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The strong force (nuclear force), fission

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- 1. Today: More about half-life, why atoms are radioactive, fission Diagram of neutron-induced U-235 fission
- 2. Half-life is the average amount of time for ½ of the atoms to decay 100% remaining 50% remaining 25% remaining 12.5% remaining Time Isotope with long half-life Isotope with short half-life “ Exponential Decay”
- 3. Radium-226 Where does it come from? <ul><li>Half life of Radium-226 approximately 1600 years </li></ul><ul><li>A 30 milligram sample has a radioactivity of ~ 1 billion Bq = 10 9 Bq = 10 9 decays per second </li></ul><ul><li>48,000 years is about 30 half lives . Activity would decrease by a factor of 2 30 = about 1 billion. So, activity 48,000 years from now would be 1 Bq . </li></ul><ul><li>4.8 billion years ago is about 3 million half-lives. So there is no way the Radium-226 we have today existed when Earth was formed </li></ul>
- 4. Radium-226 Where does it come from? <ul><li>So we know Radium-226 comes from somewhere else…where? </li></ul>It turns out it comes from the radioactive decay of uranium in natural deposits. Where does the Uranium come from??? http://www.youtube.com/watch?v=6SxzfZ8bRO4&feature=related
- 5. Why are some atoms radioactive? First: Some brainstorming Form small groups, with a spokesperson How many different forces in nature can you think of? Can you classify them into similar groups?
- 6. Brainstorming results <ul><li>-> Gravity, </li></ul><ul><li>Electrical, magnetic </li></ul><ul><li>Force that keeps protons close together in the nucleus </li></ul><ul><li>(All the electrical force) Intermolecular Interactions: Wind , friction, firehose, dams, earthquake forces, forces during volcanic eruptions, </li></ul><ul><li>Evaporation (opposing gravity somehow) </li></ul><ul><li>(momentum, not a force) </li></ul><ul><li>Metaphysical forces </li></ul>
- 7. First, we need to wonder why a nucleus can exist at all! <ul><li>The postive charges (protons) in the nucleus are very close together! There is a LOT of electrical repulsion between two neighboring protons. </li></ul><ul><li>So there MUST be something else holding them together! </li></ul><ul><li>What holds the quarks together inside protons and neutrons? </li></ul>Same charges repel each other http://www.youtube.com/watch?v=NiwBwSAeDu0 Force Charge1 * Charge2 distance 2
- 8. The strong force holds quarks together <ul><li>The strong force also accounts for the nuclear force </li></ul><ul><ul><li>attracts two nucleons only when they are very close </li></ul></ul><ul><ul><li>(like velcro only attracts when very close together) </li></ul></ul><ul><li>The strong force is one of four fundamental forces . </li></ul><ul><li>Strong force </li></ul><ul><li>Electrical force </li></ul><ul><li>Gravitational </li></ul><ul><li>Weak force (this accounts for beta decay) </li></ul>
- 9. Only gravity and electrical force act at long distances <ul><li>Inverse-square law for gravity and electrical force. </li></ul><ul><li>Strong and weak die off exponentially and are only very short range (so only manifest in nuclear reactions) </li></ul><ul><li>Strong force is the strongest and can dominate at short range. </li></ul><ul><li>Gravity is the weakest force and is negligible in nuclear physics. </li></ul>
- 10. Clicker question—fundamental forces <ul><li>Which of the following is not one of the four fundamental forces known in physics? </li></ul><ul><li>Gravity </li></ul><ul><li>Minor </li></ul><ul><li>Strong </li></ul><ul><li>Electrical </li></ul><ul><li>Weak </li></ul>
- 11. Why are some atoms radioactive? <ul><li>Recall how excited atoms can release photons as electron goes from high potential energy to low potential energy. (Fluorescence, phosphorescence, …) </li></ul><ul><li>The process of emitting a photon was random for regular fluorescence / phosphorescence . In fact, it also shows exponential decay behavior. </li></ul><ul><li>Total energy is conserved …potential energy of the electron decreases, but a photon of energy = h * frequency is created and released. </li></ul>chemeducator.org
- 12. Why are some atoms radioactive? <ul><li>Some atoms are also temporarily “trapped” in a higher-potential energy state. </li></ul><ul><li>Via radioactive decay, they can release energy and move to a lower potential energy. </li></ul><ul><li>Consider alpha-decay of Uranium-238: </li></ul>U 238 92 Th 234 90 He 4 2 The alpha particle is moving at 15,000 km/s, Having a LOT of kinetic energy Alpha-decay
- 13. Why are some atoms radioactive? <ul><li>Being trapped in a higher-energy state is a common theme in physics, chemistry, biology. </li></ul>U 238 92 Th 234 90 He 4 2 The alpha particle is moving at 15,000 km/s, Having a LOT of kinetic energy Alpha-decay (heat up reaction) Height of “activation barrier” determines reaction rate
- 14. Why are some atoms radioactive? The nuclear (residual strong) force presents an activation barrier Quantum mechanics allows some probability that an alpha particle can “tunnel” through the barrier Once through, electrostatic force dominates (imagine ripping apart velcro)
- 15. Take home message for “why radioactivity” <ul><li>The real physics is complicated quantum mechanics… </li></ul><ul><li>The nucleus is held together by the nuclear force (stronger at short range than electrical) </li></ul><ul><li>Some heavy atoms can be in a lower potential energy if they radioactively decay. </li></ul><ul><li>How large the activation barrier is determines the half-life of the random process </li></ul>
- 16. Alpha decay can be though of as a very common but special form of nuclear fission . <ul><li>Nuclear fission more generally: </li></ul><ul><li>An unstable nucleus splits into two “daughter nuclei”. </li></ul><ul><li>Except for alpha-decay, usually emits free energetic neutrons </li></ul>
- 17. As nuclei get larger, electrostatic force “wins” Strong force large between close protons. Strong force much lower between distal protons But electrostatic force still very high! (Nucleons on the surface feel less strong attraction.)
