Lesson 9: Not All That Glows is Radioactive

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Prerequisite Knowledge

  1. Radioactive decay is when an unstable atom releases a particle to reach a lower-energy, more stable state.
  2. Know what an alpha particle is and its properties like size, charge, and penetrating distance.

Description

This lab focuses on myth-busting and addressing false perceptions surrounding the nuclear field--namely, what things in nuclear science glow and why? It will first introduce what light is and what it means for something to glow. It then addresses some examples of non-radioactive and radioactive things glowing. It allows students to paint something with glow-in-the-dark paint and to "see" alpha decay with a special device called a spinthariscope.

Learning Objectives

  1. "Glowing" is when something emits light, and many different mechanisms can cause something to glow.
  2. Light is a spectrum that appears to us as different colors and different kinds of light carry different amounts of energy. Light, like all energy, cannot be created from nothing. If light is emitted, some energy must have changed forms.
  3. Some, but not all, radioactive materials appear to glow.
  4. Some, but not all, things that glow are radioactive. 
  5. Just because something glows does not mean that thing is dangerous or radioactive. Some glows can be explained by chemistry, physics, or biology.
  6. We can observe nuclear decay directly in the form of flashes of light in a special device called a spintharoscope.

Materials

Material Preparation

  • The glow-in-the-dark paint will glow most evenly if you stir (not shake) it thoroughly before using it,
  • The glow-in-the-dark paint will be brightest when "charged" under direct light, like a bright window or a powerful lamp.
  • The spinthariscope contains an alpha source. Alpha particles are not penetrating and will not escape the device unless broken and smashed open. If that were to happen, the most important thing is to avoid consuming any of the pieces. Wearing gloves and putting all the pieces into a plastic bag before disposal will help protect you because the alphas cannot penetrate the gloves or even the outer layer of your skin.
  • As will be discussed in the lab, the spinthariscope glows very faintly, and you will need to adapt your eyes to low light conditions to see its glimmer. Fifteen minutes in a very dark room should be sufficient. For some with specific visual impairments, the spinthariscope may not be visible regardless of how long they adjust. The video supplement should help show the spinthariscope for those cases.

Background Information

What do you think of when you hear "nuclear" and "glow"? It's something like the images below: a dangerous glowing green ooze. Why do we think this, and is there any truth to these images? This background section will explain why things glow and give examples in and out of nuclear science.

[green glowing ooze photo]

Anytime something glows, light must be released. For our purposes, "glows" is synonymous with "emits visible light." Light is a spectrum that includes many colors and different energies. Think about where you and your students see light in your everyday life; what kind of light is used for these applications according to the spectrum below?

[Insert image of the light spectrum]

As wavelengths decrease and frequency increases, the energy of light increases. Because light is energy and energy cannot be created or destroyed (it only changes form), light always has a source or some underlying mechanism creating it. While the light itself may look the same to us, the cause of the light can vary dramatically. Let's look at some examples.

Light can come from thermal energy, which we see in incandescent lightbulbs or red-hot stove burners. Humans are constantly releasing heat as light in the infrared spectrum. You're glowing right now, but can't see it! As the temperature of something increases, its atoms have more energy, releasing higher energy light until it reaches a spectrum we can see.

Chemical reactions can also release energy in the form of light. As chemical bonds break, energy is released. Often, this energy is released in the form of heat, but it can also be in the form of visible light. When a chemical reaction produces light, this is called chemiluminescence. Glowsticks and fireflies are prime examples.

Finally, some materials can absorb and release light based on structural properties. The most common example is items that glow under a blacklight. Blacklight is in the ultraviolet spectrum, so to our eyes, they are not very bright or perceptible because it's on the edge of our visible range; however, many materials absorb this UV light and release it at slightly longer frequencies (which we can see) making them seem to glow different colors. This is called fluorescence; the glow will stop immediately or soon after removing the light source. Rocks, jellyfish, tonic water, some laundry detergents, and many kinds of dyes all have this property. In some materials, the absorbed photons (or light energy) can be stored in the defects and structure of the material. This allows these materials to release that energy over a longer period. This is called phosphorescence and is how "glow-in-the-dark" plastics and other materials work.

