Radiation Physics and Image Formation · 3 min read
Atom Structure
Basic Atomic Theory
All matter is composed of atoms. Understanding atomic structure is fundamental to comprehending how x-rays are produced and how they interact with biological tissues.
An atom consists of three primary subatomic particles: protons, neutrons, and electrons. Protons carry a positive electrical charge and are located within the nucleus at the center of the atom. Neutrons carry no electrical charge and are also found within the nucleus alongside protons. Electrons carry a negative electrical charge and orbit the nucleus in defined energy levels called electron shells.
The nucleus contains virtually all of the atom's mass because protons and neutrons are approximately 1,836 times heavier than electrons. Despite this mass concentration, the nucleus occupies only a tiny fraction of the atom's total volume. The remainder of the atomic volume consists of the electron cloud surrounding the nucleus.
Atomic Number and Mass Number
The atomic number (Z) represents the number of protons in an atom's nucleus. This number defines the element and determines its position on the periodic table. For example, hydrogen has an atomic number of 1, carbon has an atomic number of 6, and tungsten (used in x-ray tube targets) has an atomic number of 74.
The mass number (A) equals the total number of protons and neutrons in the nucleus. Atoms of the same element may have different numbers of neutrons, creating isotopes. Isotopes share identical chemical properties but differ in atomic mass.
In a neutral atom, the number of electrons equals the number of protons, resulting in no net electrical charge. When an atom gains or loses electrons, it becomes an ion with a net positive or negative charge.
Electron Shells and Binding Energy
Electrons occupy specific energy levels or shells designated by letters K, L, M, N, and so forth, moving outward from the nucleus. The K-shell is closest to the nucleus and can hold a maximum of 2 electrons. The L-shell can hold up to 8 electrons. The M-shell can hold up to 18 electrons. Each subsequent shell can accommodate progressively more electrons.
Binding energy refers to the amount of energy required to remove an electron from its shell. Electrons in inner shells (closer to the nucleus) have higher binding energies because they experience stronger attraction from the positively charged protons. K-shell electrons have the highest binding energy, while outer-shell electrons have progressively lower binding energies.
The binding energy concept is critical for understanding x-ray production. When high-speed electrons from the cathode strike the anode target, they can dislodge inner-shell electrons from target atoms if they possess sufficient energy. This interaction produces characteristic x-rays.
Electron Transitions
When an inner-shell electron is ejected from an atom, the atom becomes unstable. To restore stability, an electron from an outer shell drops down to fill the vacancy. This transition releases energy in the form of electromagnetic radiation. If the energy released falls within the x-ray portion of the electromagnetic spectrum, characteristic radiation is produced.
The energy of the emitted radiation equals the difference in binding energies between the two shells involved in the transition. For tungsten, a K-shell vacancy filled by an L-shell electron produces approximately 59 keV of characteristic radiation.
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