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The Science of Radiation

Energy
Types of Radiation
Structure of Atoms
Effects of Ionizing Radiation on Atoms
Radioactive Decay
Half-life
Types and Sources of Ionizing Radiation

Figures & Tables

Figure 1: The Electromagnetic Spectrum
Table 1: Basic Types of Radiation
Figure 2: Structure of an Atom
Figure 3: Radioactive Decay
Figure 4: Types of Ionizing Radiation
Figure 5: Penetrating Power of Different Types of Radiation

Energy

Radiation is energy — the primal energy of the universe, originally created billions of years ago. Ionizing radiation is emitted as the unstable atoms of radioactive materials constantly emit alpha, beta, gamma, or other forms of radiation as they "decay" to a stable state. This process can take from a fraction of a second to billions of years, depending on the material. Radioactive materials (called radioisotopes or radionuclides) and the radiation they produce are everywhere — in the soil, in our food and water, and in our bodies.

There is an important difference between radiation and radioactivity (although the terms are often mistakenly used interchangeably):

Radiation is energy in the form of waves or particles sent out over a distance. (A simple example is the ripples of water radiating outward in a pond after a pebble is dropped into the water.) There are many different types of radiation.
Radioactivity is a property of a substance, such as uranium or plutonium, which emits high-energy (ionizing) radiation. Radiation travels over distances ranging from fractions of a millimeter to billions of light-years. This energetic quality of radiation makes life possible but also presents threats of danger and destruction.

To better understand radiation it is important to remember that:

Not all radiation is the same.
Different kinds of radiation affect living things in different ways.

Types of Radiation

The most basic distinction scientists make between types of radiation is the amount of energy involved (Figure 1).

Figure 1. The Electromagnetic Spectrum

Source: The Ohio State University Extension

Radiation with lower energy levels is called nonionizing; radiation with higher energy levels is called ionizing.

This guidebook sometimes uses the generic term "radiation" to refer to ionizing radiation. Keep the differences between the two types in mind as you consider the benefits and risks of the various types of radiation.

Nonionizing Radiation

Nonionizing radiation has lower energy levels and longer wavelengths. Examples include radio waves, microwaves, visible light, and infrared rays from a heat lamp.

Table 1: Basic Types of Radiation
Type	Examples
Non-Ionizing Electromagnetic Radiation	Radio waves, Microwaves, Infrared (heat), Visible Light (color)
Ionizing Electromagnetic Radiation	X-rays, Gamma rays, Cosmic rays
Ionizing Atomic Particle Radiation	Beta radiation, Alpha radiation, Neutrons

Our senses can detect some types of nonionizing radiation: we can see visible light, and feel the burning effects of infrared radiation.

Nonionizing radiation is strong enough to influence the atoms it contacts, but not strong enough to affect their structure. For example, microwave radiation is used to heat the water in food by causing water molecules to vibrate.

Living tissue can generally be protected from harmful nonionizing radiation by devices such as goggles, protective clothing, and shielding around radiation-generating equipment. However, concern has been raised about possible health effects from nonionizing radiation produced by such things as cell phones and electric power lines. (See Electric and Magnetic Fields.)

Ionizing Radiation

Ionizing radiation has higher energy levels. Examples include X-rays and cosmic rays.

Ionizing radiation has enough energy to directly affect the structure of atoms of the materials, including human tissue, which it passes through. A description of the structure of atoms will help in understanding the effects of ionizing radiation. (Table 1)

Structure of Atoms

All substances are composed of atoms that are made up of three subatomic particles: protons, neutrons, and electrons except hydrogen (which may have no neutrons). The protons and neutrons are tightly bound together in the positively charged nucleus at the center of the atom, while a cloud of negatively charged electrons orbits the nucleus. (Figure 2)

Figure 2. Structure of an Atom

Source: U.S. Environmental Protection Agency

The number of protons in the nucleus determines its atomic element. The simplest element, hydrogen, has only one proton in its nucleus. Oxygen has eight protons. Heavier elements, such as uranium and plutonium, have more than 90 protons.

Elements may have various isotopes. An isotope is one of two or more atoms that have the same number of protons but different numbers of neutrons in their nuclei.

Most atoms are stable because the nuclear forces holding the protons and neutrons together are strong enough to overcome the electrical energy that tries to push the protons apart. (The energy pushing protons apart is like two magnets with the same charge that push each other apart.)

When the number of neutrons in the nucleus is above a certain level, however, the atom becomes unstable or radioactive, and some of its excess energy begins to escape. This energy is ionizing radiation.

Effects of Ionizing Radiation on Atoms

When ionizing radiation passes through material, such as human tissue, it may "knock" one or more negatively charged electrons out of orbit around the nuclei of atoms of the material. If this happens, this causes the atoms to become positively charged (ionized). When this occurs in our bodies, molecules and cells may be damaged. The health effects of this damage may be immediate or appear gradually over many years.

Detection of Ionizing Radiation

Ionizing radiation is generally not detectable by our senses: we cannot see, smell, hear, or feel it. This, together with its unpredictable health effects, may explain why it causes so much anxiety.

However, ionizing radiation is relatively easy to detect and measure using electronic equipment. Instruments such as Geiger counters can detect radiation and help us track the amount of radiation exposure. These instruments can tell us if we are too close to a source that can harm us and warn us of a release of radiation.

