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  • The four main types of radiation are due to alpha particles, electrons (negatively charged beta particles or positively charged positrons), gamma-rays, and X-rays.

  • Alpha particles are helium nuclei (consisting of two protons and two neutrons), with a charge of +2, that are ejected from the nucleus of an atom.

  • Beta particle decay occurs when a neutron in the nucleus of an element is effectively transformed into a proton and an electron, which is ejected.

  • Gamma-ray emission occurs in combination with alpha, beta, or positron emission or electron capture. Whenever the ejected particle does not utilize all the available energy for decay, the excess energy is released by the nucleus as photon or gamma-ray emission coincident with the ejection of the particle.

  • The Compton Effect occurs when a photon scatters at a small angle from its original path with reduced energy because part of the photon energy is transferred to an electron.

  • Ionizing radiation loses energy when passing through matter by producing ion pairs (an electron and a positively charged atom residue).

  • Radiation may deposit energy directly in DNA (direct effect) or may ionize other molecules closely associated with DNA, hydrogen, or oxygen, to form free radicals that can damage DNA (indirect effect).


Ionizing radiations such as γ-rays and X-rays are radiations that have sufficient energy to displace electrons from molecules. These freed electrons then have the capability of damaging other molecules and, in particular, DNA. Atoms of the DNA target may be directly ionized or indirectly affected by the creation of a free radical that can interact with the DNA molecule. In particular, the hydroxyl radical is predominant in DNA damage. Thus, the potential health effects of low levels of radiation are important to understand in order to be able to quantify their effects. Cancer has been the major adverse health effect of ionizing radiation. National Council on Radiation Protection (NCRP) Report 160 gives a summary breakdown of exposure sources in Figure 25–1.


Percent contribution of total effective dose to individuals (Reproduced with permission from NCRP Report No. 160. Ionizing Radiation Exposure of the Population of the United States. Bethesda, MD: National Council on Radiation Protection and Measurements; 2009.


Types of Ionizing Radiation

When ionizing radiation passes through matter, it has the energy to ionize atoms so that one or more of its electrons can be dislodged and chemical bonds broken. Ionizing radiation is of two types: particulate and electromagnetic waves. Particulate radiation may either be electrically charged (α, β, proton) or have no charge (neutron). Ionizing electromagnetic radiation (photons) in the form of X-rays or γ-rays has considerably more energy than nonionizing radiation, such as ultraviolet and visible light. Radionuclides (i.e., radioactive atoms), being unstable, release ...

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