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Nuclear Reactions Chapter 18 Supernova 1999gi in NGC 3184
Credit:Al Kelly (JSCAS/NASA & Arne Henden(Flagstaff/USNO)
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Alpha radiation particle He - 4 nucleus
Decrease the atomic number by 2 Decreases the mass number by 4
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Beta radiation particle Electron Increases the atomic number by 1
Leaves the mass number unchanged
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Gamma radiation emission
High energy photons Does not effect mass or atomic number Omitted in equations
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Positron emission Positive electron Decreases the atomic number by 1
Leaves the mass number unchanged Usually occur in nuclei that have too many protons to be stable Protons are converted to neutrons
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K-capture An electron in the innermost energy level falls into the nucleus Decreases the atomic number by 1 Leaves the mass number unchanged Usually occur in heavy nuclei
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Write nuclear reactions for:
Alpha emission by Th-230 K-capture of Ar-37 Beta emission by Pb-210
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Write nuclear reactions for:
Alpha emission by Th-230 K-capture of Ar-37 Beta emission by Pb-210
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Bombardment reactions
Smack a stable nucleus with a high energy particle Neutron, alpha particle, positron. . . The stable nucleus is converted into a radioactive nucleus Which in turn decays forming stable products
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Uses Medicine Co-60 is used to eliminate malignant cells left after tumor removal I-131 is used to treat thyroid cancer Glucose containing C-11, a positron emitter, is given to patients with brain disorders. A PET scanner is then used to see what’s going on Xe-133 is used for lung imaging
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Neutron activation analysis
Can be used to find trace amounts of elements Archaeology Determine dead thing’s diet Plants contain more Sr than animals so. . .
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Smoke detectors Am-241 ionizes air molecules. If smoke gets in the chamber the flow of ions is impeded, the current drops, and the alarm sounds
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Irradiated food
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Rate of radioactive decay (first order)
Rate = kX k= rate constant X = amount of radioactive species present at time, t ln Xo/X = kt Xo = the amount at t = 0 k =0.693 / t1/2 A = kN A = activity Curies (Ci) = 3.700x1010 atoms/s N= the number of radioactive nuclei present
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Calculate the activity for 1mg of radium when k = 1.37 x 10-11 s-1
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The half-life of U-238 is 4. 5x109 yr
The half-life of U-238 is 4.5x109 yr. How long does it take for 60% of a U-238 sample to decay?
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Age of organic material ( C-14 dating)
Formation of C-14 C-14 is incorporated into CO2. Living plants and animals consume the CO2. When death occurs consumption stops and the C-14 decays. t1/2 = 5730 y
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Mass-Energy relations
Photo: Albert Einstein The Hebrew University of Jerusalem
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Calculate the energy change for the decay of 8
Calculate the energy change for the decay of 8.2g Pu-239 releasing an alpha particle
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Mass defect A nucleus weighs less than the individual protons and neutron of which it is composed Mass defect = (n + p) – (mass of nucleus)
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Calculate the mass defect for C-12
Mass defect = (6n + 6p) – (C-12)
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Binding energy The amount of energy that must be absorbed in order to decompose nuclei Measures nuclear stability. Maximum values are reached by nuclei with intermediate mass.
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Calculate the binding energy for C-12
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Fission Many different isotopes are formed
Nuclei produced have too many neutrons and are very radioactive Sr-90 is incorporated into the bones of animals where it stays forever More neutrons are produced than consumed Chain rxn Excess neutrons are absorbed by Cd rods
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Nuclear reactors
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Why nuclear energy Fossil fuels are quickly being depleted
Produces tons of energy Can be built anywhere
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Why not nuclear energy Expensive Accidents
About 4 cents more than fossil fuel Accidents Three mile island Chernobyl Japan tsunami Disposal of radioactive waste Yucca Mountain, Nevada? Utah????? Heat pollution
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Fusion Abundant fuel! Clean Very high activation energy
Needed to overcome the H-2:H-2 electrostatic repulsion Temp required is about 109 C
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