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Nuclear Binding, Radioactivity. Physics 1161: Lecture 25. Sections 32-1 – 32-9. X-Rays emitted by cathode ray tube. Polonium and radium. Radioactivity. Marie Curie 1867 - 1934. Wilhelm Roentgen 1845 - 1923. Spontaneous emission of radiation from the nucleus of an unstable isotope. - PowerPoint PPT Presentation

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Nuclear Binding, Radioactivity

Nuclear Binding, RadioactivitySections 32-1 32-9Physics 1161: Lecture 25

11RadioactivitySpontaneous emission of radiation from the nucleus of an unstable isotope.

Marie Curie1867 - 1934

Wilhelm Roentgen1845 - 1923X-Raysemittedby cathoderay tubePolonium and radium

Antoine Henri Becquerel1852 - 1908UraniumproducedX-raysNucleus = Protons+ NeutronsnucleonsA = nucleon number (atomic mass number)Gives you mass density of elementZ = proton number (atomic number) Gives chemical properties (and name)N = neutron number A=N+ZNuclear Physics

AZPeriodic_Table3Lead IsotopeCheckpointA material is known to be an isotope of lead, although the particular isotope is not known. Which of the following can be specified? The atomic mass numberThe neutron numberThe number of protonsZ=82Chemical properties (and name) determined by number of protons (Z)

But protons repel one another (Coulomb Force) and when Z is large it becomes harder to put more protons into a nucleus without adding even more neutrons to provide more of the Strong Force. For this reason, in heavier nuclei N>Z.# protons = # neutrons

5Lead IsotopeCheckpointWhere does the energy released in the nuclear reactions of the sun come from?covalent bonds between atoms binding energy of electrons to the nucleus binding energy of nucleons

Strong Nuclear ForceActs on Protons and Neutrons

Strong enough to overcome Coulomb repulsion

Acts over very short distancesTwo atoms dont feel force7What keeps protons together?Hydrogen atom: Binding energy =13.6eVBinding energy of deuteron = or 2.2Mev! Thats around 200,000 times bigger!

Simplest Nucleus: Deuteron=neutron+protonneutronprotonVery strong forceCoulomb forceelectronprotonStrong Nuclear Force(of electron to nucleus)8Binding EnergyEinsteins famous equation E = m c2Proton: mc2 = 938.3MeVNeutron: mc2= 939.5MeVDeuteron: mc2 =1875.6MeVAdding these, get 1877.8MeVDifference is Binding energy, 2.2MeVMDeuteron = MProton + MNeutron |Binding Energy|Example9

Iron (Fe) has the most binding energy/nucleon. Lighter have too few nucleons, heavier have too many.BINDING ENERGY in MeV/nucleon

10Binding Energy PlotFissionFusionFusion = Combining small atoms into largeFission = Breaking large atoms into small10Mass/Nucleon vs Atomic Number

FusionFission11E = mc2E: energym: massc: speed of lightc = 3 x 108 m/s12E = mc2Mass can be converted to energyEnergy can be converted to massMass and energy are the same thingThe total amount of mass plus energy in the universe is constant13Mass Defect in FissionWhen a heavy element (one beyond Fe) fissions, the resulting products have a combined mass which is less than that of the original nucleus.14Mass Defect of Alpha Particle

Mass difference = 0.0304 u Binding energy = 28.3 MeVFusion product has less mass than the sum of the parts.15Which of the following is most correct for the total binding energy of an Iron atom (Z=26)?

9 MeV234 MeV270 MeV504 Mev

BINDING ENERGY in MeV/nucleonWhich of the following is most correct for the total binding energy of an Iron atom (Z=26)?

