114 Introduction to Radioactivity and Nuclear Physics

The discovery that atoms contain a tiny, dense nucleus transformed our understanding of matter. Although the nucleus occupies only a small fraction of an atom’s volume, it contains nearly all of the atom’s mass and stores enormous amounts of energy. Unlike the electrons surrounding the nucleus, some nuclei are inherently unstable. These unstable nuclei spontaneously transform into more stable forms by emitting energetic particles or electromagnetic radiation in a process known as radioactivity.

Radioactivity is a natural phenomenon that occurs throughout our environment. Radioactive materials are found in rocks and soil, cosmic rays continuously reach Earth from space, and naturally occurring radioactive isotopes are even present within the human body. Understanding radioactive decay allows scientists to determine the ages of archaeological artifacts and rocks, study geological processes, investigate the origin of the elements, and explore the structure of atomic nuclei.

Nuclear physics is also central to modern medicine. Radioactive isotopes are routinely used to produce diagnostic images, monitor organ function, and treat diseases such as cancer. The concepts introduced in this chapter provide the foundation for understanding these applications, which are explored in greater detail in the following chapters.

In this chapter, you will learn the basic properties of atomic nuclei and radioactivity, examine the different types of radioactive decay, and discover how the study of the nucleus led to many of the fundamental ideas of modern physics. Later chapters will build on these concepts to explore nuclear medicine, radiation therapy, and the elementary particles that make up matter.