 Phy107 Lect312protonsneutronsPopulating nucleon states• Various quantum states for nucleons in the nucleus• Similar to the hydrogen atom:one electron in each quantum state.• Two states at each energy (spin up & spin down)HeliumThis is 4He, with2 neutrons and2 protonsin the nucleusFri. Apr. 13, 2006 Phy107 Lect313Other helium isotopesprotonsneutronsprotonsneutronsToo few neutrons, ->protons too close together.High Coulomb repulsion energyToo many neutrons, requireshigher energy states.Fri. Apr. 13, 2006 Phy107 Lect314Radioactivity• Most stable nuclei have about same number ofprotons as neutrons.• If the energy gets too high, nucleus willspontaneously try to change to lower energyconfiguration.• Does this by changing nucleons inside the nucleus.• These nuclear are unstable, and are said to decay.• They are called radioactive nuclei.Fri. Apr. 13, 2006 Phy107 Lect315Radioactive nuclei~ equal #neutrons andprotonsFri. Apr. 13, 2006 Phy107 Lect316Radioactive decay• Decay usually involves emitting someparticle from the nucleus.• Generically refer to this as radiation.• Not necessarily electromagnetic radiation,but in some cases it can be.• The radiation often has enough energy tostrip electrons from atoms, or to sometimesbreak apart chemical bonds in living cells.2Fri. Apr. 13, 2006 Phy107 Lect317Detect ionizing radiation• Ionizing radiationstrips electrons fromatoms.• Makes free chargesthat can neutralizeexisting charges• Can discharge anelectroscopeFri. Apr. 13, 2006 Phy107 Lect318Radioactive tracersWorked on radioactivityas student with Ernest Rutherford.Lodged in nearby boarding home.Suspected his landlady was serving mealslater in the week ‘recycled’ from the Sunday meat pie.His landlady denied this!deHevesy described his first forayinto nuclear medicine:George de Hevesy“The coming Sunday in an unguarded moment Iadded some radioactive deposit [lead-212] to thefreshly prepared pie and on the following Wednesday,with the aid of an electroscope, I demonstrated to thelandlady the presence of the active deposit in the soufflé.”Fri. Apr. 13, 2006 Phy107 Lect319Geiger counter• A Geiger counter• Radiation ionizes (removes electrons) atomsin the counterLeaves negativeelectrons andpositive ions.Ions attracted toanode/cathode,current flow ismeasuredFri. Apr. 13, 2006 Phy107 Lect3110A random process• The particle emission is a random process– It has some probability of occurring.• For every second of time,there is a probability that the nucleus will decayby emitting a particle.• If we wait long enough, all the radioactive atomswill have decayed.Fri. Apr. 13, 2006 Phy107 Lect3111Radioactive half-life• Example of random decay.• Start with 8,000 identical radioactive nuclei• Suppose probability of decaying in one second is 50%.t=0 t=1sect=2sect=3secThe half-lif e is onesecondEvery second, halfthe atoms decayUndecayednucleiFri. Apr. 13, 2006 Phy107 Lect3112Radioactive decay questionA piece of radioactive material is initially observedto have 10,000 radioactive nuclei.3 hours later, you measure 1,250 radiaoctive nuclei.The half-life isA. 1/2 hourB. 1 hourC. 3 hoursD. 8 hoursIn each half-life,the number of radioactive nuclei,and hence the number of decays / second,drops by a factor of two.After 1 half life, 5000 are left undecayed.After 2 half lives, 1/2 of these are left: 2,500After 3 half lives there are 1,250 left.3Fri. Apr. 13, 2006 Phy107 Lect3113Another example•232Th has a half-lifeof 14 billion years• Sample initiallycontains 1 million232Th atoms• Every 14 billionyears, the number of232Th nuclei goesdown by a factor oftwo.Fri. Apr. 13, 2006 Phy107 Lect3114Radioactive decay questionA piece of radioactive material is initially observedto have 1,000 decays/sec.It’s half life is 2 days.Four days later, you measureA. 1,000 decays / secB. 500 decays / secC. 250 decays / secD. 125 decays / secFri. Apr. 13, 2006 Phy107 Lect3115Nuclear half-livesNumber of neutronsNumber of protons (Z)Fri. Apr. 13, 2006 Phy107 Lect3116Different types of radioactivityUnstable nuclei decay by emitting some formof energy,• Three different types of decay observed:Alpha decayBeta decayGamma decay(First three letters of Greek alphabet).Ernest Rutherford (1899): "These experiments show that the uraniumradiation is complex and that there are present at least two distinct typesof radiation - one that is very readily absorbed, which will be termed forconvenience the alpha-radiation, and the other of more penetrativecharacter which will be termed the beta-radiation."Fri. Apr. 13, 2006 Phy107 Lect3117Penetrating power of radiation• Alpha radiation very weak• Beta radiation penetrates farther• Gamma radiation hardest to stopFri. Apr. 13, 2006 Phy107 Lect3118Is the radiation charged?• Alpha radiation positively charged• Beta radiation negatively charged• Gamma radiation uncharged4Fri. Apr. 13, 2006 Phy107 Lect3119Alpha radiation• Alpha radiation now knownto be a helium nucleus(2 protons, 2 neutrons)Piece of atom (alpha particle) is brokenfrom heavy nucleus and ejectedFri. Apr. 13, 2006 Phy107 Lect3120A new element• When a nucleus emits an alpha-particle,it loses two neutrons and two protons.• It becomes a different element (the numberof protons in the nucleus has changed).• Example: ! 92238U "24He +90234Th92 protons146 neutrons90 protons144 neutrons2 protons2 neutronsAlpha particleThorium is the element with 90 electrons(and hence 90 protons in the nucleus)Fri. Apr. 13, 2006 Phy107 Lect3121Why?• Why does a piece come out of the atom?– All nucleons (neutrons & protons)attracted by short-range strong force– Protons forced apart by long-range Coulomb force.– Smaller nuclei will be more stable• Why is the ejected piece an alpha-particle,and not something else?– Helium nucleus is much more stable than other light nucleiFri. Apr. 13, 2006 Phy107 Lect3122Decay questionRadium was isolated by Marie Curie in 1898.It has a half-life of 1,600 yearsand decays by alpha-emission.The


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UW-Madison PHYSICS 107 - Physics 107 Lecture 31

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