- 18. You can imagine large nuclei like large soap bubbles…very floppy Small Medium Large
- 19. If the nucleus randomly deforms, it can become favorable to split into two! <ul><li>This is called “spontaneous” fission, and for most isotopes is very rare . </li></ul>& free neutrons!!! http://www.youtube.com/watch?v=jk6Hm1QoDYY
- 20. However, free neutrons can be absorbed and induce fission <ul><li>Once physicists discovered the neutron products, they quickly realized a chain reaction was possible. http://www.youtube.com/watch?v=HmbzJGf90Xc&feature=related (mousetraps) http://www.youtube.com/watch?v=tQa4LONy9XM&feature=related </li></ul>
- 21. Naturally occurring radium is a mixture of mostly U-238 and a small amount of U-235. <ul><li>When U-238 absorbs a neutron, it beta-decays to Neptunium and then Plutonium: </li></ul><ul><ul><li>No quick emission of neutrons & no chain reaction. </li></ul></ul><ul><li>When U-235 absorbs a neutron, it very quickly undergoes fission </li></ul><ul><ul><li>producing on average 2.5 more neutrons </li></ul></ul><ul><ul><li>Chain reaction! </li></ul></ul>
- 22. Because of mix of mostly U-238, natural uranium is not suitable for a chain reaction. <ul><li>Two ways to go: </li></ul><ul><li>Separate the U-235 to make high-purity fissile material </li></ul><ul><li>Do something to make the neutrons more likely to be absorbed by U-235 </li></ul>
- 23. For U-238 and U-235, probability of absorbing neutron depends on neutron energy <ul><li>Analogous to photon absorption. </li></ul><ul><li>Fast neutrons (shorter wavelength): </li></ul><ul><li>Absorbed better by U-238 </li></ul><ul><li>Slow neutrons (longer wavelength): Absorbed better by U-235 </li></ul>
- 24. Clicker question – Nuclear Reactor Physics <ul><li>U-235 is more likely than U-238 to absorb a slow neutron. </li></ul><ul><li>U-238 is more likely to absorb a fast neutron. </li></ul><ul><li>Based upon the natural abundance of U-235 and U-238, what would need to be done in a nuclear reactor to increase chance of a chain reaction? </li></ul><ul><li>Try to make all neutrons very fast </li></ul><ul><li>Try to slow down neutrons </li></ul>
- 25. Clicker question – Nuclear Reactor Physics <ul><li>U-235 is more likely than U-238 to absorb a slow neutron. </li></ul><ul><li>U-238 is more likely to absorb a fast neutron. </li></ul><ul><li>Based upon the natural abundance of U-235 and U-238, what would need to be done in a nuclear reactor to increase chance of a chain reaction? </li></ul><ul><li>Try to make all neutrons very fast </li></ul><ul><li>Try to slow down neutrons </li></ul>This was the key to creating the first man-made controlled nuclear chain reaction…neutrons were slowed down by graphite
- 26. Clicker question—Ratio of U-238 to U-235 <ul><li>Half-life of U-238 is about 4.5 billion years </li></ul><ul><li>Uranium-235 is about 700 million years. </li></ul><ul><li>Currently natural deposits are about 0.7% U-235. </li></ul><ul><li>Was the proportion of U-235 more or less 1 billion years ago? </li></ul><ul><li>More </li></ul><ul><li>Less </li></ul>
- 27. Clicker question—Ratio of U-238 to U-235 <ul><li>Half-life of U-238 is about 4.5 billion years </li></ul><ul><li>Uranium-235 is about 700 million years. </li></ul><ul><li>Currently natural deposits are about 0.7% U-235. </li></ul><ul><li>Was the proportion of U-235 more or less 2 billion years ago? </li></ul><ul><li>More </li></ul><ul><li>Less </li></ul>Proportion was high enough To help with random creation of Nuclear reactors! (About 3% U-235)

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