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| 35 Section 1 Radionuclides and Human Health Effects Written by Michael R. Greenberg and Bernadette M. West, based in part on interviews with Kathryn Higley, Michael Stabin, and Michael Gochfeld, with comments by Niel Wald Background From a young age many people have been taught that atoms form the basic building blocks of matter. In our everyday world atoms are the indivisible piece that defines elements—such as sodium, chlorine, oxygen, and hydrogen. Combinations of these elements go on to build chemical compounds—such as water (H2 0) or table salt (NaCl). When we deal with issues of atomic and nuclear physics, we need to understand that atoms are actually built from multiple components: neutrons, electrons, and protons (and even smaller particles—but that’s for another discussion ). Electrons orbit at a considerable distance around a positively charged nucleus that contains approximately equal numbers of protons and neutrons. The components of the nucleus are in a delicate balance. The protons, which are positively charged, repel each other (opposites attract and likes repel). The neutrons, which have no charge, act as a buffer—they are the shield between the positive charges and so allow multiple protons to be packed into the nucleus . However, if the proportion of neutrons to protons is not exactly right, or if a nucleus gets bombarded by other particles, this balance is disturbed. The nucleus then seeks to move to a more energetically favorable (stable) state. A chemical element in such an unstable form is called a radionuclide. It is also referred to as a radioisotope. As these radionuclides rearrange their internal structure, they often give off radiation—energy traveling through space. The radiation emitted from a nuclear rearrangement can be in the form of either waves (like light) or particles (like electrons or helium atoms). When unstable atoms give off energy in the form of particles or waves, the process is called “radioactive decay” and what is emitted or given off from this process is called “ionizing radiation.” When ionizing radiation strikes anything—wood, iron, the human body—it 36 | The Reporter’s Handbook: Briefs creates electrically charged particles called ions, which can have effects on matter , including living things. Some radionuclides take a very long time to undergo this nuclear rearrangement . For others, the process is very quick (measured in nanoseconds). We use the term half-life to measure the time required for half of the atoms of a particular type to lower their energy level through nuclear rearrangement. In other words, a half-life is the time it takes for the material to lose one half of its radioactivity. The radioactive decay process continues until a nonradioactive stable element forms. To understand radioactive decay, it is important to keep in mind that radionuclides occur naturally but they can also be produced artificially (by people). Naturally occurring radionuclides can be sorted into three categories: primordial radionuclides, chain decay radionuclides, and cosmogenic radionuclides . Primordial radionuclides have extraordinarily long half-lives. Among these are uranium-238, thorium-232, and potassium-40. They have been on the earth since its formation from interstellar dust and were produced from supernova and reactions taking place in the interior of stars. Uranium is the oldest natural radioactive element. It has a half life of 4.5 billion years. This means that a given quantity of uranium-238 will lose half of its strength in 4.5 billion years. It will lose half of its remaining strength in another 4.5 billion years. Chain decay radionuclides are the radioactive elements that result from the decay of several of the primordial elements. They often have very short half-lives. For example, radon-222, a radioactive gas that is a product of uranium-238, has a short half-life of 3.8 days. A classic example of radioactive decay is uranium in potassium uranyl sulfate, which emits alpha particles. This decay results in thorium, which itself is radioactive and emits beta particles. The third group, cosmogenic radionuclides, are continually being produced because of the interaction with cosmic radiation in our atmosphere and surface soils. For example, carbon-14 is continuously formed in the atmosphere by cosmic rays. Radionuclides can be produced artificially in a variety of ways—in nuclear power plants during the process of fission (when the nucleus of an atom is split), in particle accelerators, and through the detonation of nuclear weapons. Accelerators speed up particles to very high energies and then smash them into specially constructed targets, producing “bits” that provide...

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