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Applications of nuclear physics have become an integral part of modern life. From a bone scan that detects a cancer to a radioiodine treatment that cures another, nuclear radiation has many diagnostic and therapeutic applications in medicine. In addition nuclear radiation is used in other useful scanning applications, as seen in [link] and [link] . The fission power reactor and the hope of controlled fusion have made nuclear energy a part of our plans for the future. That said, the destructive potential of nuclear weapons haunts us, as does the possibility of nuclear reactor accidents.

Nuclear physics revealed many secrets of nature, but full exploitation of the technology remains controversial as it is intertwined with human values. Because of its great potential for alleviation of suffering and its power as a giant-scale destroyer of life, nuclear physics is typically viewed with ambivalence. Nuclear physics is a classic example of the truism that applications of technology can be good or evil, but knowledge itself is neither.

This chapter focuses on medical applications of nuclear physics. The sections on fusion and fission address the ideas that objects and systems have properties, such as mass (Big Idea 1), and that interactions between systems can result in changes in those systems (Big Idea 4). The changes that occur as a result of interactions always satisfy conservation laws (Big Idea 5). The mass conservation (Enduring Understanding 1.C) and energy conservation (Enduring Understanding 5.B) are replaced by the law of conservation of mass-energy.

In nuclear fusion and fission reactions, so much potential energy is lost that the mass of the products of a reaction are measurably less than the mass of the reactants (Essential Knowledge 1.C.4, Essential Knowledge 4.C.4) in accordance with the equation $E=m{c}^2$ . This equation explains that mass is part of the internal energy of an object or system (Essential Knowledge 5.B.11). In addition, the number of nucleons is conserved in these nuclear reactions (Enduring Understanding 5.G), and that determines which nuclear reactions are possible (Essential Knowledge 5.G.1).

Big Idea 1 Objects and systems have properties such as mass and charge. Systems may have internal structure.

Enduring Understanding 1.C Objects and systems have properties of inertial mass and gravitational mass that are experimentally verified to be the same and that satisfy conservation principles.

Essential Knowledge 1.C.4 In certain processes, mass can be converted to energy and energy can be converted to mass according to $E=m{c}^2$ , the equation derived from the theory of special relativity.

Big Idea 4 Interactions between systems can result in changes in those systems.

Enduring Understanding 4.C Interactions with other objects or systems can change the total energy of a system.

Essential Knowledge 4.C.4 Mass can be converted into energy and energy can be converted into mass.

Big Idea 5 Changes that occur as a result of interactions are constrained by conservation laws.

Enduring Understanding 5.B The energy of a system is conserved.

Essential Knowledge 5.B.11 Beyond the classical approximation, mass is actually part of the internal energy of an object or system with $E=m{c}^2$ .

Enduring Understanding 5.G Nucleon number is conserved.

Essential Knowledge 5.G.1 The possible nuclear reactions are constrained by the law of conservation of nucleon number.