How Nuclear Radiation Works

­L­e­t's start at the beginning and understand where the word "nuclear" in "nuclear radiation" comes from. Here is something you should already feel comfortable with: Everything is made of atoms. Atoms bind together into molecules. So a water molecule is made from two hydrogen atoms and one oxygen atom bound together into a single unit. Because we learn about atoms and molecules in elementary school, we understand and feel comfortable with them. In nature, any atom you find will be one of 92 types of atoms, also known as elements. So every substance on Earth -- metal, plastics, hair, clothing, leaves, glass -- is made up of combinations of the 92 atoms that are found in nature. The Periodic Table of Elements you see in chemistry class is a list of the elements found in nature plus a number of man-made elements.

Inside every atom are three subatomic particles: protons, neutrons and electrons. Protons and neutrons bind together to form the nucleus of the atom, while the electrons surround and orbit the nucleus. Protons and electrons have opposite charges and therefore attract one another (electrons are negative and protons are positive, and opposite charges attract), and in most cases the number of electrons and protons are the same for an atom (making the atom neutral in charge). The neutrons are neutral. Their purpose in the nucleus is to bind protons together. Because the protons all have the same charge and would naturally repel one another, the neutrons act as "glue" to hold the protons tightly together in the nucleus.

The number of protons in the nucleus determines the behavior of an atom. For example, if you combine 13 protons with 14 neutrons to create a nucleus and then spin 13 electrons around that nucleus, what you have is an aluminum atom. If you group millions of aluminum atoms together you get a substance that is aluminum -- you can form aluminum cans, aluminum foil and aluminum siding out of it. All aluminum that you find in nature is called aluminum-27. The "27" is the atomic mass number -- the sum of the number of neutrons and protons in the nucleus. If you take an atom of aluminum and put it in a bottle and come back in several million years, it will still be an atom of aluminum. Aluminum-27 is therefore called a stable atom. Up to about 100 years ago, it was thought that all atoms were stable like this.

Many atoms come in different forms. For example, copper has two stable forms: copper-63 (making up about 70 percent of all natural copper) and copper-65 (making up about 30 percent). The two forms are called isotopes. Atoms of both isotopes of copper have 29 protons, but a copper-63 atom has 34 neutrons while a copper-65 atom has 36 neutrons. Both isotopes act and look the same, and both are stable.

The part that was not understood until about 100 years ago is that certain elements have isotopes that are radioactive. In some elements, all of the isotopes are radioactive. Hydrogen is a good example of an element with multiple isotopes, one of which is radioactive. Normal hydrogen, or hydrogen-1, has one proton and no neutrons (because there is only one proton in the nucleus, there is no need for the binding effects of neutrons). There is another isotope, hydrogen-2 (also known as deuterium), that has one proton and one neutron. Deuterium is very rare in nature (making up about 0.015 percent of all hydrogen), and although it acts like hydrogen-1 (for example, you can make water out of it) it turns out it is different enough from hydrogen-1 in that it is toxic in high concentrations. The deuterium isotope of hydrogen is stable. A third isotope, hydrogen-3 (also known as tritium), has one proton and two neutrons. It turns out this isotope is unstable. That is, if you have a container full of tritium and come back in a million years, you will find that it has all turned into helium-3 (two protons, one neutron), which is stable. The process by which it turns into helium is called radioactive decay.

Certain elements are naturally radioactive in all of their isotopes. Uranium is the best example of such an element and is the heaviest naturally occurring radioactive element. There are eight other naturally radioactive elements: polonium, astatine, radon, francium, radium, actinium, thorium and protactinium. All other man-made elements heavier than uranium are radioactive as well.

­Radioactive decay is a natural process. An atom of a radioactive isotope will spo­ntaneously decay into another element through one of three common processes:

In the process, four different kinds of radioactive rays are produced:

Americium-241, a radioactive element best known for its use in smoke detectors, is a good example of an element that undergoes alpha decay. An americium-241 atom will spontaneously throw off an alpha particle. An alpha particle is made up of two protons and two neutrons bound together, which is the equivalent of a helium-4 nucleus. In the process of emitting the alpha particle, the americium-241 atom becomes a neptunium-237 atom. The alpha particle leaves the scene at a high velocity -- perhaps 10,000 miles per second (16,000 km/sec).

