Video: Detecting and Blocking Different Types of Radiation

In this video, we will learn how to detect alpha, beta, and gamma radiation and what materials are effective at stopping different types of radiation.

17:14

Video Transcript

In this video, we will be learning how we can detect and block different types of radiation, specifically different types of ionizing radiation. So let’s first start by recalling what we mean by ionizing radiation. Ionizing radiation is radiation that has sufficient energy to turn atoms and molecules it interacts with into ions. In other words, it’s radiation that is ionizing, funnily enough.

And we can recall that an ion is an atom which has a different number of electrons to the number of protons in its nucleus. In other words, the number of negative charges, the number of electrons, is not equal to the number of positive charges, the number of protons. And hence, an ion has a net charge. It is a charged particle.

So ionizing radiation can turn neutral atoms, atoms which do have the same number of electrons as the number of protons in the nucleus, into ions. And most commonly, ionizing radiation does this by removing electrons from atoms. We will shortly see how that happens. But for now, let’s recall that there are three main kinds of ionizing radiation. There’s alpha radiation, beta radiation, and gamma radiation. Let’s also recall what each one of these kinds of radiation is physically.

Firstly, an alpha particle, also known as the nucleus of a helium atom, consists of two protons, labeled here in blue, and two neutrons, labeled in green. In other words then, alpha radiation is highly positively charged because it contains two protons, which are positively charged particles, but no negatively charged particles. Remember that neutrons are neutral. And for this reason, if alpha particles were to interact with some medium.

So let’s say we’ve got a radioactive source that emits alpha particles. And they were to interact with molecules of air around our radioactive source. We’ll represent the molecules of air with pink dots. Then the alpha particles, being so positively charged, will attract the electrons in the molecules of air. Because, remember, electrons are negatively charged and alpha particles, being positive, will attract the opposite charge. And often alpha particles will steal electrons from these molecules of air or, for that matter, from atoms or molecule making up any medium that they interact with.

So let’s imagine that our alpha particle here has now stolen two electrons. That turns it into a neutral helium atom, because now it’s got two electrons which balance out the two protons in the nucleus. However, the molecule of air in this case that the alpha particle stole the electrons from is now ionized. And alpha particles are very good at ionizing any material that they interact with. And the reason for this is because they’re so strongly positively charged.

Each alpha particle contains two units of positive charge because it contains two protons. And so not only does any alpha particle steal electrons from any molecule or atom it interacts with, because an alpha particle is so positively charged, it can attract electrons from atoms or molecules that are relatively far away from itself. And therefore, alpha particles steal electrons very easily. And hence, they ionize other atoms or molecules very easily.

Therefore, we can say that alpha particles have very high ionizing power, where ionizing power refers to how easily the radiation can ionize any material it interacts with. And once again, this higher ionizing power is due to the very strong positive charge on an alpha particle.

But then, a consequence of an alpha particle’s high ionizing power is that if we think about our radioactive source once again, emitting alpha particles. And this time, we only consider alpha particles moving in this direction, just to make life easier for ourselves. Then we can see that because they’re so strongly positively charged, they will very quickly interact with air molecules. This means that alpha particles cannot actually travel very far into our, let’s say, layer of air before they end up ionizing the air molecules.

So let’s say that these two alpha particles ionize these two air molecules here, which means that we won’t see very many alpha particles moving towards the right. In other words, they cannot penetrate very deep into our layer of air. And this logic doesn’t just apply to air. If we were to place, let’s say, a solid block of wood next to our radiation source, then the alpha particles emitted by the source would ionize some of the molecules on the surface of the wood. But they would not be able to penetrate very far into the wood because alpha particles are so strongly ionizing.

In fact, even a thin sheet of paper is enough to block most alpha particles. Because the alpha particles would quickly ionize molecules on the surface of the sheet of paper but not be able to make it through the paper. We could place a radiation detector on this side of the sheet of paper. And we would detect particles. But on this side, we would detect very few alpha particles. And hence, we can say about alpha radiation that it is mostly stopped by a thin sheet of paper, which means that it’s got low penetrating power. In other words, it cannot penetrate very deep into a material that it’s interacting with. And remember, that’s mostly because it has such a high ionizing power.

Okay, so now that we’ve discussed alpha radiation, let’s have a quick chat about beta radiation. Let’s recall that a beta minus particle is simply an electron. We won’t talk about beta plus here. But let’s realize that if beta minus radiation is an electron, then it is a charged particle as well, just like we saw with alpha radiation. However, the difference is that alpha radiation carried two units of positive charge, two protons, whereas each beta particle carries only one unit of negative charge, because it’s an electron.

So let’s once again consider our radiation source. But this time, it emits beta particles. And these beta particles are going to be interacting with air molecules. So let’s say that the air molecules are in pink once again. If we zoom in a little bit, then let’s imagine that this is our air molecule, which we’ve drawn for simplicity as just an atom. And let’s say here is a beta particle.

