Chapter Review
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Chapter 32: Nuclear Physics and Nuclear Radiation Chapter Review |
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Chapter Review
32-5 Nuclear Fission
Under certain conditions, nuclei can break apart into smaller nuclei in a process called nuclear fission. The energy released in nuclear fission is very large compared to the energy released in chemical reactions. The fission of one nucleus can indirectly initiate the fission of other nuclei causing a chain reaction. The ability to control the chain reaction of 23592U has lead to our modern use of nuclear power.
32-6 Nuclear Fusion
Given enough energy, light nuclei can combine to form a more massive nucleus in a process known as nuclear fusion. This larger nucleus has less mass than the individual nuclei that fused to form it. The mass difference is released as energy. As a potential energy source, nuclear fusion is even more powerful than fission. Because of the strong electric force of repulsion, or "Coulomb repulsion," between the nuclei when they are close together the process is called a thermonuclear fusion reaction. It requires a temperature of about 107 K to ignite thermonuclear fusion of protons (hydrogen nuclei) into helium nuclei. This latter process is called the proton-proton cycle and is the main energy source of the sun.
32-7 Practical Application of Nuclear Physics
(a) Biological Effects of Radiation
The high energy particles given off by radioactive decay can be very damaging to living tissue by ionizing its molecules, and therefore changing its structure. Several quantities have been defined in order to quantify the biological effects of radiation. The first of these quantities is the roentgen (R) which measures the amount of ionization caused by X-rays and g-rays. A dosage of one roentgen of X-rays or g-rays is the amount that produces 2.58 x 10-4 C of charge, from ionized molecules, in 1 kg of dry air at STP. That is,
1 R = 2.58 x 10-4 C/kg.
Another quantity is the Radiation Absorbed Dose, or rad. A dosage of 1 rad of any type of radiation delivered to a material is the amount of radiation that results in 0.01 J of absorbed energy by a 1 kg sample of material. So,
1 rad = 0.01 J/kg.
Still another quantity is the Relative Biological Effectiveness, or RBE. The relative in the RBE dose means relative to a 1-rad dose of 200 keV X-rays. A dose of 1 RBE of a particular type of radiation occurs when the ratio of the dose of 200 keV X-rays to the dose of the given type of radiation, that would produce the same biological effect, is 1. Thus, for a given biological effect,
.
RBE is a dimensionles quantity. Another quantity that combines the rad and the RBE is called the biologically equivalent dose. This dose is measured in a unit called rem, which stands for Roentgen Equivalent Man. The definition of dosage in rem is
dose in rem = dose in rad x RBE.
With this definition, 1 rem of any type of radiation produces the same biological damage.
(b) Magnetic Resonance Imaging
The interaction of nuclei with magnetic fields is used in magnetic resonance imaging, or MRI. An important advantage of MRI is that the radiation produced is low energy radiation. Because the photons have low energy, they cause very little tissue damage.
(c) PET Scans
In a Positron-Emission Tomography scan, or PET scan for short, a patient is given a radioactive substance that undergoes b+ decay, giving off a positron. This positron almost immediately annihilates with an electron within the patient's body. The gamma rays given off by this radiation penetrate through, and leave the body where they can be detected during the scan. The analysis of the results from such a scan can provide important biological information.
32-8 Elementary Particles
One of the many goals of physics is to understand the world at its most fundamental level. This endeavor has lead to the study of elementary particles which are thought to be "building blocks" of all matter. To the best of our current understanding elementary particles interact through the four fundamental forces of nature. These forces are called the strong nuclear force, the electromagnetic force, the weak nuclear force, and gravity. You have seen the need for each of these except the weak nuclear force; an explanation of the weak force is beyond the scope of this text, but the weak force is active in the process of beta decay.
Particles that experience the weak nuclear force, but not the strong nuclear force are called leptons. There are six known leptons of which the electron is one. As far as we know, no lepton has internal structure, so these are truly elementary particles. Particles that experience both the weak and strong nuclear forces are called hadrons; protons and neutrons fall into this category. All hadrons are made up of elementary particles called quarks. Some hadrons consist of two quarks, these are called mesons, and some hadrons consist of three quarks, these are called baryons. There are six quarks; these quarks are grouped into pairs that are called flavors. The flavors of the quarks are up (u) and down (d), charm (c) and strange (s), and finally, top (t) and bottom (b). Quarks carry electric charges that are fractions of e, either 
(2/3)e or (1/3)e depending on the quark. Quarks also carry another kind of charge called color. There are three different quark colors usually referred to as red, green, and blue. The theory that describes the interaction of quarks via the color change is called quantum chromodynamics (QCD).
32-9 Unified Forces and Cosmology
It is widely believed that at the beginning of the universe, at the big bang, there was only one fundamental force of nature, called the unified force. As the universe evolved, it is believed that the four forces we detect today separated from each other during processes that can be thought of as cosmological phase transitions. Today, scientists are trying to work backward to develop the theory of this unified force. One successful example is the electroweak theory which has demonstrated that the electromagnetic and the weak nuclear forces are really just different aspects of the same basic force.
32-10 Gravity Waves
In the early 1900s Einstein developed a new theory of gravity called general relativity. This theory predicts the very interesting phenomenon of gravity waves. Basically, gravity waves are waves in space and time that travel at the speed of light. The direct detection of gravity waves has not yet occured although there is strong indirect observational evidence. Today a major effort is underway to both detect gravity waves and use the information to gain deeper insight into the universe. This effort relies on the construction of a Laser Interferometer Gravitational Wave Observatory, or LIGO. Actually, there are several such observations being built. If they are successful, they will create an entirely new branch of science - gravitational wave astronomy.
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