Video: Positrons

In this video, we will learn how to describe the properties of positrons, the differences between positrons and electrons, and the differences between matter and antimatter.

13:59

Video Transcript

In this video, we we will be studying particles known as positrons. We will be looking at their properties and behaviours. But first, let’s look at how positrons are related to a particle we may be already quite familiar with, the electron.

Now the electron, as we can recall, is a fundamental particle. In other words, it cannot be split into even smaller particles. As well as this, we can recall that electrons have a mass, which we will call 𝑚 subscript e⁻, of about 9.1 times 10 to the power of negative 31 kilograms. And the reason that an electron is labelled e⁻ is because of the charge on an electron. This charge, which we will call 𝑞 subscript e⁻, is equal to negative 1.6 times 10 to the power of negative 19 coulombs. In other words, then, electrons are negatively charged particles with this magnitude of charge.

Now the reason we bring all of this up in a lesson about positron is because positrons are very closely related particles to electrons with very similar properties. A positron, with symbol e⁺, is also a fundamental particle just like an electron. But not only that, the mass of a positron, which we will call 𝑚 subscript e⁺, is the same as the mass of an electron, roughly 9.1 times 10 to the power of negative 31 kilograms. As well as this though, the charge on a positron has the same magnitude, or size, as the charge on an electron. But the only difference is that a positron is positively charged. And so, the charge on a positron is positive 1.6 times 10 to the power of negative 19 coulombs.

What this means is that in many ways electrons and positrons are very closely related. They both have the same mass and the same magnitude of charge. But the positron is positively charged and the electron is negatively charged. Now we might look at this charge, positive 1.6 times 10 to the power of negative 19 coulombs, and immediately think of a proton. Where, of course, a proton is often found in the nucleus of atoms. However, positrons and protons are very different particles. And this is because protons actually have a much larger mass than electrons or positrons.

Recall that the mass of a proton is about 1.7 times 10 to the power of negative 27 kilograms, much heavier than the mass of an electron or positron. Although a proton does have the same charge as a positron, 1.6 times 10 to the power of negative 19 coulombs. But that’s not relevant. We can think about positron as almost the positively charged version of an electron. Now the technical term for this is that the positron is the antiparticle of an electron. Or, conversely, we could say that the electron is the antiparticle of the positron as well.

Now antiparticles are particles which have the same mass as each other and the same magnitude, or size, of charge, but the sign on those charges is the opposite sign. And so, as we’ve seen, a positron is the antiparticle of an electron. Now it turns out that other particles that we may be familiar with, such as protons and neutrons, also have their own antiparticles. The antiparticle of a proton is known as the antiproton. And even the neutron has an antiparticle known as the antineutron.

But hang on. Didn’t we just say that the antiparticle must have the same mass as the particle and the same magnitude of charge but the opposite sign? And aren’t neutrons neutral particles, so they have zero charge? Well, yes, this is true. Both the neutron and the antineutron have a charge of zero. But the reason we can tell them apart is because neutrons, remember, are made up of quarks, specifically, one up quark and two down quarks. And those quarks are charged particles. So, an antineutron, although neutral overall, is actually made up of the antiparticles of those quarks. It’s made up of one antiup quark and two antidown quarks. And that’s how we can tell the difference between a neutron and an antineutron.

But anyway, coming back to our electron and positron, we’ve seen that we denote an electron with an e⁻ and a positron with an e⁺. Where in both cases, the signs are written in the top right-hand corner above the letter e. But another common way to denote an electron is simply using the letter e. And a positron will then be denoted as the letter e with a bar on top of it. And this is usually a more general notation. Anything with a bar on top is the antiparticle of the thing that doesn’t have a bar on top. So, coming back to our proton example, we denote a proton with the letter p and an antiproton as a p with a bar on top of it.

And actually, if we now zoom out a little bit on this discussion, we can recall that all of these ordinary particles that we know about, the electron, the proton, the neutron, form what we call matter. Matter is just another fancy way of talking about the ordinary physical stuff that makes up our universe, which is primarily made up of atoms, which are made up of protons, neutrons, and electrons, and so on. And the equivalent collection of antiparticles, so antiprotons, antineutrons, antielectrons, and so no, this collection is known as antimatter. Just like how for a specific case, we can say that the positron is the antiparticle of an electron.

