For many of the sources you encounter in everyday life, the amount of radiation is so low that you don’t need to worry about it. But ionizing radiation can also be dangerous, because these free electrons interact with the molecules in the cells and tissues of the human body. Adding an extra electron can break the chemical bonds that hold molecules together. That is why the radioactive substances associated with nuclear weapons and power plant meltdowns can raise the risk of cancer.
There are four types of ionizing radiation: alpha, beta, gamma, and neutron radiation. Here’s what’s going on with each type and how they can be detected.
In 1896, no one really knew anything about radiation. They didn’t know if it was a particle or some type of electromagnetic wave, like light. So they decided to use the term “rays” in the generic sense—like light rays. That’s how we get holdover terms like alpha rays or gamma rays.
But—SPOILER ALERT—alpha rays are not waves. They are actually electrically charged particles. An alpha particle is made of two protons and two neutrons. This means that an alpha particle is a helium atom without the electrons. (Yes, they should have called them “helium particles,” but no one knew what was going on.)
How can you tell that it’s alpha radiation, and not some other type? The answer is that alpha particles can easily be blocked by something as thin as a sheet of paper. So if you have a source that produces alpha particles, you can shield the detector—like photographic film—with a very small amount of material.
The reason that alpha particles are so easily blocked is that, because they are so heavy, they are often ejected from the radioactive source with a relatively slow speed. Also, with an electrical charge equal to two protons, there is a significant electrostatic force between the alpha particle and the positive nucleus of the shielding paper. (We call this a charge of 2e, where e is the fundamental charge of an electron or proton.) It doesn’t take too many of these atoms in the paper to essentially bring the alpha particle to a stop.
Do you know what else can stop an alpha particle? Human skin. That’s why alpha radiation is often considered to be the least harmful of the radiation types.
In 1899, Ernest Rutherford classified three types of radiation: alpha, beta, and gamma. While the alpha particles were easily stopped, beta and gamma particles could go through some amount of metal shielding, penetrating further into material because they are much lower mass. In fact, beta particles are electrons—the fundamental particles with a negative charge. The mass of an alpha particle is more than 7,000 times larger than that of a beta particle. This means that very low-mass beta particles can be emitted with very high speeds that give them the ability to penetrate objects, including the human body.
Gamma rays are actually rays, not particles. They are the third class of radiation, and a type of electromagnetic wave—just like visible light.
However, the light that you can see with your eyes has a wavelength between 400 and 700 nanometers, while gamma rays have a much smaller wavelength. A typical gamma ray might have a wavelength of 100 picometers. (Note: 1 picometer = 10-12 meter, and 1 nanometer = 10-9 meter.) This means that the wavelength of gamma radiation can be around 1,000 times smaller than visible light. With such a small wavelength, and a very high frequency, gamma rays can interact with matter at very high energy levels. They can also penetrate quite deep into most materials, so it usually takes a large chunk of lead to block this radiation.