Video Transcript
What type of radiation does a
radioactive isotope need to emit to be useful as a radioactive tracer? A) Gamma and beta radiation. B) Neutron radiation only. C) Beta radiation only. D) Alpha radiation only. E) Gamma radiation only.
Okay, so to answer this question,
let’s first recall what we mean by a radioactive tracer. Well, a radioactive tracer is a
radioactive substance that’s injected into the human body or ingested with some food
or drink that then flows around the body. For example, if it’s injected into
the body, then it flows with the blood. And because the substance is
radioactive, it gives off radiation as it moves through the body. Because of this, we can place a
detector outside the body that detects the radiation coming from inside the
body. And that allows us to see that the
flow of the substance that we’re testing — in this case, the blood — is normal
inside the body.
If something is wrong, however, for
example, if we say that there’s a blockage in the flow of the blood here in the body
of the patient, then when we place a detector in that region, we will find that lots
of radiation is coming from there as lots of blood piles up there and therefore lots
of radioactive tracer parts up there. And in other regions of the body,
for example, after the blockage, we might only get a few tracers of radiation. So that’s the purpose of a
radioactive tracer.
Now, importantly, the radiation
coming from the radioactive tracer needs to pass through the body so that it’s
detected outside the body. In other words, the type of
radiation that the tracer must emit must have a high penetrating power. It must be able to penetrate quite
deep into any material because remember, the radiation itself needs to penetrate
through a lot of flesh and skin in order to escape the body. Now, at this point, we can see that
four different kinds of radiation have been mentioned in the options given to us in
the question. We’ve seen alpha radiation, beta
radiation, gamma radiation, and neutron radiation, otherwise known as free
neutrons.
Well, we can recall that alpha
radiation has a very low penetrating power. It cannot penetrate very deep into
materials before it interacts with them and is absorbed by material. And on top of this, alpha radiation
is highly ionizing. But if we were to inject a
substance into the body that produces highly ionizing radiation, then the radiation
would very quickly cause a lot of damage inside the body. It would ionize a lot of the cells
in the body. And as well as this because alpha
radiation cannot penetrate very far, most of it would not be able to escape the
body. And so, our detectors would be
useless at that point. Therefore, we do not want anything
that emits alpha radiation.
And for a similar reason, we can
eliminate beta radiation as well. Because even though beta radiation
can penetrate further than alpha radiation and is not quite as strongly ionizing as
alpha radiation, it’s still not ideal because quite a lot of beta radiation would be
absorbed before it got out of the body. And beta radiation is still
strongly ionizing enough that it would cause a fair amount of damage in the
body. And so, we can eliminate anything
that produces beta radiation as well.
Now, when we think about free
neutrons, well different substances interact differently with a beam of free
neutrons. Nevertheless, neutrons are
generally known to have a high penetrating power. They can get quite far through a
material. So we might think that a
neutron-emitter is a useful isotope in this case. But actually, it turns out that
neutrons are quite hard to detect. So even if neutrons were to make
their way out of the body, detecting them would be quite difficult. And as well as this, some neutrons
would be absorbed by the body, which means that these neutrons would interact with
the atoms in the body.
This is problematic because
sometimes when neutrons interact with some material, they cause other forms of
radioactive decay. In other words, a neutron
interacting with the body could result in alpha or beta or gamma radiation being
emitted. And this is problematic because, as
we’ve already seen, we do not want any alpha or beta radiation within the body for
the purposes of simply diagnosing how well a particular substance is moving around
inside the body. And for these reasons, we do not
want neutron-emitters to be used as radioactive tracers.
And so, it looks like gamma
radiation is the best of all worlds. It’s highly penetrating, so we’re
likely to detect it outside the body, and also relatively weakly ionizing, so it
won’t cause a lot of damage. And hence, our answer is that a
radioactive isotope used as a radioactive tracer must emit gamma radiation only.
