Hello, my name's Dr.

George.

This lesson is called Ionising Radiation and it's part of the unit Nuclear Physics.

The outcome of the lesson is, I can describe the properties of alpha, beta.

and gamma radiation.

I'll be using these keywords, which I'll explain as they come up.

You can come back to this slide anytime if you need to remind yourself of the definitions.

This lesson has two parts.

They're called radioactive sources and nuclear radiation and alpha, beta, and gamma radiation.

Radioactive sources are hazardous.

That's because they emit, give out, radiation that can damage living tissue and sensitive electronics.

And the types of radiation that they give out are called ionising radiation.

There's a hazard warning sign for ionising radiation that's put on or near radioactive sources.

Radioactive sources contain radioactive particles, atoms with unstable nuclei that decay over time.

And this decay releases ionising radiation.

Different radioactive sources emit different types of radiation.

Some emit alpha radiation, some beta, and some gamma.

In fact, most sources emit more than one of these types.

Here are a few examples and the way they're written with a hyphen and then a number, that number represents the nucleon number, the total number of protons and neutrons in that particular isotope.

Now we need to be careful about the language we use here because alpha and beta particles themselves aren't radioactive, and nor is gamma radiation because they don't decay and release radiation.

And that's what radioactive means.

Let's look a little bit more at alpha, beta, and gamma.

Here's what they actually are.

An alpha particle is two protons and two neutrons stuck together.

A beta particle is a fast moving electron and gamma radiation is high frequency electromagnetic radiation.

And now a quick question for you.

Why are alpha particles not radioactive? And when I ask a question, I'll wait five seconds, but if you need longer, press pause and press play when you have your answer ready.

And the reason we don't call them radioactive is because they can't decay any further.

And a radioactive material is something that can decay and release radiation, whereas the fast moving alpha particles are the radiation.

Radioactive sources, which are sometimes used in school experiments and also in hospitals for certain procedures, are stored in lead-lined boxes.

This reduces the amount of radiation that people could be exposed to if they go anywhere near these stored sources.

Lead is used because it's a dense metal and it absorbs most of the radiation that comes from the source.

And that means less radiation can reach the environment.

Why should a radioactive source be placed back into a lead-line box immediately after use? And only one of these statements is correct.

And it's C, lead absorbs the emitted radiation, reducing the amount that can escape from the box.

Now, can you fill in the gaps in these sentences using only the words radioactive particles or radiation in each gap? The first sentence should say, all radioactive sources contain radioactive particles.

They don't contain radiation, but radioactive particles emit radiation when they decay.

Alpha, beta, and gamma are all types of radiation that can be emitted by radioactive particles.

And radioactive sources are often stored in a wooden lead-line box because lead absorbs radiation.

Well done if you've got those right.

There's often some confusion between radiation and particles that emit radiation, so try to make sure you're clear on that.

Now let's move on to learn a bit more about alpha, beta, and gamma radiation.

Some radioactive sources emit alpha particles and they contain two protons and two neutrons.

Each proton's positively charged and so an alpha particle has a larger electrical charge than the single proton.

It has double the charge.

Alpha particles themselves aren't radioactive because they can't decay.

Alpha particles are one of the forms of ionising radiation.

Let's see what that means.

As they pass near atoms, their electric charge exerts an electrostatic force on the outer electrons of the atoms, and that force can be strong enough to pull electrons away from their atoms. Here's what that might look like.

Here's a couple of neutral atoms with some outer electrons shown and an alpha particle moving past.

It attracts an electron that's nearby, moving it out of its atom, and it can do the same again as it passes another atom.

And that's what we mean by ionisation.

It means turning an atom into an ion and an ion is an atom that has lost or gained electrons, so that it's no longer neutral.

These two atoms have just lost electrons, so they're now positively charged.

Alpha particles are strongly ionising compared to other types of radiation.

As we've seen, an alpha particle has two protons.

It has a relative charge of plus two, and that gives it a stronger electric field to force electrons off atoms compared with other types of radiation.

It also has a much bigger mass than an electron.

So when an alpha particle interacts with an outer electron of an atom, the electron speed changes much more than the speed of the alpha particle does.

But alpha particles are slowed down a little bit each time they cause ionisation.

And because they easily ionise atoms that they pass, alpha particles slow down quickly in a short distance 'cause they do a lot of ionising.

As a result, alpha particles are only weakly penetrating.

So in terms of what they can travel through, they're stopped by just a few centimetres of air or by a single sheet of paper or by the outer dead layer of a person's skin.

So they don't get far because they lose their energy by doing a lot of ionising over a short distance.

Now can you select one option from each of the two columns here to describe alpha particles? In the first column, only radiation is correct.

So fast moving alpha particles coming out of a radioactive source, they are the radiation.

