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
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
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
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
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
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
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
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.