Lesson Video: Neutron Emission Physics

In this lesson, we will learn how to describe the process of unstable atoms decaying to a lower energy state by emitting neutrons.


Video Transcript

In this video, we’re talking about neutron emission. This is one way that the nucleus of an atom can undergo radioactive decay. Whenever an atomic nucleus gives off an uncharged particle, a neutron, that’s neutron emission. And we can get a better sense of how this works by considering a specific example.

Say that we have an atom of iodine represented with this symbol, capital I. Iodine has an atomic number of 53, and we’ll say the mass number of this atom is 137. We can recall that the mass number of an atom is equal to the number of protons it has in its nucleus plus the number of neutrons it has. While the atomic number simply reports the number of protons. This means if you want to solve for the number of neutrons in an atomic nucleus, we just need to subtract the atomic number from the mass number. 137 minus 53 is equal to 84. So that means that this particular atom of iodine has 84 neutrons in its nucleus.

We can see then that there are significantly more neutrons in the nucleus than there are protons. We could describe this atomic nucleus as neutron rich and proton poor. Atomic nuclei like these are strong candidates for neutron emission. Let’s say that our iodine atom goes through this type of nuclear decay. It gives off a neutron. The resulting atom will still be iodine. That is, it will have 53 protons in its nucleus. But since it’s lost one neutron, its mass number will no longer be 137 but 136. The neutron emitted in this process is symbolized this way, using a lower case n. Lower case because a capital N represents the element nitrogen. And we can see that this neutron has an atomic number of zero because it has no protons or electric charge to it. But it has a mass number of one because there’s one neutron in it.

So this, then, is a nuclear equation that shows us neutron emission. And notice that both atomic number and mass number are the same on either side of this reaction. Regarding atomic number, on the left-hand side, we have 53. And on the right-hand side, we have 53 plus zero. And then, with respect to mass number, we have 137 on the left and 136 plus one, or 137, on the right. So then, here’s what we can say has happened to this atomic nucleus once it undergoes neutron emission.

Neutron emission does not change the atomic number involved. That stays the same. It also doesn’t affect the charge of the nucleus involved because a neutron has no electric charge. However, neutron emission does change the mass number of an atom. It decreases it by one for every neutron emitted.

Now, one way to represent neutron emission or other types of radioactive decay is through a graph called a nuclear decay graph. These graphs are typically set up so that the number of neutrons in a nucleus is plotted against the number of protons it has. Now, let’s say we had some particular atomic nucleus with a specific number of protons and neutrons. And let’s say that it fit right here on our graph. That is, it has this many protons in its nucleus, however many that is. And it has this many neutrons, however many that is. As we’ll see, the numbers themselves aren’t as important as how those numbers change. And indeed, if this atomic nucleus undergoes some sort of radioactive decay, then those numbers will change.

We know there are a number of different radioactive decay possibilities. A nucleus could emit an alpha particle or could emit a beta plus or a minus particle or, and we’ll assume this is the case for us, it could emit a neutron. If that happens, if this nucleus gives off one neutron, then that means the number of neutrons it has will go down by one. However, because it’s only neutron emission and nothing else is given off, the number of protons in the nucleus will stay the same. This means that if we want to represent this transition on our graph, we do it by drawing an arrow from where our nucleus originally was to where it transitions to following this decay event.

As we consider the starting and ending location of our nucleus, we can see that what we’re indicating is a loss of one neutron while maintaining the number of protons in this nucleus. The decay event that corresponds to this is neutron emission. Now, notice that the atomic nucleus we have here is not the same as the one we have here. It’s true the element hasn’t changed because the number of protons in the nucleus is the same. But because the number of neutrons is different, what we have are atomic isotopes of one another. Now, let’s say that our resulting isotope, this nucleus right here, also underwent neutron emission. That would mean that, once again, a neutron is lost from the nucleus, while the number of protons in it remains the same.

We would indicate that transition with a line like this. And our new nucleus would be here on our decay graph. So then, on a nuclear decay graph, which plots the number of neutrons in the nucleus versus the number of protons. Anytime we see vertically downward transitions like we’ve seen here, that indicates the process of neutron emission. That’s the kind of decay that must be going on.

Now, recall that we mentioned that when a nucleus changes, when it undergoes radioactive decay, there are different kinds of particles it can give off. One of those is called an alpha particle, also known as a helium nucleus because an alpha particle has two protons just like the nucleus of a helium atom. Because an alpha particle has two protons and no electrons, that means it has a net or a relative charge of positive two. This strong relative charge makes an emitted alpha particle very likely to interact with other matter. This means that alpha particles are unlikely to travel very far at all into a solid material because they quickly interact with that material.

We say then that the penetration depth of an alpha particle is very low. It’s easily blocked even by fairly thin layers of material, such as a sheet of paper. So alpha particles don’t penetrate into material very far. But what about something else that might be emitted in a decay process? For example, what about a beta particle, in particular, a beta minus particle which is an electron? Knowing that an electron is much smaller than an alpha particle in size, we would expect it to have a better penetration depth. One thing going against it, though, is that the beta particle, like the alpha particle, has a net charge. This means it, too, is likely to interact with matter rather than passing right through.

