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The The Science section explains what Radioactive Material is and the terminology surrounding it. Since most of the concepts of radioactive material were explained in the Introduction section, this section will go over the concepts surrounding the material itself, not radiation and the different kinds.
Radioactive material is simply material that emits radiation. Consider the sun. The sun beams energy down upon the earth. Energy from the sun reaches earth in the form of electromagnetic radiation of many different types. One familiar type is ultra-violet light. Ultra-violet light is energy in the same family as radio waves, x-rays, and gamma rays. As anyone who has stayed out too long in the sun without sun block knows, ultra-violet radiation can burn the skin; the longer your skin remains exposed to the sun, the more it gets burned. As anyone who has experienced sunburn knows, the damage is usually repaired by the body in a relatively short time. The severity of a sunburn will depend upon how strong the sun is; what the sunlight must penetrate before reaching the skin (clouds, sun block, etc.) and how long you remain exposed to the sun. In very severe cases a sunburn can overwhelm the body’s ability to repair itself and death can result.
Gamma rays affect the body in way similar to the way ultra-violet radiation affects the skin. Gamma rays can penetrate through the body and cause their effects deep within the body. That’s why doctors can use gamma-rays to treat cancer tumors. If your body absorbs gamma radiation, the amount of damage will depend upon how much gamma radiation is absorbed. The severity of any effect of gamma radiation depends on how strong the gamma source is; how far away it is; whether the gamma rays must pass through anything before reaching the body (lead shielding for example); and how long you remain exposed. The body can repair damage from gamma ray exposure just like it can recover from sunburn. Serious exposures to gamma rays can overwhelm the body’s ability to repair itself and death can result.
While we’re on the subject it is worth noting that exposures from radioactive materials normally found in nature and at levels found in the Schuylkill River would not cause detectable effects in the bodies of humans or other animals or plants.
In addition to electromagnetic radiation, it is necessary to discuss particulate radiation. Particulate radiation, for our purposes, consists primarily of beta and alpha particles. A beta particle is simply a fast moving electron traveling through space while not connected to or “orbiting” any atom. An alpha particle is a helium nucleus (i.e. a nucleus with 2 protons and 2 neutrons) without any electrons likewise traveling through space. Both of these particulate types of radiation make their effects known by causing ionization. That is, they cause atoms to have too many or too few electrons. Once an atom is “ionized” its chemical behavior is changed. That can change its function within a molecule which, in turn, can cause structural or functional problems within a material. Because beta particles are small, fast moving, and have a relatively weak charge, they are capable of penetrating a bit deeper into matter than the more massive and highly charged alpha particles. Beta particles can be effectively stopped by heavy cardboard, while alpha particles can be stopped by the outer layers of dead skin. For this reason, particulate radiation is more of a concern when it is emitted by material deposited within the body. For particulate radiation to cause harm it usually must be ingested, inhaled, or injected.
Both gamma rays and beta and alpha particles are radiation. Radioactive Material refers to atomic matter that emits radiation. Radioactive Material was discovered long ago. One of the early pioneers, Madame Curie, determined that a piece of radium exactly one gram in weight emits radiation 37 trillion times every second. When we speak about radioactive material, we describe the material in terms of its decay rate. For a given material, there is a constant relationship between the decay rate and the amount of material required to produce that decay rate. When any radioactive material decays at the rate of 37 trillion times per second, we say we have one Curie of material. Radioactive atoms ordinarily decay only once. As a consequence, if we do have a Curie of material one minute, the next minute we will have less than a Curie because some of the atoms will have decayed and will no longer be available to contribute to the decay rate. The amount of material (as measured in Curies) will continue to gradually fall off. The rate at which the material decays is dependent upon the type of material decaying. Some materials decay slowly, like Uranium and Plutonium, which can take millions of years to decay. Other materials decay very rapidly, existing for only fractions of a second.
The speed at which radioactive materials decay is measured in half-life. Half-life is defined as the time taken for half of a material to decay. For example, if you start with one curie of Cobalt 60, whose half-life is 5.26 years, and then wait 5.26 years, you will be left with exactly half of the original amount of Cobalt-60: half a curie. The half life will then be in effect again – in another 5.26 years you will only have a quarter of a curie.
There are a few more than 100 different chemical elements. Elements have names like Hydrogen, Helium, Carbon, Oxygen, Chlorine, Iron, Cobalt, etc.) Every atom of a given element shares the same number of protons. In fact, we use the number of protons to tell the difference between the elements. Each element then, has a fixed number of protons in the nucleus. For example, Cobalt has 27 protons in its nucleus. Ordinary, stable, non-radioactive Cobalt has 32 neutrons in its nucleus. We refer to this kind of Cobalt as 27Co59 or more commonly Cobalt-59. The former method of expression is referred to as ZXA notation. The combination of 27 protons and 32 neutrons is stable. Other combinations of neutrons with 27 protons can be unstable or “radioactive”. For example Co-60, an atom of Cobalt with 33 neutrons is a very common example of a radioactive atom often created in nuclear reactors. Co-60 decays, turning itself into another more stable atom – Nickel-60 – by a process called beta decay. In doing so, Co-60 emits two gamma rays and one beta particle. Before decaying, Cobalt-60 is said to be in an “excited” state. After decaying to Nickel-60, the new atom is said to be stable or at “ground” state.
So, that's it for radioactive material. To see how radiation affects the world around it and how people keep track of it, visit the next page, Bringing It Together .
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Last updated: Thursday, March 5, 2009 9:19 AM