If you were looking at an individual americium-241 atom, it would be impossible to predict when it would throw off an alpha particle. However, if you have a large collection of americium atoms, then the rate of decay becomes quite predictable. For americium-241, it is known that half of the atoms decay in 458 years. Therefore, 458 years is the half-life of americium-241. Every radioactive element has a different half-life, ranging from fractions of a second to millions of years, depending on the specific isotope. For example, americium-243 has a half-life of 7,370 years.

Tritium (hydrogen-3) is a good example of an element that undergoes beta decay. In beta decay, a neutron in the nucleus spontaneously turns into a proton, an electron, and a third particle called an antineutrino. The nucleus ejects the electron and antineutrino, while the proton remains in the nucleus. The ejected electron is referred to as a beta particle. The nucleus loses one neutron and gains one proton. Therefore, a hydrogen-3 atom undergoing beta decay becomes a helium-3 atom.

In spontaneous fission, an atom actually splits instead of throwing off an alpha or beta particle. The word "fission" means "splitting." A heavy atom like fermium-256 undergoes spontaneous fission about 97 percent of the time when it decays, and in the process, it becomes two atoms. For example, one fermium-256 atom may become a xenon-140 and a palladium-112 atom, and in the process it will eject four neutrons (known as "prompt neutrons" because they are ejected at the moment of fission). These neutrons can be absorbed by other atoms and cause nuclear reactions, such as decay or fission, or they can collide with other atoms, like billiard balls, and cause gamma rays to be emitted.

Neutron radiation can be used to make nonradioactive atoms become radioactive; this has practical applications in nuclear medicine. Neutron radiation is also made from nuclear reactors in power plants and nuclear-powered ships and in particle accelerators, devices used to study subatomic physics.

In many cases, a nucleus that has undergone alpha decay, beta decay or spontaneous fission will be highly energetic and therefore unstable. It will eliminate its extra energy as an electromagnetic pulse known as a gamma ray. Gamma rays are like X-rays in that they penetrate matter, but they are more energetic than X-rays. Gamma rays are made of energy, not moving particles like alpha and beta particles.

While on the subject of various rays, there are also cosmic rays bombarding the Earth at all times. Cosmic rays originate from the sun and also from things like exploding stars. The majority of cosmic rays (perhaps 85 percent) are protons traveling near the speed of light, while perhaps 12 percent are alpha particles traveling very quickly. It is the speed of the particles, by the way, that gives them their ability to penetrate matter. When they hit the atmosphere, they collide with atoms in the atmosphere in various ways to form secondary cosmic rays that have less energy. These secondary cosmic rays then collide with other things on Earth, including humans. We get hit with secondary cosmic rays all of the time, but we are not injured because these secondary rays have lower energy than primary cosmic rays. Primary cosmic rays are a danger to astronauts in outer space.

­Alth­ough they are "natural" in the sense that radioactive atoms naturally decay and radioactive elements are a part of nature, all radioactive emissions are dangerous to living things. Alpha particles, beta particles, neutrons, gamma rays and cosmic rays are all known as ionizing radiation, meaning that when these rays interact with an atom they can knock off an orbital electron. The loss of an electron can cause problems, including everything from cell death to genetic mutations (leading to cancer), in any living thing.

Because alpha particles are large, they cannot penetrate very far into matter. They cannot penetrate a sheet of paper, for example, so when they are outside the body they have no effect on people. If you eat or inhale atoms that emit alpha particles, however, the alpha particles can cause quite a bit of damage inside your body.

Beta particles penetrate a bit more deeply, but again are only dangerous if eaten or inhaled; beta particles can be stopped by a sheet of aluminum foil or Plexiglas. Gamma rays, like X-rays, are stopped by lead.

Neutrons, because they lack charge, penetrate very deeply, and are best stopped by extremely thick layers of concrete or liquids like water or fuel oil. Gamma rays and neutrons, because they are so penetrating, can have severe effects on the cells of humans and other animals. You may have heard at some point of a nuclear device called a neutron bomb. The whole idea of this bomb is to optimize the production of neutrons and gamma rays so that the bomb has its maximum effect on living things.

As we have seen, radioactivity is "natural," and we all contain things like radioactive carbon-14. There are also a number of man-made nuclear elements in the environment that are harmful. Nuclear radiation has powerful benefits, such as nuclear power to generate electricity and nuclear medicine to detect and treat disease, as well as significant dangers.