Question Video: Understanding the Properties of Different Types of Radiation Physics

Which type of ionizing radiation is most easily absorbed? [A] Alpha radiation [B] Beta radiation [C] Gamma radiation [D] Free neutrons


Video Transcript

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.

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