9 MeV234 MeV270 MeV504 Mev Total B.E 56x9=504 MeV

BINDING ENERGY in MeV/nucleonFor Fe, B.E./nucleon 9MeV

has 56 nucleonsa particles: nucleii

b- particles: electronsg : photons (more energetic than x-rays) penetrate!3 Types of RadioactivityEasily StoppedStopped by metal

Radioactive sourcesB field into screendetector18Alpha DecayAlpha decay occurs when there are too many protons in the nucleus which cause excessive electrostatic repulsion.An alpha particle is ejected from the nucleus.An alpha particle is 2 protons and 2 neutrons.An alpha particle is also a helium nucleus.Alpha particle symbol:

Beta DecayBeta decay occurs when neutron to proton ratio is too bigA neutron is turned into a proton and electron and an antineutrinoThe electron and the antineutrino are emitted

Gamma DecayGamma decay occurs when the nucleus is at too high an energyNucleus falls down to a lower energy levelHigh energy photon gamma ray - is emitted

: example

recall: example Decay RulesNucleon Number is conserved.Atomic Number (charge) is conserved.Energy and momentum are conserved.g: example

238 = 234 + 4Nucleon number conserved92 = 90 + 2Charge conserved

Needed to conserve energy and momentum.

Example22A nucleus undergoes decay. Which of the following is FALSE?

Nucleon number decreases by 4 Neutron number decreases by 2 Charge on nucleus increases by 2 A nucleus undergoes decay. Which of the following is FALSE?

Nucleon number decreases by 4 Neutron number decreases by 2 Charge on nucleus increases by 2 decay is the emission of

Z decreases by 2(charge decreases!)A decreases by 4The nucleus undergoes decay. Which of the following is true?

The number of protons in the daughter nucleus increases by one. The number of neutrons in the daughter nucleus increases by one.

decay involves emission of an electron: creation of a charge -e. In fact, inside the nucleus, and the electron and neutrino escape.

Radioactive Decay

4.5 x 109 yr half-life24 day half-life1.17 min half-life

250,000 yr half-life

U 238 DecayDecay SeriesNuclear Decay Linkshttp://physics.bu.edu/cc104/uudecay.htmlhttp://www.physics.umd.edu/lecdem/honr228q/notes/U238scheme.gifhttp://www.physics.umd.edu/lecdem/honr228q/notes/fourdecschemes.gif

Which of the following decays is NOT allowed?

Which of the following decays is NOT allowed?

238 = 234 + 492 = 90 + 2214 = 210 + 484 = 82 + 214 = 14+06 7+040 = 40+0+019 = 20-1+0Decays per second, or activity:If the number of radioactive nuclei present is cut in half, how does the activity change?

No. of nuclei presentdecay constantIt remains the same It is cut in half It doubles Decays per second, or activityStart with 16 14C atoms.After 6000 years, there are only 8 left.How many will be left after another 6000 years?

No. of nuclei presentdecay constantEvery 6000 years of atoms decay046

time

Decay Function33Instead of base e we can use base 2:

Survival:No. of nuclei present at time tNo. we started with at t=0

whereThen we can write

Half lifeRadioactivity Quantitatively

No. of nuclei presentdecay constantDecays per second, or activity34Carbon DatingCosmic rays cause transmutation of Nitrogen to Carbon-14

C-14 is radioactive with a half-life of 5730 yearsIt decays back to Nitrogen by beta decay

The ratio of C-12 (stable) atoms to C-14 atoms in our atmosphere is fairly constant about 1012/1This ratio is the same in living things that obtain their carbon from the atmosphere

You are radioactive!One in 8.3x1011 carbon atoms is 14C which b- decays with a life of 5730 years. Determine # of decays/gram of Carbon.

Example36Carbon DatingWe just determined that living organisms should have a decay rate of about 0.23 decays/ gram of carbon.

The bones of an ice man are found to have a decay rate of 0.115 decays/gram. We can estimate he died about 6000 years ago.Example37SummaryNuclear ReactionsNucleon number conservedCharge conservedEnergy/Momentum conserved a particles = nuclei b- particles = electrons g particles = high-energy photons

DecaysHalf-Life is time for of atoms to decay

Survival:

38Mass/Nucleon vs Atomic Number

FusionFissionFusionFission39U-235 -- Fissile

40Abundance of U-235

41U-235 Fissionby Neutron Bombardment

42Possible U-235 Fission

43How Stuff Works SiteVisit the How Stuff Works Site to learn more details about nuclear energy44Chain Reaction

45Plutonium Production

46U-238 Not Fissile

47Breeder Reaction

48Breeder ReactorSmall amounts of Pu-239 combined with U-238Fission of Pu frees neutronsThese neutrons bombard U-238 and produce more Pu-239 in addition to energy49

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