Radioactive Decay

When the nucleus of a radioactive isotope decays, emitting ionizing radiation, the nucleus is transformed into a different isotope, called a decay product. The new isotope may be stable or unstable. If it is unstable, it will continue to decay, changing its nucleus and emitting more ionizing radiation. Several decays may occur before a stable isotope is produced. (Figure 3)

Figure 3. Radioactive Decay

Source: The Ohio State University Extension

Half-life

The half-life is the time it takes for one-half of a radioactive isotope's atoms to decay. For example, suppose that several atoms of a radioactive isotope with a half-life of three hours were isolated and observed. After three hours, one-half of those radioactive atoms would remain. The other half would have decayed into different isotopes. After three more hours, only half of the remaining radioactive atoms (one-fourth of the original number) would remain unchanged. The half-life can vary substantially from one isotope to another, ranging from a fraction of a second for plutonium-214, to 8 days for Iodine-131, to 24,000 years for plutonium-239, to billions of years for uranium-238. The half-life of an isotope determines the longevity of its radioactivity. The longer the half-life, the more atoms it takes to give a certain amount of radioactivity. However, the half-life of a radioactive material is not a direct measure of the risk associated with the material. (See Determining Levels of Risk.)

Types and Sources of Ionizing Radiation

Ionizing radiation can take two different forms:

Electromagnetic waves which spread out in all directions through space at the speed of light.

High-energy particles which travel through space at various rates.

Examples of ionizing radiation include:

Xrays (used in medicine and for scientific research) and
Gamma rays (emitted by some materials, including the sun and stars and soil).

The major types of ionizing radiation emitted as a result of radioactive decay are alpha and beta particles and gamma rays. (Figure 4) Xrays, another important type of radiation, arise from processes outside of the nucleus.

Figure 4. Types of Ionizing Radiation

Source: The Ohio State University Extension

Alpha Radiation

An alpha particle is composed of two neutrons and two protons in a tight positively-charged bundle that has escaped from the nucleus of a heavy radioactive element, such as uranium or radium, during radioactive decay.

Alpha radiation is relatively slow-moving, has little penetrating power and can be stopped by a single sheet of notebook paper or the dead outer layer of skin tissue. (Figure 5) Therefore, alpha-emitting radioisotopes are not usually a hazard outside the body.

However, when alpha-emitting materials are ingested or inhaled, energy from the alpha particles is deposited in internal tissues such as the lungs and can be harmful. (See The Health Effects of Radon.)

Figure 5. Penetrating Power of
Different Types of Radiation

Beta Radiation

Beta particles are fast-moving free electrons emitted during radioactive decay. They can be either negatively or positively charged. A positively charged beta particle is called a positron.

A beta particle is small — less than 1/7000 of the weight of an alpha particle-and it travels farther through solid material than alpha particles. Beta particles can travel significant distances in air. However, most beta particles can be reduced or stopped by a layer of clothing, eyeglasses, or a few millimeters of a substance such as aluminum. (See Figure 5)

Although more penetrating than alpha particles, beta particles are less damaging over the same distance. Some beta particles can penetrate the skin and cause tissue damage especially to the eyes. However, both alpha and beta emitters are generally more hazardous when they are inhaled or ingested.

Humans can be exposed to beta particles from both manmade and natural sources. Tritium, carbon-14, and strontium-90 are examples of radionuclides that emit beta particles upon decay. (See Major Uses of Radioisotopes.)

Gamma Radiation

Like visible light and xrays, gamma rays are photons-weightless packets of energy. Gamma rays often are emitted from a radioactive nucleus along with alpha or beta particles. They have neither a charge nor mass and are very penetrating.

Most gamma rays can pass completely through the human body. This may cause ionization and possible health effects in any organ of the body. Most gamma rays lose almost all their energy in a few feet of soil, three feet of concrete, or six inches of lead.

A naturally-occurring source of gamma rays in the environment is potassium-40. Manmade sources include iodine-131 (produced in nuclear reactors, accelerators, and nuclear explosions) and cobalt-60 (also created in nuclear reactors) which is used in food irradiation. (See Food Irradiation and Major Uses of Radioisotopes.)

Xrays

Xrays are emitted from processes occurring outside the nucleus. They have essentially the same properties as gamma rays, but are generally lower in energy and therefore less penetrating than gamma rays. A few millimeters of lead can stop xrays.

Xray machines are widely used in medicine for diagnosis and treatment, and in industry for examinations, inspections, and process controls. Because of this heavy use, xrays are the largest source of manmade radiation exposure. Due to their very short wavelength, xrays can pass through materials, such as wood, water, and flesh. They can be most effectively stopped by heavy materials like lead or by substantial thickness of concrete.

Neutrons

One source of ionizing radiation results from the release of neutrons during nuclear fission. Neutrons are released during nuclear fission, which may occur spontaneously or during a nuclear reaction, when a free neutron collides with a nucleus.

Neutrons have a neutral electrical charge, so they may be readily absorbed by the nuclei of other atoms, creating new radioactive isotopes. Fission fragments and neutron-activated material are responsible for the intense radioactivity on the inside surfaces of nuclear reactors.

(Material for this page is adapted from What Is Radioactive Material and How Does It Decay? (RER-20) and What Is Ionizing Radiation? (RER-21), Ohio State University Extension.)

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November 27, 2002