elementary particles that have sufficient energy to interact with and transfer some of their energy to objects or materials that intercept their path.

Ionizing Radiation

Many different types of interactions can take place when radiation strikes an object. For instance, atoms in an irradiated object are neutral; they each consist of a positively charged nucleus (made up of protons and neutrons) surrounded by negatively charged electrons. The process of removing an orbital electron from an atom is called ionization.

Some types of radiation can transfer energy in a manner that creates ionization in the object. X rays and gamma rays are particles called photons that can create ionization. Microwaves, ultraviolet radiation, visible light, and infrared are also photons, but they do not result in ionization and are referred to as nonionizing radiation.

Ionization created by radiation in living systems can have unique biologic consequences that are different from those caused by nonionizing radiation. RECA is related specifically to diseases found to have an association with exposure to ionizing radiation.

The process that accelerates particles to form radiation can occur naturally. For example, the sun continuously emits particles that reach the atmosphere and result in a continuous shower of elementary particles on the surface of the earth. Some sources of radiation are man-made, such as x-ray machines, particle accelerators used for cancer therapy, and nuclear power reactors used to generate electricity.

Radioactivity

Radioactivity is another important source of ionizing radiation. Every element such as hydrogen, oxygen, or iron are defined by the number of protons in the nucleus. However, atoms of the same element can have a different number of neutrons in the nucleus. These are called isotopes. Isotopes are identified by the name of the element and the total number of protons and neutrons in the nucleus. For example, the element hydrogen has one proton, 1H. There is another isotope of hydrogen with one proton and one neutron, 2H, called deuterium and also one proton and two neutrons, 3H, called tritium. Some nuclei are unstable, and these can transform (decay) into more stable nuclei by emitting particles—a process called radioactive decay. The emitted particles are a form of radiation originating from radioactivity.

Every element in the periodic chart has at least one isotope that is radioactive. For instance, sodium-23 (23Na) is stable, but sodium-22 (22Na) and sodium-24 (24Na) are radioactive; similarly, iodine-127 (127I) is stable, and iodine-131 (131I) is radioactive. A salt containing natural potassium will always contain some radioactive potassium-40 (40K). Potassium is an essential mineral in our



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