They're not the radioactive source, and they themselves are particles, but they're not radioactive, they don't decay.

And the only correct option on the right is they can ionise atoms. They don't make other atoms radioactive and they don't emit radiation themselves.

Well done if you picked out both of those.

Now, some radioactive sources emit beta particles, and these are high speed electrons that are emitted from an unstable nucleus in an atom.

Electrons are negatively charged, so a beta particle has a negative electrical charge.

Beta particles themselves, like alpha particles, aren't radioactive, they don't decay, they don't emit radiation.

Fast moving beta particles are radiation and they're a form of ionising radiation as well.

As they pass new atoms, their electric charge exerts an electrostatic force on outer electrons, repulsion this time, and it's strong enough to pull electrons away from their atoms. Here's what that might look like.

Here's a neutral atom with some outer electrons shown.

A beta particle moves past and this time, it repels an electron.

And in doing that, it can make the electron leave its atom, in which case it's ionised that atom, which has become a positive ion.

Now in these two columns, can you select one option from each that describes beta particles? In the first column, only radiation is appropriate.

And in the second, they can ionise atoms, but they can't do either of the other things.

Beta particles are not as strongly ionising as alpha particles.

Beta particles have a relative charge of minus one, whereas alpha particles have a relative charge of plus two, twice the charge on a proton.

The electric field of a beta particle then is weaker than the one around an alpha particle, so it doesn't ionise atoms as easily.

Also, the mass of a beta particle is much smaller than the mass of an alpha particle.

It's about 1/8000th the mass of an alpha particle.

Like alpha particles, beta particles are slowed down as they ionise, but because they're not strongly ionising, they may pass by many atoms before they ionise one.

As a result, beta particles are moderately penetrating.

They can get through materials more easily than alpha particles.

They can be stopped by a few metres of air or a thin sheet of aluminium, and they can travel through the outer layer of a person's skin.

What happens to a beta particle after it has ionised an atom? And the only correct answer here is it moves a little bit less quickly.

It's lost some of its energy.

Some radioactive sources emit gamma radiation.

And gamma radiation isn't electrically charged, it's neither positive nor negative.

It's an electromagnetic wave, like light, microwaves, x-rays, and radio waves.

So it travels at the speed of light, which is faster than alpha or beta particles can travel.

And gamma radiation itself isn't radioactive.

It can't decay and release more radiation.

Gamma radiation as well is ionising.

As it passes nearby atoms, its electromagnetic field can interact with the outer electrons of atoms and it can give them enough energy to escape, producing positive ions.

So here's some gamma radiation passing near an atom, and it might free an outer electron, but gamma radiation is only very weakly ionising compared with alpha or beta particles.

Being electromagnetic radiation, it has no overall electric charge, but it does have a weak electric field that's constantly changing.

So it can ionise atoms, but not so easily.

Gamma radiation is absorbed when it ionises, but because it's only very weakly ionising, it can pass by very many atoms before its ionises one.

And that makes it highly penetrating.

It travels a long way.

It's not easily stopped at all by air.

It is stopped or rather greatly reduced by a few centimetres of lead or a few metres of concrete, and it can travel straight through a person.

Now, can you select one answer from each of these two columns to describe gamma radiation? And in the first column, travels at the speed of light and it's electromagnetic radiation.

Explain why alpha, beta, and gamma radiation are each able to travel different distances through air and try to include as much detail in your answer as you can.

Press pause while you're writing your answer down and press play when you're ready to check it.

And here's an example answer.

Alpha radiation is highly ionising because it easily ionises atoms that it passes.

Each time an alpha particle ionises an air molecule, or you could say atom, it is slowed down.

As it ionises so many air molecules over a short distance, it can only travel a few centimetres through air.

Beta radiation is not so highly ionising as alpha.

It ionises fewer particles that it passes near to, and it's not slowed down so quickly by air.

It can travel a few metres through air before it's stopped.

Gamma radiation is the least ionising and so travels furthest through air.

Don't worry if your answer wasn't worded in quite the same way as that, but check that you included some of the same points.

And now we've reached the end of this lesson.

And here's a summary.

Some materials emit radiation because they contain radioactive particles and three types of radiation it can emit.

Alpha particles interact easily with atoms to form ions and are highly ionising.

They're not very penetrating and can travel only a few centimetres in air.

They're stopped by a sheet of paper.

Beta particles interact less easily with atoms and are less ionising.

They're moderately penetrating and can travel a few metres through air.

They're stopped by a sheet of aluminium.

Gamma radiation is least ionising.

It is highly penetrating and is not really stopped by air.

It can travel through a human body and is stopped by thick sheets of lead or by several metres of concrete.

So well done for working through this lesson.

I hope you found it interesting, and I hope to see you again in a future lesson.

Bye for now.