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Chapter 31 Nuclear Energy; Effects and Uses of Radiation
Contents of Chapter 31 Nuclear Reactions and the Transmutation of Elements Nuclear Fission; Nuclear Reactors Nuclear Fusion Passage of Radiation through Matter; Radiation Damage Measurement of Radiation Dosimetry
Contents of Chapter 31 Radiation Therapy Tracers in Research and Medicine Emission Tomography: PET and SPECT Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI)
31-1 Nuclear Reactions and the Transmutation of Elements A nuclear reaction takes place when a nucleus is struck by another nucleus or particle. If the original nucleus is transformed into another, this is called transmutation. An example:
31-1 Nuclear Reactions and the Transmutation of Elements Energy and momentum must be conserved in nuclear reactions. Generic reaction: (31-1) The reaction energy, or Q-value, is the sum of the initial masses less the sum of the final masses, multiplied by c 2 : (31-2a)
31-1 Nuclear Reactions and the Transmutation of Elements If Q is positive, the reaction is exothermic, and will occur no matter how small the initial kinetic energy is. If Q is negative, there is a minimum initial kinetic energy that must be available before the reaction can take place.
31-1 Nuclear Reactions and the Transmutation of Elements Neutrons are very effective in nuclear reactions, as they nave no charge and therefore are not repelled by the nucleus.
31-2 Nuclear Fission; Nuclear Reactors After absorbing a neutron, a uranium-235 nucleus will split into two roughly equal parts. One way to visualize this is to view the nucleus as a kind of liquid drop.
31-2 Nuclear Fission; Nuclear Reactors The mass distribution of the fragments shows that the two pieces are large, but usually unequal in size.
31-2 Nuclear Fission; Nuclear Reactors The energy release in a fission reaction is quite large. Also, since smaller nuclei are stable with fewer neutrons, several neutrons emerge from each fission as well. These neutrons can be used to induce fission in other nuclei, causing a chain reaction.
31-2 Nuclear Fission; Nuclear Reactors In order to make a nuclear reactor, the chain reaction needs to be self-sustaining it will continue indefinitely but controlled. A moderator is needed to slow the neutrons; otherwise their probability of interacting is too small. Common moderators are heavy water and graphite. Unless the moderator is heavy water, the fraction of fissionable nuclei in natural uranium is too small to sustain a chain reaction, about 0.7%. It needs to be enriched to about 2 3%.
31-2 Nuclear Fission; Nuclear Reactors Neutrons that escape from the uranium do not contribute to fission. There is a critical mass below which a chain reaction will not occur because too many neutrons escape.
31-2 Nuclear Fission; Nuclear Reactors Finally, there are control rods, usually cadmium or boron, that absorb neutrons and can be used for fine control of the reaction, to keep it critical but just barely.
31-2 Nuclear Fission; Nuclear Reactors Some problems associated with nuclear reactors include the disposal of radioactive waste and the possibility of accidental release of radiation. An atomic bomb also uses fission, but the core is deliberately designed to undergo a massive uncontrolled chain reaction when the uranium is formed into a critical mass during the detonation process.
31-3 Nuclear Fusion The lightest nuclei can fuse to form heavier nuclei, releasing energy in the process. An example is the sequence of fusion processes that change hydrogen into helium in the Sun. They are listed here with the energy released in each: (31-6a) (31-6b) (31-6c)
31-3 Nuclear Fusion The net effect is to transform four protons into a helium nucleus plus two positrons, two neutrinos, and two gamma rays. (31-7) More massive stars can fuse heavier elements in their cores, all the way up to iron, the most stable nucleus.
Another processes:: Nuclear Fusion in the Sun Layers of the Sun: Core (produces energy), T~15.5 10 6 K Radiative zone Convention zone
Sun s lifetime The sun will continue to shine as it currently does for approximately 5 billion years more. This is shown by estimating that the sun will stay in its current evolutionary tract until approximately 10% of its hydrogen is converted to helium. Because hydrogen to helium releases approximately 0.007*m*c 2 of energy, and given that it releases approximately 3.846 x 10 26 J/s: When the core runs out of hydrogen fuel, it will contract under the weight of gravity. However, some hydrogen fusion will occur in the upper layers. As the core contracts, it heats up. This heats the upper layers, causing them to expand. As the outer layers expand, the radius of the star will increase and it will become a red giant.
