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Objectives
At the conclusion of this unit the student should be able to --
- Distinguish between natural and man-made radiation.
- Describe the principle of half-life of radioactive materials and demonstrate how half-lives can be calculated.
- Identify and discuss the different types of radiation.
Resources:
Lesson
Radiation is all around us. It comes from the Earth and from outer space. Many forms of radiation are invisible -- we can't feel it, see it, taste it, or smell it. Yet, it can be detected and measured when present. We measure ionizing radiation in units called millirems. But what is radiation? Radioactive materials are composed of atoms that are unstable. An unstable atom gives off its excess energy until it becomes stable. The energy emitted is radiation. We can classify radiation as being either natural or man-made.
As I mentioned a moment ago, the Earth is surrounded by radiation. Every day, for example, we are exposed to radon, a radioactive gas from uranium found in soil dispersed in the air; from radioactive potassium in our food and water; from uranium, radium, and thorium in the Earth's crust; and from cosmic rays and the sun.
These types of radiation are called natural or background radiation. In the U.S. we are exposed to an average of 300 millirems of natural radiation each year (a millirem is a unit of measure for exposure to radiation). This amounts to natural radiation accounting for nearly 82 percent of our total annual exposure. Where does the remaining 18 percent come from? Man-made sources.
Doses in Our Daily Lives
Man-made radiation sources that people can be exposed to include tobacco, television, medical x-rays, smoke detectors, lantern mantles, nuclear medicine, and building materials.
Adding it all up, the average American is exposed to a total of about 360 millirems a year from natural and man-made radiation. The sources of radiation are shown in Classroom Activity 1 .
Doses from Medical Procedures
In addition to natural background radiation , Americans receive an average dose of about 0.06 rem (60 millirem ) per year from man-made sources of radiation , including medical, commercial, and industrial sources. Of these sources, medical procedures provide the largest contribution to human exposure to man-made radiation. For example, a chest x-ray typically gives a dose of about 0.006 rem (6 millirem), as shown in the table to the left.
Among these medical procedures, x-rays, mammography, and CT use radiation or perform functions similar to those of radioisotopes. However, they do not involve radioactive material and, hence, are not regulated by the U.S. Nuclear Regulatory Commission (NRC). Instead, most of these procedures are regulated by State health agencies. In fact, among these procedures, the NRC and its Agreement States only license and regulate the possession and use of radioactive materials for nuclear medicine. |
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Radioactivity in Food
All organic matter (both plant and animal) contains some small amount of radiation from radioactive potassium-40 ( 40 K), radium-226 ( 226 Ra), and other isotopes. In addition, all water on Earth contains small amounts of dissolved uranium and thorium. As a result, the average person receives an average internal dose of about 30 millirem of these materials per year from the food and water that we eat and drink, as illustrated by the following table. (Amounts are shown in picocuries per kilogram.)
Generally, when we think of exposure to radiation, we need to look at radioactive atoms produced in nuclear reactors and described as being unstable. They are unstable because they undergo a disintegrating process called decaying. During this process, unstable atoms becomes stable, throwing off (emitting) radiation in the form of rays and/or particles. |
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How fast a radioactive atom decays into a stable atom depends on the atom itself. For example, the range in the rate of decay among isotopes goes from fractions of seconds to several billion years (e.g., uranium).
Let's take a look at uranium-238 to illustrate the decay chain.
As U-238 decays it changes into thorium-230, which changes into radium-226, which changes into radon-218, which changes into bismuth-214, and finally into lead-206 (a stable element).
One peculiar thing about radioactive atoms is that no one knows exactly when the element will decay and give off radiation. There is, however, a pattern relating to how long it takes for an isotope to lose half of its radioactivity. The pattern is called half-life . If an atom, for example, has a half-life of 10 years, half of its atoms will decay in 10 years. Then in another 10 years half of that amount will decay and so on. While there are several different forms of radiation, we're going to concentrate on just three that result from the decay of radioactive isotopes: alpha, beta, and gamma.
Alpha, Beta, and Gamma Radiation
Beta particles are high energy electrons. Both alpha and beta particles are emitted from unstable isotopes. The alpha particle, consisting of two protons and two neutrons, is relatively large compared to beta particles. Gamma rays have no mass.
Because of its size and electrical charge (+2), the alpha particle has a relatively slow speed and low penetrating distance (one to two inches in air). Alpha particles are easily stopped by a thin sheet of paper or the body's outer layer of skin. Since they do not penetrate the outer (dead) layer of skin, they present little or no hazard when they are external to the body.
However, alpha particles are considered internal hazards, because when they come into contact with live tissue they cause a large number of ionizations to occur in small areas, thus causing damage to tissues and cells.
Beta radiation , while faster and lighter than alpha radiation, can travel through about 10 feet of air and penetrate very thin layers of materials such as aluminum foil. However, while clothing will stop most beta particles, they can penetrate the live layers of skin tissue. Therefore, beta radiation is considered to be both an internal and external (to skin only) hazard. Thin layers of metals and plastics can be used to shield individuals from beta radiation.
