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Ionizing Radiation Theory

Ionizing radiation has the ability to remove electrons from atoms, creating ions; hence, the term "ionizing radiation". The result of ionization is the production of negatively charged free electrons and positively charged ionized atoms. There are four types of ionizing radiation involved that can be classified into two groups: 1) photons, such as gamma and x-rays, and 2) particles, such as beta particles (positrons or electrons), alpha particles (similar to helium nuclei, 2 protons and 2 neutrons), and neutrons (particles with zero charge, electrically neutral). Photons are electromagnetic radiation having energy, but no mass or charge; whereas particles have typically both mass and charge as well as energy. Neutrons have mass and energy, but no charge, and are typically produced by man with machines, such as cyclotrons. All types of ionizing radiation can remove electrons, but interact with matter in different ways.

Particles are more highly ionizing; excitation and ionization are the primary interaction with matter, and potential for ionization increases as mass and charge increase. The range in tissue (depth to which the radiation may penetrate) for particles decreases as mass and charge increase. Photons, because they have no mass or charge, are less ionizing but more penetrating in matter.

Ionized atoms (free radicals), regardless of how they were formed, are much more active chemically than neutral atoms. These chemically active ions can form compounds that interfere with the process of cell division and metabolism. Also, reactive ions can cause a cascade of chemical changes in the tissue. The degree of damage suffered by an individual exposed to ionizing radiation is a function of several factors: type of radiation involved, chemical form of the radiation, intensity of the radiation flux (related to the amount of radiation and distance from the source), energy, and duration of exposure.

Radioactive Decay

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