Concept Summary

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Topic | Discussion | Learning Aids
Ionizing radiation | 32.1 BIOLOGICAL EFFECTS OF IONIZING RADIATION Ionizing radiation consists of photons and/or moving particles that have enough energy to ionize an atom or molecule. Exposure is a measure of the ionization produced in air by X-rays or rays. When a beam of X-rays or rays is sent through a mass m of dry air (0 °C, 1 atm pressure) and produces positive ions whose total charge is q, the exposure in coulombs per kilogram (C/kg) is |
Exposure (in C/kg) |

| With q in coulombs (C) and m in kilograms (kg), the exposure in roentgens is |
Exposure (in roentgens) |

| (32.1)

| The absorbed dose is the amount of energy absorbed from the radiation per unit mass of absorbing material: |
Absorbed dose |

| (32.2)

| The SI unit of absorbed dose is the gray (Gy); 1 Gy = 1 J/kg. However, the rad (rd) is another unit that is often used: 1 rd = 0.01 Gy. |
| The amount of biological damage produced by ionizing radiation is different for different types of radiation. The relative biological effectiveness (RBE) is the absorbed dose of 200-keV X-rays required to produce a certain biological effect divided by the dose of a given type of radiation that produces the same biological effect. |
Relative biological effectiveness |

| (32.3)

| The biologically equivalent dose (in rems) is the product of the absorbed dose (in rads) and the RBE: |
Biologically equivalent dose |

| (32.4)

Induced nuclear reaction Induced nuclear transmutation | 32.2 INDUCED NUCLEAR REACTIONS An induced nuclear reaction occurs whenever a target nucleus is struck by an incident nucleus, an atomic or subatomic particle, or a -ray photon and undergoes a change as a result. An induced nuclear transmutation is a reaction in which the target nucleus is changed into a nucleus of a new element. |
Nuclear reactions obey conservation laws | All nuclear reactions (induced or spontaneous) obey the conservation laws of physics as they relate to mass/energy, electric charge, linear momentum, angular momentum, and nucleon number. |
Shorthand notation for nuclear reactions | Nuclear reactions are often written in a shorthand form, such as . The first and last symbols and denote, respectively, the initial and final nuclei. The symbols within the parentheses denote the incident particle (on the left) and the small emitted particle or proton p (on the right). |
Thermal neutron | A thermal neutron is one that has a kinetic energy of about 0.04 eV. |
Nuclear fission Chain reaction Controlled chain reaction | 32.3 NUCLEAR FISSION Nuclear fission occurs when a massive nucleus splits into two less massive fragments. Fission can be induced by the absorption of a thermal neutron. When a massive nucleus fissions, energy is released because the binding energy per nucleon is greater for the fragments than for the original nucleus. Neutrons are also released during nuclear fission. These neutrons can, in turn, induce other nuclei to fission and lead to a process known as a chain reaction. A chain reaction is said to be controlled if each fission event contributes, on average, only one neutron that fissions another nucleus. |
Nuclear reactor Fuel elements Neutron moderator Control rods Critical state
Subcritical state
Supercritical state
32.4 NUCLEAR REACTORS A nuclear reactor is a device that generates energy by a controlled chain reaction. Many reactors in use today have the same three basic components: fuel elements, a neutron moderator, and control rods. The fuel elements contain the fissile fuel, and the entire region of fuel elements is known as the reactor core. The neutron moderator is a material (water, for example) that slows down the neutrons released in a fission event to thermal energies so they can initiate additional fission events. Control rods contain material that readily absorbs neutrons without fissioning. They are used to keep the reactor in its normal, or critical, state, in which each fission event leads to one additional fission, no more, no less. The reactor is subcritical when, on average, the neutrons from each fission trigger less than one subsequent fission. The reactor is supercritical when, on average, the neutrons from each fission trigger more than one additional fission.
Nuclear fusion Magnetic confinement Inertial confinement | 32.5 NUCLEAR FUSION In a fusion process, two nuclei with smaller masses combine to form a single nucleus with a larger mass. Energy is released by fusion when the binding energy per nucleon is greater for the larger nucleus than for the smaller nuclei. Fusion reactions are said to be thermonuclear because they require extremely high temperatures to proceed. Current studies of nuclear fusion utilize either magnetic confinement or inertial confinement to contain the fusing nuclei at the high temperatures that are necessary. |
Photon family Lepton family Hadron family | 32.6 ELEMENTARY PARTICLES Subatomic particles are divided into three families: the photon family (which consists only of the photon), the lepton family (which includes the electron), and the hadron family (which includes the proton and the neutron). |
Elementary particles | Elementary particles are the basic building blocks of matter. All members of the photon and lepton families are elementary particles. |
Quark theory | The quark theory proposes that the hadrons are not elementary particles but are composed of elementary particles called quarks. Currently, the hundreds of hadrons can be accounted for in terms of six quarks (up, down, strange, charmed, top, and bottom) and their antiquarks. |
The standard model | The standard model consists of two parts: (1) the currently accepted explanation for the strong nuclear force in terms of the quark concept of “color” and (2) the theory of the electroweak interaction. |
Cosmology | 32.7 COSMOLOGY Cosmology is the study of the structure and evolution of the universe. Our universe is expanding. The speed v at which a distant galaxy recedes from the earth is given by Hubble’s law: |
Hubble’s law |

| (32.5)

Hubble parameter | where H = 0.022 m/(s·light-year) is called the Hubble parameter and d is the distance of the galaxy from the earth. |
Big Bang theory | The Big Bang theory postulates that the universe had a definite beginning in a cataclysmic event, sometimes called the primeval fireball. The radiation left over from this event is in the microwave region of the electromagnetic spectrum, and it is consistent with a perfect blackbody radiating at a temperature of 2.7 K, in agreement with theoretical analysis of the Big Bang. |
Standard cosmological model | The standard cosmological model for the evolution of the universe is summarized in Figure 32-16. |

Use Self-Assessment Test 32.1 to evaluate your understanding of Sections 32.1, 32.2, 32.3, 32.4, 32.5, 32.6 and 32.7 .

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