A practical review of basic atomic and nuclear physics is essential to understand the origins of radiations, as well as their interactions with matter. The nature and type of emissions are determined by the structural character of the atom and nucleus. The ways in which radiation interacts with matter have a direct relationship with imaging and radiation safety. The types of radiation and how they interact with matter are the foundation of radionuclide imaging and radiation safety. This chapter will focus on atomic and nuclear structure and the interaction of radiations with matter as they relate to radionuclide imaging.
ATOMIC AND NUCLEAR STRUCTURE
Matter is composed of atoms and the characteristics of a specific form of matter are determined by the number and type of atoms that make it up. How atoms combine is a function of their electron structure. The electron structure is determined by the nuclear architecture. As we have yet to image the atom, its structure is based on a "most-probable" model that fits physical behaviors we observe. The probabilistic approach is based on the model of the atom proposed by Niels Bohr in 1913. The Bohr atom proposed a positively charged nucleus, surrounded by negatively charged electrons. A neutral atom is one in which the positive and negative charges are matched. A mismatch in these charges determines the ionic character of the atom, which is the basis for its chemistry. The electron configuration is also a source for emissions used in radionuclide imaging.
These emissions, or radiations, may be in either of the two forms: particulate or electromagnetic. The origins of either type of radiation may be from the nucleus or the electron structure.
Electrons are arranged around the nucleus in shells. The number of shells is determined by the number of electrons, which is, in turn, determined by the number of protons in the nucleus. The force exerted on these shells, called binding energy, is determined by the proximity of the shell to the nucleus. Higher binding energies are exerted on shells closest to the nucleus and conversely, lower binding energy for those more distant from the nucleus. The innermost shell is named the "K" shell and electrons in this shell are subject to the highest binding energy. The magnitude of that energy is dependent on the positive forces, which is determined by the number of protons in the nucleus. The shells more distant from the nucleus are named L, M, N, and so on. Each of these shells has lower binding energies as a result of their distance from the nucleus (Fig. 1-1).
Atomic structure. The nucleus is surrounded by electron shells. The binding energy decreases as the distance from the nucleus increases (K > L > M).