Now a beta particle will end up ionizing the material it’s interacting with, in this case an atom in the air, by firstly repelling electrons inside the atom. Because, remember, a beta particle is an electron, so it’s negatively charged. And so it will repel other negatively charged particles. And secondly, as the beta particle approaches the electron, sometimes the electron will get knocked out of the atom itself. And so the electron itself has flown off in this direction. And let’s say the beta particle has flown off somewhere in this direction.

What we’re then left with is an ion which is missing an electron. And that’s how beta particles ionize any material they interact with. But remember, because beta particles only contain one unit of charge, one electron, the strength with which a beta particle will repel other electrons is not as high as the strength with which an alpha particle will attract electrons. Basically, all that last statement is saying is that an alpha particle contains two protons. So it will very strongly attract electrons. Whereas a beta particle is only one electron. So it will not so strongly repel other electrons.

In other words then, beta particles have slightly lower ionizing power than alpha particles. We’re going to label this as medium ionizing power. And the reason we’re not labeling it as low ionizing power is because we haven’t discussed gamma radiation yet. But anyway, so beta particles have medium ionizing power, whereas alpha particles have high ionizing power.

But a consequence of the fact that beta particles have slightly lower ionizing power than alpha particles is that they can penetrate further into materials. In fact, if we took our radiation source once again, this time emitting beta particles, and we placed a sheet of paper next to it, like we did with our alpha source earlier, the sheet of paper would not be enough to stop the beta particles traveling through. We would still be able to detect beta particles on this side of the sheet of paper. And this is because beta particles can penetrate further into materials, because they don’t straightaway ionize the materials that they interact with. Hence, they can travel a bit further. What we would need to stop beta particles is something like a sheet of aluminum.

And hence, we can say that beta radiation is stopped, or at least mostly stopped, by a sheet of aluminum. And hence, we can label it as having medium penetrating power. And once again, we’re calling it medium penetrating power because, as we’ll see, gamma radiation actually has very high penetrating power.

And so, to very quickly recap, if we want to block alpha radiation, all we need is a thin sheet of paper. If we want to block beta radiation, a thin sheet of paper will not suffice. We’ll need a sheet of aluminum, or maybe a block of wood or something like that, generally something thicker or more dense than paper.

And finally, if we move on to gamma radiation, then we can recall that it is an electromagnetic wave. And because it’s an electromagnetic wave, it does not carry any charge. This is different to alpha and beta particles. Remember that each alpha particle has two units of positive charge and each beta particle has one unit of negative charge. The fact that alpha particles and beta particles are charged help them in ionizing materials that they interact with. Because their attraction or repulsion to other charged objects allows them to more easily ionize those materials. Whereas with gamma rays, they don’t have this ability. They’re not charged at all.

And so if a gamma ray has to ionize, let’s say, an atom in the air that it’s interacting with, then it has to directly collide with one of the electrons in the atom in order to knock the electron out of the atom. Now this looks similar to how beta particles interacted with the electrons. But remember, a beta particle didn’t need to be on a direct collision course with the electron in the atom. It could have even been passing by the atom. And yet the mutual repulsion between the beta particle and the electron would’ve caused the electron to fly out of the atom. Because the electron and the beta particle, both being the same thing, are charged particles.

Hence, beta particles are more likely to cause ionization than gamma rays. And so we can say that gamma rays have low ionizing power. But then, as we can gather from the trend that we’ve already seen, a low ionizing power will mean that gamma rays can penetrate deep into materials. In fact, if we have a radioactive source, now emitting gamma rays, then a sheet of paper or a sheet of aluminum is not sufficient to block these gamma rays. What we need is a rather thick block of lead.

So, once again, we can state that gamma radiation is stopped by a thick block of lead. And hence, it has a very high penetrating power. We need something very thick and very dense, like a block of lead, to block this radiation.

So now that we’ve discussed how we would block alpha, beta, and gamma radiation, let’s quickly discuss how we would know if we were blocking these types of radiation effectively. In other words, how can we detect these different kinds of radiation to make sure that our blocking is effective?

Well, we can use a device known as a Geiger counter. A Geiger counter is a device that comes attached, most commonly, to a little tube. And that tube is what allows us to detect ionizing radiation. In other words, every time an alpha particle, a beta particle, or a gamma ray enters the little tube on the Geiger counter, the counter on the Geiger counter increases. Now a Geiger counter can’t tell the difference between alpha, beta, and gamma radiation. But it can tell us the overall level of radiation present in the environment around the Geiger counter.

Quite often, Geiger counters won’t just measure the total number of ionizing radiation particles and waves that it detects. But rather, they’ll show a reading of how many particles they detect per second, which allows us to gauge how radioactive the region that we’re in actually is. Because if the counter detects more particles of radiation per second, then the area is very radioactive, and vice versa.

So we’ve just seen how to detect different types of ionizing radiation. We use a Geiger counter. And we’ve already seen earlier how to block each different kind of ionizing radiation, which means that we should now get some practice and look at an example question.

Which of the following types of radiation is negatively charged? A) Alpha radiation, B) beta radiation, C) gamma radiation, D) free neutrons.

