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# 10: Nuclear Physics

In this chapter, we study the composition and properties of the atomic nucleus. The nucleus lies at the center of an atom, and consists of protons and neutrons. A deep understanding of the nucleus leads to numerous valuable technologies, including devices to date ancient rocks, map the galactic arms of the Milky Way, and generate electrical power.

• 10.0: Prelude to Nuclear Physics
The Sun is the main source of energy in the solar system. The Sun is 109 Earth diameters across, and accounts for more than 99%99% of the total mass of the solar system. The Sun shines by fusing hydrogen nuclei—protons—deep inside its interior. Once this fuel is spent, the Sun will burn helium and, later, other nuclei. Nuclear fusion in the Sun is discussed toward the end of this chapter. In the meantime, we will investigate nuclear properties that govern all nuclear processes, including fusion.
• 10.1: Properties of Nuclei
The atomic nucleus is composed of protons and neutrons. Protons and neutrons have approximately the same mass, but protons carry one unit of positive charge  and neutrons carry no charge. These particles are packed together into an extremely small space at the center of an atom. According to scattering experiments, the nucleus is spherical or ellipsoidal in shape, and about 1/100,000th the size of a hydrogen atom. Protons and neutrons within the nucleus are called nucleons.
• 10.2: Nuclear Binding Energy
The mass defect of a nucleus is the difference between the total mass of a nucleus and the sum of the masses of all its constituent nucleons. The binding energy (BE) of a nucleus is equal to the amount of energy released in forming the nucleus, or the mass defect multiplied by the speed of light squared. A graph of binding energy per nucleon (BEN) versus atomic number A implies that nuclei divided or combined release an enormous amount of energy.
In the decay of a radioactive substance, if the decay constant $$(\lambda)$$ is large, the half-life is small, and vice versa. The radioactive decay law, $$N = N_0 e^{-\lambda t}$$, uses the properties of radioactive substances to estimate the age of a substance. Radioactive carbon has the same chemistry as stable carbon, so it mixes into the ecosphere and eventually becomes part of every living organism.
• 10.4: Nuclear Reactions
Early experiments revealed three types of nuclear “rays” or radiation: alpha (α) rays, beta (β) rays, and gamma  (γ) rays. These three types of radiation are differentiated by their ability to penetrate matter. Alpha radiation is barely able to pass through a thin sheet of paper. Beta radiation can penetrate aluminum to a depth of about 3 mm, and gamma radiation can penetrate lead to a depth of 2 or more centimeters.
• 10.5: Fission
The splitting of a nucleus is called fission. Energy changes in a nuclear fission reaction can be understood in terms of the binding energy per nucleon curve. U-235 fission can produce a chain reaction. In a compound consisting of many U-235 nuclei, neutrons in the decay of one U-235 nucleus can initiate the fission of additional U-235 nuclei. This chain reaction can proceed in a controlled manner, as in a nuclear reactor at a power plant, or proceed uncontrollably, as in an explosion.
• 10.6: Nuclear Fusion
Nuclear fusion is a reaction in which two nuclei are combined to form a larger nucleus; energy is released when light nuclei are fused to form medium-mass nuclei. The amount of energy released by a fusion reaction is known as the Q value. Nuclear fusion explains the reaction between deuterium and tritium that produces a fusion (or hydrogen) bomb; fusion also explains the production of energy in the Sun, the process of nucleosynthesis, and the creation of the heavy elements.
• 10.7: Medical Applications and Biological Effects of Nuclear Radiation
Radioactive compounds are used in to identify cancer, study ancient artifacts, and power our cities. Nuclear fusion also powers the Sun, the primary source of energy on Earth. The focus of this chapter is nuclear radiation. In this section, we ask such questions as: How is nuclear radiation used to benefit society? What are its health risks? How much nuclear radiation is the average person exposed to in a lifetime?