Sun s lifetime The radius of the red giant sun will be just beyond the Earth's orbit. At some point after this, the core will become hot enough to cause the helium to fuse into carbon. When the helium fuel runs out, the core will expand and cool. The upper layers will expand and eject material that will collect around the dying star to form a planetary nebula. Finally, the core will cool into a white dwarf and then eventually into a black dwarf. This entire process will take a few billion years.
31-3 Nuclear Fusion There are three fusion reactions that are being considered for power reactors: (31-8a) (31-8b) (31-8c) These reactions use very common fuels deuterium or tritium and release much more energy per nucleon than fission does.
31-3 Nuclear Fusion A successful fusion reactor has not yet been achieved, but fusion, or thermonuclear, bombs have been built. Several geometries for the containment of the incredibly hot plasma that must exist in a fusion reactor have been developed the tokamak, which is a torus; and inertial confinement, which is tiny pellets of deuterium ignited by powerful lasers.
31-4 Passage of Radiation Through Matter; Radiation Damage Radiation includes alpha, beta, and gamma rays; X rays; and protons, neutrons, pions, and other particles. All these forms of radiation are called ionizing radiation, because they ionize material that they go through. This ionization can cause damage to materials, including biological tissue.
31-5 Measurement of Radiation Dosimetry Radiation damages biological tissue, but it can also be used to treat cancer and other diseases. It is important to be able to measure the amount, or dose, of radiation received. The source activity is the number of disintegrations per second, often measured in curies, Ci. 1 Ci = 3.70 10 10 disintegrations per second The SI unit for source activity is the becquerel (Bq): 1 Bq = 1 disintegration/s
31-5 Measurement of Radiation Dosimetry Another measurement is the absorbed dose the effect the radiation has on the absorbing material. The rad, a unit of dosage, is the amount of radiation that deposits energy at a rate of 1.00 10 2 J/kg in any material. The SI unit for dose is the gray, Gy: 1 Gy = 1 J/kg = 100 rad
31-5 Measurement of Radiation Dosimetry The effect on tissue of different types of radiation varies, alpha rays being the most damaging. To get the effective dose, the dose is multiplied by the relative biological effectiveness.
31-5 Measurement of Radiation Dosimetry If the dose is measured in rad, the effective dose is in rem; if the dose is grays, the effective dose is in sieverts, Sv. Natural background radiation is about 0.3 rem per year. The maximum for radiation workers is 5 rem in any one year, and below 2 rem per year averaged over 5 years. A short dose of 1000 rem is almost always fatal; a short dose of 400 rem has about a 50% fatality rate.
31-6 Radiation Therapy Cancer is sometimes treated with radiation therapy to destroy the cells. In order to minimize the damage to healthy tissue, the radiation source is often rotated so it goes through different parts of the body on its way to the tumor.
31-7 Tracers in Research and Medicine Radioactive isotopes are widely used in medicine for diagnostic purposes. They can be used as non-invasive scans, or tools to check for unusual concentrations that could signal a tumor or other problem. The radiation is detected with a gamma-ray detector.
31-8 Emission Tomography: PET and SPECT Radioactive tracers can also be detected using tomographic techniques, where a three-dimensional image is gradually built up through successive scans.
31-9 Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) A proton in a magnetic field can have its spin either parallel or antiparallel to the field. The field splits the energy levels slightly; the energy difference is proportional to the field magnitude.
31-9 Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) The object to be examined is placed in a static magnetic field, and radio frequency (RF) electromagnetic radiation is applied. When the radiation has the right energy to excite the spin-flip transition, many photons will be absorbed. This is nuclear magnetic resonance. The value of the field depends somewhat on the local molecular neighborhood; this allows information about the structure of the molecules to be determined.
31-9 Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) Magnetic resonance imaging works the same way; the transition is excited in hydrogen atoms, which are the commonest in the human body.
31-9 Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) Giving the field a gradient can contribute to image accuracy, as it allows determining the origin of a particular signal.
31-9 Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) Here is a summary of the medical imaging techniques we have discussed.
Summary of Chapter 31 Nuclear reaction occurs when nuclei collide and different nuclei are produced Reaction energy or Q-value: Fission: heavy nucleus splits into two intermediate-sized nuclei Chain reaction: neutrons emitted in one fission reaction trigger another, and so on Critical mass: minimum needed to sustain chain reaction
Summary of Chapter 31 Moderator: slows neutrons Fusion: small nuclei combine to form larger ones Sun s energy comes from fusion reactions Useful fusion reactor has not yet been built Radiation damage is measured using dosimetry Effect of absorbed dose depends on type of radiation