Thanks to its relatively smaller net charge and relatively smaller size, the beta particle does have a greater penetration depth than the average alpha particle. But it turns out that neither of these compares with the penetration depth achieved by emitted neutrons. Neutrons are able to travel so far into matter without interacting with it, largely because they have no electric charge. This means they’re neither attracted to nor repelled by any other electric charges. And recalling that atoms are mostly empty space, a neutron is typically able to pass right on through. But when a neutron doesn’t pass right through but instead interacts with the nucleus of an atom, something interesting can happen.

When a neutron runs into an atomic nucleus, and we could say is captured by it, then as a result of that capture, the nucleus becomes heavier. And it can enter an excited state. A nucleus in an excited state is an unstable nucleus. That is, one prone to radioactive decay. So here’s what this means. By emitting a neutron, which was then captured by this nucleus, we’ve created radioactive material. This process, where a neutron is captured by a nucleus which then becomes heavier and enters an excited state, is called neutron activation. And the result, as we saw, is radioactive material.

This is one reason why emitted neutrons can be considered dangerous. They’re capable of creating radioactive material where there was none before. Along with this, emitted neutrons are capable of creating ions in the atoms they collide with. Not directly because a neutron has no electric charge but indirectly through, say, colliding with an electron and bumping it off an atom. So despite their neutral electric charge, neutrons can actually have quite an impact on atomic nuclei and overall atomic charge. This is one reason why in radioactive decay scenarios that involve emitted neutrons, shielding is carefully installed. Now that we understand a bit about neutron emission and what causes it, let’s get a bit of practice with these ideas through an example.

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

Okay, so in this question, we have an unstable nucleus. Let’s say that this is our unstable nucleus, jiggling and shaking because it’s about to decay. We’re told that as this nucleus decays, it gives off a neutron. And we want to know by how much does this change the atomic number of the nucleus. Now, we can recall that given some atomic element, let’s call it element X, that element has a certain number of protons, sometimes symbolized Z. And it has a certain number of protons plus neutrons, often symbolized N. Now, this number here — what we’ve called Z, the number of protons in the nucleus — is also known as the atomic number. It’s literally how many protons there are in the nucleus of an X atom, whatever X is.

So then, going back over to our unstable nucleus, we’ve drawn the neutrons in green and the protons, the positively charged particles in blue. And in this emission process, we’ve lost a green particle, that is a neutron. But we’ve kept all the blue ones, the protons. Since it’s the protons that make up the atomic number of our nucleus, that means the atomic number of this nucleus hasn’t changed. Another way to say it is, its change is zero. That’s because, as we saw, we emitted a neutron. But that has no effect on the atomic number of the nucleus. So when an unstable nucleus goes through neutron emission, the atomic number of the nucleus does not change.

Let’s look now at a second example exercise.

When an unstable nucleus emits a neutron, by how much does the mass number of the nucleus change?

So in this example, we have an unstable atomic nucleus. We can show that here, with the blue circles representing positively charged protons and the green circles representing neutrally charged neutrons. And our problem statement tells us that this nucleus is unstable and emits a neutron. So we could represent that this way, with our nucleus giving off a green circle, a neutron. And the question is, how much does this emission change the mass number of the nucleus?

Well, let’s remind ourselves what the mass number of any given nucleus is. Say that we have some atomic element. We don’t know what element it is, so we’ll just give it the symbol X to represent that element. We know this element has some number of protons in its nucleus. We represent that with Z, which stands for the atomic number of the element. And if we add together the number of protons with the number of neutrons in its nucleus, we get what’s called its mass number. So Z, the atomic number, is the number of protons in the nucleus. And N, the mass number, is the number of protons plus the number of neutrons.

All this to say, if we were interested in solving for the number of neutrons in an atom, we could subtract Z, the atomic number, from N, the mass number. But anyway, for our purposes, we don’t want to know about the number of neutrons in the nucleus. But rather, how much does the mass number of the nucleus change. And this change, as we saw, happens as a result of neutron emission.

Now, since the mass number of any nucleus, including the unstable nucleus we have here, is equal to the number of protons plus the number of neutrons in the nucleus. That means if we subtract away a neutron — as a result of neutron emission, like we did here — then that mass number will decrease by one. That’s because we’ve kept the same number of protons, but we’ve lost a neutron. And that, then, is our answer to this question. When an unstable nucleus emits a neutron, its mass number decreases by one.

Now, let’s summarize what we’ve learned about neutron emission. Starting off, we saw that neutron emission is a form of radioactive decay, where a nucleus gives off one or more neutrons. We saw that representing neutron emission in a nuclear equation involves using a lower case n to represent a neutron. And it has an atomic number of zero and a mass number of one.

Based on this, we saw that emitting a neutron does not change the atomic number of the emitting nucleus, but it does change the mass number. We saw further that graphs called nuclear decay graphs depict nuclear transitions, including neutron emission. And lastly, we saw that neutrons, as emitted particles, can penetrate farther than alpha or beta particles. And they can create radioactive material through a process called neutron activation.

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