Lesson Video: Gamma Radiation Physics • 9th Grade

In this video, we will learn how to describe the process of unstable atoms decaying to a lower energy state by emitting high-energy gamma rays.

10:38

Video Transcript

In this video, we’re talking about gamma radiation. As this image suggests, gamma radiation is a form of nuclear decay. When an atomic nucleus is excited and needs to decay to a stabler, lower-energy state, one way it can do this is by giving off gamma radiation. Now, as a bit of background, we know that when an atomic nucleus becomes unstable, it’s a strong candidate for experiencing radioactive decay. When this decay takes place, the nucleus emits something. It gives something off. This something could be an alpha particle, two protons and two neutrons. Or, it could be a beta particle, an electron. It could be a single neutron. And it could also be what’s called a gamma ray or gamma radiation.

Each one of these different types of atomic emission has the same goal: to stabilize an unstable nucleus. And yet, each radiation type has its own unique characteristics. When it comes to gamma radiation, what makes this different is that there’s no mass involved. Gamma radiation is purely energy. In other words, in a gamma ray, there are no protons, neutrons, or electrons. Instead, a gamma ray, which is often symbolized using a squiggly line like this, is what’s called a photon, a packet of electromagnetic radiation. This symbol that we’ve drawn here that represents gamma radiation is simply the Greek letter 𝛾. Because gamma radiation has no mass and is purely energy, that means it has no protons or electrons, which also means it has no electric charge.

So, let’s say that we were looking at a nuclear equation involving gamma decay. Imagine we had a barium-137 isotope, and that this nucleus experienced radioactive decay through gamma radiation. As we saw, the symbol for gamma radiation is the Greek letter 𝛾. And when we write this in a nuclear equation, it has no atomic number or mass number. The reason for that, as we saw earlier, is that gamma radiation has no mass and no electric charge. This means we could represent a gamma ray with an atomic number and mass number of zero, if we wanted to. Now, we know that, in general, when it comes to nuclear equations, we need to have the same total atomic number and same total mass number on either side.

In this case, we have a mass number of 137 and an atomic number of 56 on the left, which means we must have those same numbers on the right. And that gives us a clue as to what else needs to go on the right side of this equation. It’s another barium-137 nucleus. This equation may seem confusing because we’re starting with a barium-137 nucleus, and then we’re ending with a barium-137 nucleus plus a gamma ray. It seems like we’re getting something for nothing. What’s really taking place, though, is we’re taking an excited barium-137 nucleus, and then it’s decaying into lower-energy, stabler barium-137 plus this high-energy gamma ray.

We could say that we’re stripping energy away from this nucleus, and we’re sending it away, so to speak, as gamma radiation. And then, what’s left over is a stabler version of that same isotope. So, as we can see from this equation, gamma radiation does not lead to a change in atomic number. And it also doesn’t lead to a change in mass number. Those two values remain the same. And we can see that the relative charge of the nucleus giving off the gamma ray also remains the same. This just emphasizes the point that gamma radiation is nothing but energy. Indeed, it’s the most energetic type of electromagnetic radiation.

Let’s consider a second example of a nuclear equation involving gamma radiation. This time, we’ll start with cobalt-60. When this isotope goes through gamma decay, it gives off a gamma ray, a photon. And what’s left over is a stabler, lower-energy version of the same isotope. Once again, we see that gamma radiation does not change atomic number or mass number. And we also see, in this example, that those two values, atomic number and mass number, are still balanced on the left- and right-hand sides of this equation. Now that we know a bit about gamma radiation and how to include it in nuclear equations, let’s get some practice with these ideas through an example exercise.

When an atomic nucleus emits a gamma ray, by how much does the atomic number of the nucleus change?

Okay, so in this example, we have an atomic nucleus. Let’s say that this is it. And we’re told this nucleus emits a gamma ray. And we can symbolize that this way. When a nucleus emits a gamma ray, we say that it’s giving off gamma radiation. Now, an important thing to realize about gamma radiation is that it’s purely energy. There’s no mass involved. Another way to describe a gamma ray is as a packet of electromagnetic radiation called a photon. Now, if a gamma ray has no mass, that also means it has no protons or electrons in it because those objects have a mass. So, our massless gamma ray also has no electric charge. And this fact helps us answer our question of how much the atomic number of this nucleus that gives off the gamma ray changes.