[electron transition image] 

Just like in our non-radioactive cases, when something glows in nuclear science, there must be a source of the energy causing the glow. In nuclear applications, this energy source is usually nuclear reactions rather than chemical reactions or material properties like those discussed above.

Below is an image of glowing Plutonium. Like the earlier incandescent light bulb, it shines because it is red-hot. The source of the heat, in this case, is radioactive decay. As atoms within the material decay, the radiation from those decays bounces into other atoms in the material which heats them.

[Plutonium picture]

Below is an image of a nuclear reactor in its cooling pool. One of the potential sources of the stereotypes for glowing nuclear material is that active reactors submerged in water do glow. This blue glow is called Cherenkov radiation, and it is caused by a charged particle, most commonly an electron, traveling through a dielectric medium (meaning it can be polarized electrically) with a speed greater than light's speed in that medium. The physics behind this phenomenon is a little complicated (see the additional resources if you'd like more), but it is like a sonic boom but with light instead of sound.

[cherenkov radiation picture]

The final kind of glow caused by radioactivity is radioluminescence. Just like heat or chemical reactions can excite electrons, which then fall back down to their original energy, releasing light; radioactive decay and incident radiation particles can also excite electrons and lead to light release. In the spinthariscope we will look at in the lab, zinc sulfide is used to absorb energy from alpha particles. These alpha particles excite the electrons in the zinc sulfide, which then release a photon in the visible spectrum. What we see when we look into the spinthariscope is this visible light while no alpha particles pass through the screen and lens and air between the source and the viewer's eye. Each spark represents an individual atomic decay in real time!

[Spinthariscope image and video]

Teachers can interleave covering this content by doing the lab itself, and you are welcome to cover as much or as little as you think your students will be interested in. The paint and UV light activity are great ways to introduce fluorescence and phosphorescence as a break in between the non-radioactive and radioactive sections. It is less important for students to know all the terminology than to understand that sometimes materials directly emit energy back (fluorescence under UV), and others store energy and release it over time (phosphorescence like the paint). You can then introduce different nuclear glowing concepts as you transition to the spinthariscope. 

Laboratory Instructions

[teacher vid here]

  1. In this lab, you will explore why things glow. Maybe when you think of the term "nuclear" or "radioactive," you think of a glowing green ooze. We hope this curriculum and laboratory, in particular, can help you better understand what things glow and why. 
  2. We encounter different frequencies of light every day; what are some examples you can think of? 
  3. Paint something with glow-in-the-dark paint! Where should you put your object to "charge" it the most or get it to glow brightest based on what you've learned?
  4. Adjust to the dark and look into the spinthariscope. Where does each glimmer of light come from? What is actually hitting your eye- is it the radioactive decay product or something else?

Answers for teachers

2. X-rays are needed to see our teeth and other bones. Infrared light for "heat vision" goggles and thermal imaging. Radio and TV signals are carried with light. Space imaging and satellite pictures use different light frequencies to see things our eyes can't! Of course, we see a whole spectrum of light in our daily lives, too, and they appear as different colors.

4. Each glimmer of light represents an alpha decay that just occurred. The thing our eye actually sees is not the alpha particles because there is a screen between the source and us, so the alphas don't actually reach the viewer. This screen has a material on it that releases light when hit with a big charged particle (like an alpha).

Supplemental Material

Despite our ability to see and interact with light every day, it is a complicated subject. As the lab demonstrates, there are lots of different sources of light. We don't need students to memorize these; we want them to understand that glowing things are not inherently dangerous. We also hope they will get a spark of curiosity to understand other glowing things in the world.