Video: Understanding Radioactive Tracers

Polonium-210 is a radioactive isotope of polonium with a half-life of 138 days. It decays via alpha decay to lead. Which of the following are reasons why polonium-210 cannot be used as a radioactive tracer? [A] Polonium-210 is too expensive to be used as a radioactive tracer. [B] Polonium-210 decays too slowly and thus would remain in a person’s system for too long. [C] Polonium-210 is a very rare isotope, and thus refining it for use in medicine is too difficult. [D] Polonium-210 decays too quickly and thus would not remain in a person’s system long enough to be useful. [E] Polonium-210 decays to lead, which is highly toxic. [F] Polonium-210 decays via alpha decay. Radioactive tracers have to emit gamma rays. The alpha particles released by polonium would be absorbed by the body cells.

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Video Transcript

Polonium-210 is a radioactive isotope of polonium with a half-life of 138 days. It decays via alpha decay to lead. Which of the following are reasons why polonium-210 cannot be used as a radioactive tracer?

Now, there are six possible reasons. But before we go through any of them, let’s read the question again. We can underline all the important bits, so we don’t miss out any information. So we know that polonium-210 is a radioactive isotope of polonium. We’re looking at polonium in this question. This isotope has a half-life of 138 days. We also know that it decays via alpha decay to lead. What we’re asked to do is to work out which of the following are reasons why polonium-210 cannot be used as a radioactive tracer.

So first of all, what is a radioactive tracer? Well, a radioactive tracer is a chemical that has a very useful purpose. It can be ingested into the body using various different methods at which one is allowed to flow around the body using natural processes, such as for example blood flow. This is where the radioactive part kicks in. Essentially, as the tracer travels around the body, it also releases radiation. This radiation can be detected outside the body by scanners. And then a computer can be used to put together an image which shows the distribution of the radioactive tracer within the body. This means that, using the example of blood flow again, if the blood in your body is not flowing to the right places as necessary, then there won’t be any radioactive tracer where the blood isn’t flowing. Using this logic, doctors can determine whether or not your blood flow is normal. So yes, radioactive tracers are very useful in medical physics.

Anyway, let’s look at the six options we have, to see why polonium-210 cannot be used as a radioactive tracer.

Option a: Polonium-210 is too expensive to be used as a radioactive tracer.

Now, if something is too expensive, then what better field to use it than in medicine. Quite often, when trying out new and advanced treatment methods, as well as diagnosis methods, cost of an object is generally ignored. This is because researchers generally believe that it’s important to explore every single possible avenue when it comes to treating or diagnosing an illness. Therefore, usually cost is not an issue. And in this case, this is not a reason why polonium-210 cannot be a radioactive tracer.

So let’s move on to option b: Polonium-210 decays too slowly and thus would remain in a person’s system for too long.

In order to be able to reason this one out, we need to look at some of the information we’ve been given in the question. We’ve been told that polonium’s half-life is 138 days. This means that whatever amount of polonium we injected to the body, half of it will still be polonium 138 days after it was injected. This definitely is an issue, especially for just trying to diagnose in the space of maybe a few hours or at most a few days. We don’t want anything remaining in the body for 138 days. Therefore, option b is definitely a reason why polonium-210 cannot be used as a radioactive tracer.

Next option, option c: Polonium-210 is a very rare isotope, and thus refining it for use in medicine is too difficult.

Now this is a bit similar to what we discussed in option a, which was cost. This time we’re talking about rarity. So yes, polonium-210 is a fairly rare isotope. But, this is no excuse in medicine either. If, for example, polonium-210 happened to be useful as a radioactive tracer, then there are ways of exploring how to make more of it. For example, polonium isotopes can be made by humans by radiating bismuth atoms with neutrons. And if polonium-210 happened to be useful, then something like this would have to be explored. So once again, rarity is not usually an excuse in medicine. And in this case, this is not a reason why we can’t use this as a radioactive tracer. There are much more pressing reasons as to why polonium-210 cannot be used as a radioactive tracer. And we’ll see those in a second.

Let’s move on to option d. Polonium-210 decays too quickly and thus would not remain in a person’s system long enough to be useful.

Well, this doesn’t make sense. It’s got a half-life of 138 days. Of course, it would remain in the body long enough to be useful. In fact, as we’ve discussed earlier, it would remain in the body for far too long. So this reason doesn’t even mean anything. It doesn’t make sense.

And so we can swiftly and happily move on to option e, which is that polonium-210 decays to lead which is highly toxic.

And this, absolutely, is a reason why we should not use it as a radioactive tracer. We cannot inject polonium-210 into the body because it decays to lead. Lead inside the body is highly toxic. Therefore, even if it was a good radioactive tracer, it would end up killing the patient in the space of a few days. This goes against the principle of medicine, to say the least. So yes, option e is a reason why we cannot use polonium-210 as a radioactive tracer.

And finally, option f: Polonium-210 decays via alpha decay. Radioactive tracers have to emit gamma rays. The alpha particles released by polonium would be absorbed by the body cells.

Now, whatever this statement is saying is correct. If a radioactive tracer has to be put into the body, then the radiation it emits needs to be as penetrating as possible. The reason for this is that we want as much radiation that’s being emitted by a tracer to come outside the body. This way, we can detect as much of it as possible and create an accurate picture of what’s going on within the body. However, alpha particles, which is what polonium-210 emits when it decays, are not very penetrating at all. They can travel a distance of a few centimeters before being absorbed. There is of course also the matter that they’re the most ionizing form of radiation. So they would damage a lot of the body cells from within. So yes, option f is also a reason why polonium-210 cannot be used as a radioactive tracer.

And so, at this point, we’ve exhausted all of the options on the list. So we can say that our final answer is that options b, e and f are the main reasons why polonium-210 cannot be used as a radioactive tracer.

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