The radioactivity of a substance is measured in the number of nuclei that decay per unit time. The energy associated with the radioactive decay ranges from thousands to millions of electron-volts per nucleus, which is why the decay of a single nucleus typically leads to a large number of ionizations. One joule (abbreviated J) is equivalent to the amount of energy used by a one-watt light bulb lit for one second. One electron-volt is only 1.6 x 10 -19 joules of energy, in other words, 0.16 billion-billionth of a joule. We say then that the atom is “ionized.” In the jargon, the “ionization energy” of the tightly bound electron in hydrogen is 13.6 electron volts.Įlectrons are very light objects, so we don’t expect an electron-volt to represent very much energy. It takes 13.6 electron-volts of energy to move this electron completely away from the proton. It takes energy to move this electron away from the proton. An electron is “tightly bound” in a hydrogen atom (one proton and one electron). The electron-volt (abbreviated eV) is a unit of energy associated with moving electrons around. Ionizing radiation can be measured using units of electron volts, ergs, and joules. For a more detailed explanation, see Health Effects of Exposure to Low Levels of Ionizing Radiation (BEIR V report), National Academy Press, 1990, pp. These processes variously result in the emission of gamma rays, beta radiation, and, in the case of spallation, more neutrons. Neutrons can, however, ionize indirectly in a variety of ways: elastic collisions, inelastic scattering, nonelastic scattering, capture reactions, or spallation processes. Unlike alpha and beta particles, they do not interact with electrons or cause ionization directly. Neutrons are neutral particles that have no electric charge. The term “X rays” is also sometimes used for the gamma rays emitted in the process of radioactive decay that are at the lower end of the energy spectrum of electromagnetic radiation resulting from radioactive decay. When gamma rays interact with tissue, they ionize atoms. Gamma rays can penetrate much more deeply than alpha or beta particles a high-energy gamma ray photon may pass through a person without interacting with tissue at all. A radioactive element may emit gamma rays (in discrete bundles, or quanta, called photons) if the nucleus remaining after alpha or beta decay is in an excited state. Gamma rays are electromagnetic radiation. A medium-energy beta particle travels about one meter in air and one millimeter in body tissue. Thus, it takes beta particles a longer distance than alpha particles to lose energy. For example, alpha particles do not penetrate the outer layer of human skin, but if inhaled, alpha particles can damage lung tissue.Ī beta particle is an electron or a positron and is much lighter than an alpha particle. An alpha particle can travel several millimeters in air, but in general its range decreases with increasing density of the medium. Alpha particles readily ionize material they contact and transfer energy to that material’s electrons. All have enough energy to ionize atoms, in other words, remove one or more of the atom’s electrons.Īn alpha particle consists of two protons and two neutrons, the equivalent of The four forms of ionizing radiation are alpha particles, beta particles, gamma rays, and, indirectly, neutrons. Radioactive decay occurs when the nucleus of an atom spontaneously decays by emitting a particle (an alpha particle, an electron, or one or more neutrons). Ionizing radiation is emitted when radioactive substances decay. ( Some of the terms used below are defined in IEER’s Glossary) Also see the associated Energy & Security no. 4, which includes a Glossary of Radiation-Related Terms, and information on Measuring Radiation: Devices and Methods. This resource is part of Science for Democratic Action vol.
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