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.