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INTRODUCTION

KEY POINTS

  • 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).

Gamma-rays and x-rays are ionizing radiations that have sufficient energy to displace electrons from molecules. These freed electrons then have the capability of damaging other molecules including DNA. The amount of radiation that the public receives has greatly increased due to medical applications, especially the higher doses associated with computed tomography (CT) scans, and from environmental exposure to primarily radon. Figure 25–1 summarizes exposure sources.

FIGURE 25–1

Percent contribution of total effective dose to individuals (Data from NCRP Report No. 160, Ionizing Radiation Exposure of the Population of the United States.)

Biological effects of radiation are primarily due to damage to DNA. Atoms of DNA may be directly ionized or indirectly affected by the creation of a free radical that can interact with the DNA molecule. For radiation particles such as neutrons and α particles, the damage is primarily direct, whereas for photons such as x-rays, about two-thirds of the DNA damage in mammalian cells is due to hydroxy radicals. Cancer has been the major adverse health effect of ionizing radiation. More recently, there has been a concern with possible cardiovascular effects, cataractogenesis, and possibly immunosenescence.

RADIATION BACKGROUND

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 ...

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