Gamma radiation , high energy light, is a little different. It is a type of electromagnetic wave, just like radio waves, light waves, and x-rays. Gamma radiation is a very strong type of electromagnetic wave. It is has no weight and travels at the speed of light. This is much faster than alpha and beta radiation.
Because of their penetrating capability, gamma rays are considered both internal and external hazards. Thick walls of cement, lead, or steel are needed to stop it.
Alpha, beta, and gamma radiations are also known as ionizing radiation. Ionizing radiation is especially harmful because it can change the chemical makeup of many things, including the delicate chemistry of the human body and other living organisms. For this reason it is a good idea to avoid unnecessary exposure to all ionizing radiation.
The problem is simply this: large amounts of radiation -- far above the levels encountered in daily life -- can produce cancer and genetic defects in living organisms. Radiation causes damage and alters the body's normal cells and normal cell function. This breakdown in normal cell function may result in an uncontrolled growth of cells, hence the potential for malignant/cancerous tumors.
Whether the source of radiation is natural or man-made, whether it is a small dose of radiation or a large dose, there will be some biological effects. A diagram can show the biological effect of ionizing radiation.
Radiation causes ionizations of atoms that will affect molecules that may affect cells that may effect tissues that may affect organs that may affect the whole body.
Although we tend to think of biological effects in terms of the effect of radiation on living cells, in actuality, ionizing radiation, by definition, interacts only with atoms by a process called ionization. Thus, all biological damage effects begin when radiation interacts with atoms forming the cells in the human body. As a result, radiation effects on humans proceed from the lowest to the highest level.
Sources of Radiation
Since the beginning of time, all living creatures have been, and are still being, exposed to radiation. Nonetheless, most people are not aware of all the natural and man-made sources of radiation in our environment.
A chart of the public's exposure to ionizing radiation shows that people generally receive a total annual dose of about 360 millirem . Of this total, natural sources of radiation account for about 82 percent, while man-made sources account for the remaining 18 percent.
Natural background radiation comes from the following three sources:
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Cosmic Radiation
The sun and stars send a constant stream of cosmic radiation to Earth, much like a steady drizzle of rain. Differences in elevation, atmospheric conditions, and the Earth's magnetic field can change the amount (or dose) of cosmic radiation that we receive.
Terrestrial Radiation
The Earth itself is a source of terrestrial radiation. Radioactive materials (including uranium, thorium,
and radium) exist naturally in soil and rock. Essentially all air contains radon, which is responsible for most of the dose that Americans receive each year from natural background sources. In addition, water contains small amounts of dissolved uranium and thorium, and all organic matter (both plant and animal) contains radioactive carbon and potassium. Some of these materials are ingested with food and water, while others (such as radon) are inhaled. The dose from terrestrial sources varies in different parts of the world, but locations with higher soil concentrations of uranium and thorium generally have higher doses.
Internal Radiation
All people have internal radiation, mainly from radioactive potassium-40 and carbon-14 inside their bodies from birth and, therefore, are sources of exposure to others. The variation in dose from one person to another is not as great as that associated with cosmic and terrestrial sources.
Man-Made Sources
Although all living things are exposed to natural background radiation, exposure to man-made radiation sources differs for the following groups:
Members of the Public
In general, the following man-made sources expose the public to radiation (the significant radioactive isotopes are indicated in parentheses):
- Medical Sources (by far, the most significant man-made source)
- Diagnostic x-rays
- Nuclear medicine procedures (iodine-131, cesium-137, and others)
- Consumer Products
- Building and road construction materials
- Combustible fuels, including gas and coal
- X-ray security systems
- Televisions
- Fluorescent lamp starters
- Smoke detectors (americium)
- Luminous watches (tritium)
- Lantern mantles (thorium)
- Tobacco (polonium-210)
- Ophthalmic glass used in eyeglasses
- Some ceramics
To a lesser degree, the public is also exposed to radiation from the nuclear fuel cycle, from uranium mining and milling to disposal of used (spent) fuel. In addition, the public receives some minimal exposure from the transportation of radioactive materials and fallout from nuclear weapons testing and reactor accidents (such as Chernobyl). For that reason, the U.S. Nuclear Regulatory Commission (NRC) requires its licensees to limit the maximum radiation exposure to individual members of the public to 100 mrem (1 mSv) per year. The related NRC regulations and radiation exposure limits are contained in Title 10, Part 20, of the Code of Federal Regulations ( 10 CFR Part 20).
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In general, occupationally exposed individuals work in the following areas:
Such individuals are exposed to varying amounts of radiation, depending on their specific jobs and the sources with which they work (including cobalt-60, cesium-137, americium-241, and other isotopes). For that reason, the NRC requires its licensees to limit occupational exposure to adults working with radioactive material to 5,000 mrem (50 mSv) per year. The related NRC regulations and radiation exposure limits are contained in 10 CFR Part 20. Toward that end, employers carefully monitor the exposure of these individuals using instruments called dosimeters. |
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"Nuclear Reactor Concepts" Workshop Manual , U.S. NRC. 
