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9 Radiation for Radionuclide Users e- p + n e- p + n e- p + n Hydrogen Z=1 A=1 Deuterium Z=1 A=2 Tritium Z=1 A=3 B. Fundamentals of Radioactivity and Ionizing Radiation Atomic Structure All matter contains atoms that are themselves composed of three primary particles; protons, neutrons, and electrons. Protons and neutrons are relatively massive compared to electrons and make up the dense core of the atom known as the nucleus. Protons are positively charged while neutrons, as their name implies, are neutral. The negatively charged electrons are found in an extended cloud surrounding the nucleus. The number of protons within the nucleus defines the atomic number , designated by the symbol Z. In an electrically neutral atom, Z also indicates the number of electrons within the atom. The number of protons plus neutrons in the nucleus is termed the mass number , designated by the symbol A. For lighter elements, the number of neutrons in a stable nucleus approximately equals the number of protons. The atomic number of an atom designates its specific elemental identity. For example, an atom with a Z of 1 is hydrogen, an atom with a Z of 2 is helium, and an atom with a Z of 3 is lithium. A given species of nucleus characterized by a particular atomic number and mass number is called a nuclide . A specific nuclide is represented by its chemical symbol with the mass number in superscript, e.g., 3 H, 14 C, 125 I or H-3, C-14, I-125. Nuclides with the same number of protons (i.e., the same Z) but different number of neutrons (i.e., different A) are called isotopes of that particular element. Isotopes of an element have for most practical purposes identical chemical properties. Figure 1 shows three naturally occurring isotopes of hydrogen. Radioactive Decay Depending upon the ratio of neutrons to protons within its nucleus, an isotope of a particular element may be stable or unstable. Over time the nuclei of unstable isotopes spontaneously disintegrate, or transform, in a process known as radioactive decay . As part of this process, various types of ionizing radiation may be emitted from the nucleus and/or its surrounding electrons. Nuclides which undergo radioactive decay are called radionuclides . Any material which contains measurable amounts of one or more radionuclides is a radioactive material . Isotopes of Hydrogen