1Mon. Apr. 10, 2006 Phy107 Lect311From Last Time…• Nucleus is small, tightly boundsystem of protons & neutrons.• Proton number determines the element.• Different isotopes have different # neutrons.• Some isotopes unstable, radioactively decay• Nucleus held together by the strong nuclear force– Stronger than coulomb force,– But much shorter range than coulomb force.• Strong force actually between quarks, internalconstituents of the neutron/proton.‘Leaks’ out to appear as an attractive force.Mon. Apr. 10, 2006 Phy107 Lect312Proton and Neutron states• Various quantum states for nucleons in the nucleus• Proton and neutron can be in the same stateprotonsneutronsNucleon quantum statesin the nucleusSchematic indicatingneutron & proton canoccupy same stateMon. Apr. 10, 2006 Phy107 Lect313protonsneutronsPopulating 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 nucleusMon. Apr. 10, 2006 Phy107 Lect314Other helium isotopesprotonsneutronsprotonsneutronsToo few neutrons, ->protons too close together.High Coulomb repulsion energyToo many neutrons, requireshigher energy states.Mon. Apr. 10, 2006 Phy107 Lect315Radioactivity• 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.Mon. Apr. 10, 2006 Phy107 Lect316Radioactive nuclei~ equal #neutrons andprotons2Mon. Apr. 10, 2006 Phy107 Lect317Radioactive 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.Mon. Apr. 10, 2006 Phy107 Lect318Discovery of radioactivity• Accidental discovery in 1896• Henri Becquerel was trying to discover x-rays (discovered in 1895 by Roentgen).• Exposed uranium compound to sunlight,then placed it on photographic plates• Believed uranium absorbed sun’s energy andthen emitted it as x-rays.• On the 26th-27th February, experiment"failed" because it was overcast in Paris.• Becquerel developed plates anyway, findingstrong images,• Proved uranium emitted radiation withoutan external source of energy.Mon. Apr. 10, 2006 Phy107 Lect319Detecting radiation• A Geiger counter• Radiation ionizes (removes electrons) atomsin the counterLeaves negativeelectrons andpositive ions.Ions attracted toanode/cathode,current flow ismeasuredMon. Apr. 10, 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.Mon. Apr. 10, 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 decayUndecayednucleiMon. Apr. 10, 2006 Phy107 Lect3112Radioactive decay questionA piece of radioactive material is initially observedto have 1,000 decays/sec.Three hours later, you measure 125 decays / second.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, the decays/sec drop to 500.After 2 half lives it is 250 decays/secAfter 3 half lives there are 125 decays/sec.3Mon. Apr. 10, 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.Mon. Apr. 10, 2006 Phy107 Lect3114Nuclear half-livesNumber of neutronsNumber of protons (Z)Mon. Apr. 10, 2006 Phy107 Lect3115Different 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."Mon. Apr. 10, 2006 Phy107 Lect3116Penetrating power of radiation• Alpha radiation very weak• Beta radiation penetrates farther• Gamma radiation hardest to stopMon. Apr. 10, 2006 Phy107 Lect3117Is the radiation charged?• Alpha radiation positively charged• Beta radiation negatively charged• Gamma radiation unchargedMon. Apr. 10, 2006 Phy107 Lect3118Alpha radiation• Alpha radiation now knownto be a helium nucleus(2 protons, 2 neutrons)Piece of atom (alpha particle) is brokenfrom heavy nucleus and ejected4Mon. Apr. 10, 2006 Phy107 Lect3119A 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)Mon. Apr. 10, 2006 Phy107 Lect3120Why?• 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 nucleiMon. Apr. 10, 2006 Phy107 Lect3121Decay questionRadium was isolated by Marie Curie in 1898.It has a half-life of 1,600 yearsand decays by alpha-emission.The resulting element isA. Polonium (84 electrons)B. Thorium (90 electrons)C. Radon (86 electrons) ! 88226RaMon. Apr. 10, 2006 Phy107 Lect3122Decay chain originates at 238UNumber of
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