Okay, so let’s start by recalling what each one of these different kinds of radiation actually is physically. We can recall that alpha radiation is also known as a helium nucleus. And the reason for this is that an alpha particle is made up of two protons, labeled in blue, and two neutrons, labeled in green here. And because it contains two protons, it’s a helium nucleus. And also because it contains two protons, an alpha particle must be positively charged. Because both protons are positively charged particles and there are no negatively charged particles to balance out this overall positive charge. And hence, alpha radiation is not the answer that we’re looking for.

Moving on to beta radiation then, there are two different kinds of beta radiation: beta plus and beta minus. Now we’ll focus on beta minus radiation here because we can recall that a beta minus particle is simply an electron. And electrons are negatively charged particles. Therefore, it looks like B could be our answer. But let’s quickly look at options C and D as well.

Option C says gamma radiation. And we can recall that gamma radiation is an electromagnetic wave, just like visible light or infrared rays or ultraviolet light, or so on and so forth. But then electromagnetic waves are not charged at all. So gamma rays cannot be negatively charged. And hence, that’s not the answer we’re looking for either.

So finally, option D talks about free neutrons. The neutrons, as suggested by their name, are neutral particles. In other words, they are not charged at all. And so they cannot be negatively charged. And so our answer to this question is that beta radiation is negatively charged.

Let’s now take a look at another example question.

Which type of ionizing radiation is most easily absorbed? A) Alpha radiation, B) beta radiation, C) gamma radiation, D) free neutrons.

Now the final option might seem a little bit strange. We don’t normally discuss free neutrons when talking about ionizing radiation. But it turns out that free neutrons are actually indirectly ionizing radiation. In other words, when free neutrons interact with a material, those interactions can then end up generating alpha particles or beta particles or gamma rays, which then go on to ionize the material. And hence, free neutrons are indirectly ionizing radiation.

But anyway, free neutrons are not going to be most easily absorbed because neutrons are neutral particles. Therefore, if we consider a material that our neutrons are going to interact with and we fire some neutrons towards that material, then that material, being made up of atoms, is not going to massively interact with these neutrons. Because, remember, neutrons are neutral, so they will not attract or repel any charged particles. And in many cases, they’ll be able to travel far through the material before they end up colliding with a nucleus of an atom and, for example, being absorbed by that nucleus.

Now, the exact distance that free neutrons can travel inside a material depends on the material itself. But the point is that free neutrons can still generally travel quite far before the material ends up absorbing them. So free neutrons is not the answer that we’re looking for.

Let’s now consider gamma radiation interacting with a material. And let’s recall that gamma rays are not charged because they’re electromagnetic radiation. And so one way that gamma rays can interact with a material, ending up ionizing the material, is if they manage to knock electrons out of an atom. But if we zoom in to a material slightly, the only way that our gamma ray can knock an electron out of an atom is if the gamma ray collides directly with the electron. In that situation, the electron flies out of the atom. But if a gamma ray is not on a direct collision course, then the gamma ray continues to penetrate through the material.

And so gamma rays can penetrate very far into a material. They are not very easily absorbed by the material at all. And this is a consequence of the fact that gamma radiation is not charged.

Moving on to beta radiation then, here’s our material and here’s our beta particles moving toward the material. Beta particles also need to knock electrons out of atoms in order to cause ionization. But let’s recall that beta radiation, or at least beta minus radiation, is made up of electrons, which are charged particles. And so if we zoom in to our material once again, we see that if this is the atom that our beta particle is going to interact with. And let’s say this is our beta particle moving towards the right. Then it doesn’t need to directly collide with an electron. It simply needs to move past an electron, fairly close by.

And the fact that our beta particle is negatively charged means that an electron in the atom will be repelled from the beta particle. Because the beta particle and the electron are both negatively charged, and like charges repel. This means that beta particles can cause ionization more easily than gamma rays can. And so they’re more likely to be absorbed more quickly in the material that they’re traveling through. They can’t penetrate quite as deep before causing ionization and being absorbed. So beta radiation is a better answer than gamma radiation or free neutrons.

But by far, the best answer on the board is alpha radiation. Let’s recall that alpha particles consist of two protons and two neutrons. And the fact that alpha particles are positively charged due to the two protons means that they will very strongly attract electrons from the material that they’re interacting with. Therefore, it doesn’t take them long to steal electrons and ionize the material that they’re interacting with. In other words, they cannot penetrate very deep into the material. They’re absorbed very quickly because they interact so strongly. And this means that we found the answer to our question. The type of ionizing radiation that’s most easily absorbed is alpha radiation.

So now that we’ve had a look at a couple of example questions, let’s summarize what we’ve talked about in this lesson. We firstly saw that when comparing three different kinds of ionizing radiation, alpha, beta, and gamma radiation, the alpha radiation had the highest ionizing power of the three. Beta had a medium ionizing power. And gamma had a low ionizing power. And as a result, alpha radiation had low penetrating power, beta had medium penetrating power, and gamma had high penetrating power.

And lastly, the alpha radiation can be blocked with a thin sheet of paper. Beta radiation can be blocked with a sheet of aluminum. And gamma radiation needed a thick block of lead to block it. And very finally we saw that ionizing radiation can be detected with a Geiger counter.

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