Most people interpret radiation as a bad thing—but it isn’t always. In fact, radiation is a very normal phenomenon. For now, let’s just say that radiation is when an object produces energy. When a material is radioactive, it emits energy either as particles or electromagnetic waves. The particles are usually things like electrons or atoms. The waves could be in any region of the electromagnetic spectrum. Since your Wi-Fi produces electromagnetic waves, technically your home access point is a source of radiation. So is that light bulb in the ceiling. Actually, even you are a source of radiation in the infrared spectrum, due to your temperature.
However, most people don’t think of radiation that way. What’s commonly called “radiation” is actually a special type: ionizing radiation. When an object produces ionizing radiation, it emits enough energy that when it interacts with other materials there’s a chance it could free an electron from its atom. This electron is then free to interact with other atoms, or maybe just wander off into empty space. But no matter what the electron does, once it gets away from its original atom, we call that ionization.
Ionizing radiation was discovered by accident. Before digital smartphones, when people took pictures on film, the basic idea of photography was that when film was exposed to light, it would cause a chemical reaction that would reveal a picture when the film was developed. Then in 1896, French physicist Henri Becquerel discovered radioactivity when he realized that uranium salts produced an effect on otherwise unexposed photographic film that was still in its wrapper. Somehow the uranium produced an effect similar to light, but unlike the light, it could pass through the paper wrapping.
It turns out that uranium is naturally radioactive, and this was a type of ionizing radiation. Uranium produces electromagnetic waves in the gamma spectrum. Gamma radiation is similar to visible light when it interacts with film (thus exposing it), but it’s different from visible light in that it can pass through paper.
You might not directly use uranium in your everyday life, but you will indeed encounter ionizing radiation—at safe levels—in many different applications. For example, smoke detectors use a radioactive source to detect smoke in the air. A radioactive source produces charged particles (alpha particles, in most cases) that ionize the air inside the detector, which in turn creates an electric current in the air. If tiny particles of smoke get inside the detector, it blocks this electrical current. Then the detector sends a signal to make an ear-piercing noise so that you know there’s a fire—or maybe that you burnt your dinner on the stove.
Eighteen percent of the electrical power in the US comes from nuclear power plants, and they obviously produce ionizing radiation. Medical x-ray images can produce ionizing radiation. Some ceramic dishes are coated in a uranium-based paint—yup, that produces radiation. Technically, bananas are radioactive, due to their comparatively large concentration of potassium. Ionizing radiation could even be from outer space—we call these cosmic rays.
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.
Alpha Particles
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.
Beta Particles
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
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.
(No, gamma radiation won’t turn you into the Hulk. That’s just for comic books and movies.)
Neutron Radiation
There’s a fourth type of radiation, but it’s quite different than the other three. Alpha, beta, and gamma are all types of ionizing radiation, in that they can kick an electron out of an atom. However, with neutron radiation a neutron is ejected from a radioactive nucleus.
Since neutrons have a zero net charge and are similar to protons, they don’t actually interact with electrons. Instead, when a neutron collides with an atom it can either split it into two new atoms (and a whole bunch of energy) or be absorbed into the nucleus. This will create an isotope, an atom with a different number of neutrons, which might not be stable. When the nucleus is unstable, it’s going to have radioactive decay and produce beta and gamma rays. It’s those secondary interactions that produce ionizing radiation.
Because neutrons don’t have an electrical charge, they can easily pass through a lot of material. That makes shielding rather difficult. The key to protecting things (and people) from neutron radiation is to somehow slow down the particles. It turns out you can do this with hydrogen. When a neutron interacts with molecules that contain hydrogen, like water or hydrocarbons, the collisions slow the neutron down a little bit. The more collisions, the slower the neutron becomes. Eventually, it will be going so slow as to not cause a problem.
Radiation Detection
There are several methods that we can use to detect all of these types of radiation. The one that most people are familiar with—mostly from movies—is the Geiger counter, which is also known as the Geiger-Muller counter.
Photograph: Rhett Allain
The important part of this device is the tube on top of the box. Inside this tube is a gas, such as helium or argon, with a wire running along the axis of the tube. A large electric potential difference is applied to the outer surface of the tube and the center wire. It looks something like this:
Illustration: Rhett Allain
When alpha, beta, or gamma rays pass through the gas in the tube, it can ionize an atom and create a free electron. This electron is then attracted to the positive voltage of the central wire. As the electron moves toward the wire, it increases in speed and collides with other gas molecules which results in even more free electrons. These new electrons also accelerate towards the wire and they also produce electrons. We call this an “electron avalanche,” because one electron can make a whole bunch more.
Once these electrons reach the wire, they produce an electric current that is amplified and sent to an audio input. This amplified electron avalanche makes that classic “click” sound you hear with a Geiger counter.
There’s another way you can detect radiation: a scintillator. This is a special manufactured crystal or plastic-like material. When any of the four types of radiation passes through the scintillator, it will produce a tiny amount of visible light. Then you just need a device to detect these tiny amounts of light. The most common tool for this is a photomultiplier tube. Of course, since you are using the scintillator to detect light, you need to shield the material from external light sources by covering it with something like electrical tape.
Surprisingly, you might have a radiation detector right in your pocket. It’s possible to use a smartphone to detect gamma rays (and x-rays). Here’s how it works: The camera in your phone has an image sensor. Normally, this produces a complicated electrical signal when visible light hits different parts of the sensor. This data is then turned into a digital picture of your favorite cat or dog, or whatever image you wish to capture. But this image sensor is also activated by both gamma and x-rays. So, you just need some special software and something to block the visible light from the camera, like black tape. Boom, radiation detector!
Of course, since your image sensor is quite tiny so that it can fit in your pocket, that means it’s not very efficient. But it is indeed a radiation detector. It’s just like that Geiger counter in a watch that James Bond used in the movie Thunderball—except this one is real.