Now it’s worth noting we did say earlier that the positron is the antiparticle of the electron, but also the electron is the antiparticle of the positron. In other words, these two particles are antiparticles of each other. However, this can get a bit confusing because commonly we say that ordinary matter, the stuff that we’re used to, is matter. And so, electron is just a particle. And it’s these more uncommon particles, such as the positron, that are the antiparticles.

So, this is something that we need to be warry of. Although, we could say that matter is the antimatter of antimatter, we generally say that the stuff that’s most abundant in our universe is matter. And this stuff is antimatter.

Now as we’ve said already, most of the stuff in our universe is actually made up of matter. So, can we actually observe antimatter in our universe? For example, can we detect positrons in any way? Or are they just neat theoretical concepts? Well, as it turns out, we can observe positrons. Commonly, in a process known as beta decay.

Now let’s start by recalling that there are two different kinds of beta decay. One type is called beta minus, while the other type is called beta plus. And the reason for this is the following. In beta minus decay, we start with a nucleus that’s highly unstable because it contains too many neutrons, compared to the number of protons it has in it. And so, to become stable, what it does is it changes one of these neutrons into a proton. In other words, if we look at the neutron that specifically changes from a neutron to a proton, then we can say that initially it was a neutron and then it changes into a proton, and it also emits a beta minus particle, otherwise known as an electron.

Now there are a couple of things to note here. Firstly, we can see that conservation of charge is taking place here. We start out with a neutron, which has a total charge of zero because it’s a neutral particle. And we end up with a proton, which we can say has a relative charge of positive one because it’s a positively charged particle, and an electron, which has a relative charge of negative one. So, the total charge on the right-hand side of this equation is plus one minus one, which is zero. Therefore, the total charge before the decay process is the same as the total charge after the decay process.

And so, what happens to this nucleus is that this neutron turns into a proton and also emits an electron. Now it’s worth noting, by the way, that sometimes this electron might be represented by the Greek letter 𝛽 with a negative sign, just as we do for an electron. And that’s because we call this type of decay beta decay, specifically beta minus decay. So, that’s what happens in beta minus decay.

In beta plus decay, however, we start out with a nucleus that has far too many protons this time, compared to the number of neutrons it has in it. And so, to stabilize, one of the protons turns into a neutron. In other words, we start with a proton, and then we end up with a neutron and, this time, a positron.

Now let’s start by confirming that conservation of charge is indeed in effect here. On the left-hand side of the equation the total charge is positive one because we’ve got a proton. And on the right-hand side, we’ve got a neutron, which has a charge of zero, and a positron, which we said earlier has a relative charge of positive one. Because, remember, it has the same magnitude charge as an electron but the opposite sign. And so, the total charge on the right-hand side of this equation is also positive one, same as the left-hand side. In other words then, we see this proton being converted into a neutron and, as well as this, a positron is released. And so, beta plus decay is a very common way for us to observe positrons.

Now it’s also worth noting that in both of these decays, beta minus and beta plus, an additional particle is released. In the case of beta minus decay, an antineutrino is released. But because the mass of an antineutrino is extremely small, and it has no charge, we often ignore this particle in our decay equations. Similarly, in beta plus decay, a neutrino is released. And the antineutrino we saw in the beta minus decay is the antiparticle of this neutrino. But once again, we ignore this neutrino in our decay equations because it has a very small mass and zero charge. So, we may see beta decay equations written with or without the neutrinos.

But anyway, so the point of this discussion is to realise that beta plus particles are, in fact, positrons. And that’s one way for us to observe positrons in our universe. But it’s also worth realising that there isn’t a large amount of antimatter knocking about in our universe. The vast majority of our universe is made up of matter. At this point in time, scientists don’t know why the universe is mainly made up of matter and only small amounts of antimatter exists. But the fact that there’s not much antimatter is actually probably a good thing for those of us existing in this universe.

Now the reason that we say this is because of what happens when a particle meets its antiparticle. Let’s think about the simplest example of this happening, an electron meeting a positron. When an electron collides with its corresponding antiparticle the positron, the two particles undergo a process known as annihilation. If they come together relatively slowly and then they collide with each other, then both of these particles get destroyed. And large amounts of energy is released in the form of two or more gamma rays.