The atomic number — we can call it 𝑧 — of an element is equal to the number of protons in the nucleus. Since the relative charge of a single proton is plus one, the number of protons in a nucleus and its overall relative charge are the same. Just as a side note, when it comes to other subatomic masses such as beta particles, which are electrons, this similarity between proton number and relative charge means that, often, an emitted electron is symbolized as having an atomic number of negative one. This doesn’t mean that a beta particle has negative-one protons, but rather that its relative charge is negative one.

All this to say that when we’re talking about atomic nuclei, atomic number corresponds to the relative charge of the nucleus. And as we saw earlier, the relative charge of a gamma ray is zero because there’s no charge or mass involved in this radiation. That means it has no effect on the atomic number of the nucleus from which it was emitted. So, when we talk about how much the atomic number of that nucleus changes, the answer is simply that it doesn’t change at all. The change is zero. Another way to say this is that the atomic number of a nucleus that emits a gamma ray stays the same.

Let’s look now at a second example exercise.

The following nuclear equation shows gamma radiation by radon. What is the name of element X? What element symbol should replace X?

All right, taking a look at this nuclear equation, we see that we’re starting out with this element, which we’re told is radon. The atomic number of radon is 86. It has 86 protons in its nuclear core. And its mass number, the sum of the number of protons and the number of neutrons in its nucleus, is 222. So then, we can refer to this isotope of radon as radon-222. This radon nucleus experiences nuclear decay, and it emits a gamma ray. This is the Greek symbol for that letter 𝛾 representing this radiation. Once the gamma ray is emitted from the radon nucleus, there’s this leftover element, element X.

Starting off, we want to solve for the name of this element. And then, we want to know what element symbol should replace X. These two questions are closely connected, and there are a couple of different ways to answer them. One way is to consider that what’s being emitted here is gamma radiation. And we can recall that a gamma ray, a photon, a packet of electromagnetic energy, has no mass to it. And it also has no electric charge. This means that when we consider gamma radiation as a part of an overall nuclear equation, whatever is left over after the gamma ray is emitted — in our case, it’s this element X — will have the same mass and the same electric charge as what emitted the gamma ray in the first place. In other words, this element here and this element here are the same.

Now, there’s a second way to see this. And that is by looking at the atomic number and the mass number of this unknown element. When we compare these values to the equivalent values on the left-hand side of our nuclear equation, we see that they’re the same. Both our radon nucleus and our nucleus of element X have an atomic number of 86 and a mass number of 222. This is a second way of seeing that these two elements must be the same. And therefore, the name of element X is the name of this element. Element X is radon. The next question is related. It says, what element symbol should replace X? Well, if X is radon and the element’s symbol for radon is capital Rn, then that tells us that that same symbol should go in place of capital X.

Let’s look now at one last example.

What type of particle is a gamma ray the same as?

Okay, so thinking about gamma rays, we know that these are emitted in radioactive emission events. The symbol for a gamma ray is the Greek letter 𝛾. And sometimes, we’ll see these rays represented by a squiggly line that looks something like this. And this representation can remind us that a gamma ray is electromagnetic radiation. That is, in some respects, it behaves like a wave. But in this question, we’re asked specifically what type of particle a gamma ray is the same as. We can start off by realizing what type of particles a gamma ray is not. A gamma ray, as electromagnetic radiation, has no mass, and it also has no electric charge. That means a gamma ray can’t be any particle that has mass or charge. It can’t be a proton. It can’t be a neutron. It can’t be an electron and so on.

But then, what particle is there that’s both massless and chargeless? We can think back to the identity of a gamma ray as electromagnetic radiation. The name for a packet or a single section of electromagnetic radiation is photon. A photon is considered to be a particle, and yet it satisfies the condition of having no mass and no charge. Therefore, it’s a match for a gamma ray. And therefore, this is our answer. A photon is the particle that a gamma ray is the same as.

Let’s now summarize what we’ve learned about gamma radiation. In this lesson, we saw that when radioactive nuclei decay, that is, seek out a lower-energy, more stable state, then they may emit gamma radiation. Gamma radiation is comprised of gamma rays, which are massless, chargeless particles of electromagnetic energy called photons. Finally, we saw that when a nucleus emits a gamma ray, its atomic number, mass number, and relative charge do not change. In other words, it’s the same nucleus as before, just in a lower-energy state. This is a summary of gamma radiation.

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