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Lecture PowerPoints
Chapter 31 Physics: Principles with Applications, 7th edition
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.
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%.
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.
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.
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.
More massive stars can fuse heavier elements in their cores, all the way up to iron, the most stable nucleus.
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*c2 of energy, and given that it releases approximately 3.846 x 1026 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.
• 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.
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.
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 × 1010 disintegrations per second
The SI unit for source activity is the becquerel (Bq):
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.
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.
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.
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.