Now the reason for this is that all of the mass of the electron and the positron is converted into energy, in accordance with Einstein’s famous equation 𝐸 is equal to 𝑚𝑐 squared. Where, in this case, what we’re referring to is the energy released in the form of gamma rays, which is 𝐸. And 𝑚 is the mass of the electron and positron combined. And 𝑐 is the speed of light, which happens to be about three times 10 to the power of eight metres per second.

So, to recap, all of the mass of the electron and positron when they collide with each other is converted to energy. That energy goes away from the reaction in the form of gamma rays. And this entire process is known as annihilation.

Now as we can see from this equation, just a small amount of mass will release very large amounts of energy because we’re multiplying the mass by 𝑐 squared. Now 𝑐 already is a very large number. It’s on the order of 10 to the power of eight. And so, 𝑐 squared is this very large number squared, made even bigger. And this is why we say that if there were large amounts of antimatter in our universe, then lots of it would collide with matter and annihilate and basically destroy everything in the universe whilst also releasing large chunks of energy.

And because any matter coming in contact with its corresponding antiparticles is so dangerous, that means that using antimatter in our universe is actually quite a risky process. This makes antimatter very expensive to deal with, as well as risky. And so, it’s use is generally restricted to particle accelerators and other such large physics experiments.

So, now that we’ve learnt a bit about positrons and antimatter in general, let’s take a look at an example question.

The antiparticle of a particle has the same blank as that particle but has an opposite blank.

Okay, so, this question is testing our understanding of what an antiparticle is. So, to answer it, let’s look at an example of an antiparticle. Let’s recall that the antiparticle of an electron is known as a positron. And let’s also recall that the mass of an electron, which is 9.1 times 10 to the power of negative 31 kilograms, is the same as the mass of a positron. And so, if the electron is our particle and the positron is our antiparticle, we see that they have the same mass. Hence, mass can be filled in into our first blank.

Now let’s recall a second property of electrons and positrons. Let’s recall that the charge on an electron, which we’ll call 𝑞 subscript e⁻, is negative 1.6 times 10 to the power of negative 19 coulombs because, remember, the electron is a negatively charged particle. However, the charge on a positron, 𝑞 subscript e⁺, is positive 1.6 times 10 to the power of negative 19 coulombs.

Therefore, both the electron and the positron have the same magnitude of charge. But the positron is positively charged, whereas the electron is negative. And hence, we can say that these two particles, the electron and the positron, have opposite electric charges. And we can fill electric charge into our second blank.

And now we’ve used this specific case of electrons and positrons to fill in the blanks in the statement. But actually, this applies to all particles and antiparticles. We can say that the antiparticle of a particle has the same mass as that particle but has an opposite electric charge. Okay, so, now that we’ve had a look at an example question, let’s summarise what we’ve talked about in this lesson.

We firstly saw that a particle’s antiparticle has the same mass as that particle but the opposite electric charge. We also saw that a positron, sometimes called the antielectron, is the antiparticle of an electron. We saw that when an electron is labelled e superscript minus, a positron is labelled e superscript plus. When an electron is simply labelled e, a positron can be labelled e bar. And this, by the way, is a general notation. An antiparticle has a bar on top of it.

And finally, we also saw that in beta decay an electron can be known as a beta minus particle, and a positron can be known as a beta plus particle. In fact, that last point is an important one. We can observe positrons in our universe because they’re produced during beta plus decay. Now there are other ways to produce positrons, but this is probably one of the most common.

And finally, we saw that when electrons and positrons collide, or generally when a particle collides with its antiparticle, they annihilate, and energy is released in the form of gamma rays. As an additional side note though, this total annihilation generally only occurs when the particle and its antiparticle approach each other relatively slowly.

However, if it’s a high-speed collision, then sometimes some of the kinetic energy that the particles had can be converted into other types of exotic particle. But that process also releases gamma rays. And so, annihilation is the process when large amounts of mass of these particles is converted into energy in accordance with Einstein’s famous equation 𝐸 is equal to 𝑚𝑐 squared. So, that is an overview on positrons.

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