Video: Understanding Radioactive Tracers

The table shows four different radioactive isotopes. Which isotopes would be suitable for use as radioactive tracers?


Video Transcript

The table shows four different radioactive isotopes. Which isotopes would be suitable for use as radioactive tracers?

Okay, so in this question, we’ve been given a table that shows firstly the names of isotopes, secondly the type of radioactive decay that each one of those isotopes undergoes, and thirdly the half-life of each one of these isotopes as well. So for example, iodine 129 undergoes 𝛽 and 𝛾 decay and has a half-life of 15.7 million years. And we’ve been given the same sort of information for three other isotopes too. So we need to use this information to work out which of these isotopes will be good to use as radioactive tracers.

Now, radioactive tracers are usually chemicals that are sent around the body. And they move around the body, for example, through the bloodstream. So that all of this radioactive material gets distributed around the body. Then when all of these radioactive isotopes decay via 𝛽 or 𝛾 radiation in these cases, that gets detected by an external detector outside of the body. So based on this information, we can create a picture of, for example, how blood is flowing around the body or, in general, how these radioactive isotopes are being moved around the body.

If there’s some sort of abnormality in the body, for example, if there’s a clump of cancerous tissue, then that clump of cancerous tissue may, for example, take up some extra blood, this resulting in a larger concentration of the radioactive tracer material in the cancerous area. And therefore a larger quantity of radiation would be detected to be coming from that cancerous region. Now of course, it’s not necessary that the region of abnormality will take up a larger-than-usual amount of the radioactive tracer.

Some abnormalities might actually take up less than the usual amount. But the point is that this difference in flow of radioactive tracer around the body allows us to create an image of what’s actually going on inside the body. And this is solely because radioactive tracer is distributed around the body. And then that radioactive tracer emits radiation which can be detected outside the body. So based on this information, we need to find the isotopes that are best suited for this kind of use.

The most important thing then in this case is going to be the half-life of the radioactive tracer. Because we want the half-life to be long enough, so that most of the radioactive tracer hasn’t decayed away by the time we inject it into the bloodstream. And it actually gets to the region of the body that we want it to get to. However, we also want this radioactive tracer to have a half-life that’s short enough, so that the person who’s being injected with this radioactive tracer is not going to be radioactive for a large period of time.

So let’s start by looking at Iodine-129. We see that this isotope has a half-life of 15.7 million years. Now, this is not suitable for use as a radioactive tracer because this means that the person that we inject with this isotope will be emitting radiation for a very very long period of time. And over large period of time, this radiation might actually damage them. It might end up ionising the atoms that form the cells of their body. And as well as this, because the half-life of Iodine-129 is very large, this means that we’d have to inject a lot of Iodine-129 into the person in order to get any detectable level of radiation coming out of them. Because if the half-life of an isotope is very large, then only a small amount of the isotope decays in any given unit of time. So we’d need to inject lots of this isotope to get a decent amount of radiation coming out.

At the other end of the spectrum, we’ve got Rhodium-106 with a half-life of 30 seconds. Now, this actually is way too short, because by the time you produce this isotope and get it close to the patient and ready to inject into their body, a lot of this isotope will have decayed away already. And so we won’t be able to get any meaningful radiation out of the body of the person who’s been injected with this isotope. Hence, we want an isotope which has a half-life somewhere on the order of a few hours. And in fact, both Technetium-99m and Iodine-123 have half-lives on the order of a few hours.

Therefore, Technetium-99m and Iodine-123 would both be suitable for use as radioactive tracers. Now, the other important thing is that in this table, we’ve been given the type of decay that each one of these isotopes undergoes. And we can recall that 𝛾 radiation, which is actually just electromagnetic radiation, is very mildly ionising but can penetrate large distances through materials. Therefore, if we have some radioactive tracer in a human’s body, then if that radioactive tracer emits 𝛾 radiation, then it’s highly likely that that 𝛾 radiation will be able to escape the body of the human and be detected by an external detector.

But when we think about 𝛽 radiation on the other hand, we can recall that 𝛽 radiation is actually a lot more ionising than 𝛾 radiation. And as well as this, its penetrating power is a lot lower. This means that 𝛽 radiation may not be able to escape the body of the human that’s injected with this radioactive tracers. And what’s more is that because 𝛽 radiation is so much more ionising than 𝛾 radiation, it’s highly likely that 𝛽 radiation inside the human body will cause a lot of damage.

Now, the whole purpose of radioactive tracers is to be able to detect what’s going on inside the body without damaging the body as much as possible. However, injecting a 𝛽 source into a human body is a very bad idea because 𝛽 radiation will cause lots of ionisation in the body and generally wreak havoc.

So Iodine-129 and Rhodium-106 are not good for use as radioactive tracers because they’re both 𝛽 emitters. And we don’t want 𝛽 radiation in the body. And because Iodine-129 has a half-life too large. And Rhodium-106 has a half-life too small for our purposes. And hence we can say that out of the isotopes given to us in the table, the ones that are suitable for use as radioactive tracers are Technetium-99m and Iodine-123.

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