Dialogue on Radioactivity

The further Dialogue between Reasonable Roisin and the Ostensible Oracle - this time, on the subject of radioactivity

RR I don't like all this radioactive stuff- everyone says It's dangerous.

OO Everyone does?

RR Well, my mum does. She says you can get cancer from It. Can you?

OO Yes, actually. Especially leukaemia...

RR You mean, she was right?

OO Yes. Is it that surprising?

RR I suppose not. But if it's so dangerous, how come we have to have it?

OO You don't have to have it...

RR Yes we do! It's all around us - in the air, in the ground...

OO Oh that! You can stop worrying - it's only the background radiation.

RR What do you mean, "background radiation"? And why shouldn't I worry - it's still radiation, isn't it?

OO Yes...

RR Well then!

OO But there's only a little bit of it. It won't hurt you.

RR Why not? It's still radiation.

OO But it's all a matter of degree. The danger Increases with the strength of the radiation, and the little bit that we get as background won't do you any harm. You've already lived with It all your life.

RR What?

OO Ever since you were born - before, even - you've- been exposed to the background radiation. You look OK to me.

RR But, what is background radiation?

OO It's Just radiation that occurs naturally. You were right when you said It comes from the ground, and the stuff in the air Just comes from space. That's called cosmic radiation, by the way.

RR Whereabouts in space?

OO Nobody's really sure, but the radiation that comes from the ground is easier to explain - It comes from the rocks decaying.

RR Doing what?

OO Decaying. Well, I suppose it's the atoms that are decaying really.

RR What?

OO I wish you'd stop saying "What?" It gets boring after a while.

RR But I don't understand. What do you mean, atoms decaying?

OO Exactly that. You do know about atoms, I suppose?

RR Of course! We learnt all about them in Chemistry.

OO Very good! So you know about the nucleus, and the electrons?

RR Yes.

OO And that the nucleus has electrons around it?

RR Yes.

OO And that the nucleus Is made of protons and neutrons?

RR Yes! I'm not stupid!

OO And that protons have a positive charge?

RR Yes! OK! And neutrons don't have any charge and electrons have a negative charge. Satisfied?

OO And that protons and neutrons are collectively called nucleons?

RR Er, yes... I think we did that...

OO And that protons and neutrons are made of quarks?

RR No - hold on a minute - Now you're trying to wind me up! We never did anything about, what did you call them?

OO Quarks.

RR No. Definitely not. What are these, quarks?

OO The things that protons and neutrons are made of.

RR Why do they need to be made of anything? What's wrong with just having protons and neutrons?

OO What's wrong with atoms? But you can't explain decay without quarks.

RR Oh?

OO When atoms decay, they do so in one of three different ways. They're called alpha, beta and gamma radiation for historical reasons.

RR What about cosmic radiation?

OO That's Just the same as gamma, but it comes from space.

RR So why not call it gamma?

OO Historical reasons again. Nobody knew what It was when it was first discovered, and it's too late to change the name now.

RR I don't see why? It's easy to change a name.

OO OK, we'll change yours. How about... Gertrude?

RR No way! OK, you can call them cosmic rays.

OO Thank you. All these rays cause ionization, by the way. You do know what ionisation means, I hope?

RR Of course!

OO Go on.

RR It's like, when an atom loses or gains an electron, so it becomes charged. And a charged atom is called an ion. See! I do know!

OO So you're happy to call them ionising radiation then - the alpha, beta and gamma?

RR What about X-rays?

OO What about them?

RR They cause ionisation too, so they should be included.

OO But they're not.

RR Why not?

OO Historical reasons.

RR That's always your excuse. Whenever something doesn't make sense, you blame it on history.

OO I can't- help it if people discover things in the wrong order.

RR All right! Ionising radiation. Alpha, beta, gamma. You still haven't told me what they are, or what it has to do with quarks.

OO OK. When an atom decays by alpha decay, it throws out two protons and two neutrons...

RR That's a helium atom!

OO A Helium nucleus, would be more accurate, but no. It's just identical to a helium nucleus, but It's called an alpha particle.

RR It is a helium nucleus!

OO No, because there was never any helium there to start with. It's just a consequence of all atoms being made of the same things.

RR You mean, like if you demolished a house and took away enough bricks to make a garage, they wouldn't actually be a garage?

OO A very good analogy. Well done!

RR Thanks. I told you I'm not stupid!

OO I never said you were. But, when the nucleus throws out these things, it means the nucleon number - oh. I expect you call it the mass number in Chemistry - goes down by four, and the proton number...

RR Goes down by two. I can count! And we do call it the proton number, so you needn't bother to mention the name "atomic number". OK?

OO OK. I'm glad to see they teach you something in Chemistry.

RR But, what I don't get is, what happens to the bit that's left? It can't be the same element if the proton number's different, can it?

OO Quite right. It moves two places down the periodic table.

RR But, why does it do it?

OO Some atoms are unstable; they have too much energy. And you can't predict when It's going to happen, for a particular atom. It's a totally random process, although there are some rules which govern the behaviour of large numbers of atoms.

RR Like what?

OO Like. It always takes the same time for half the atoms in a sample to decay, no matter how many you start off with. It's called the half-life.

RR That's stupid! No, wall a minute. If you have a lot to start off with, there's more chance of them decaying, so the rate of decay would be more. As the numbers go down, so does the chance of another one decaying at any particular moment. Yes! It does make sense. But I still don't see why you need quarks.

OO That's for beta decay. A neutron changes into a proton by some rearrangement of the quarks Inside - don't ask how that bit works – but then the atom would have extra positive charge but that isn't allowed so an electron gets created and comes whizzing out with loads of energy. It's called a beta particle.

RR Gets created? You mean an electron just comes into existence, by magic?

OO Not by magic. Science.

RR But there aren't any electrons in the nucleus!

OO Correct.

RR But, you just said...

OO The electron doesn't exist until the moment of decay. It comes from the nucleus but it wasn't there before and it certainly isn't there afterwards!

RR Oh, all right! And I suppose, since one of the neutrons turns into a proton, that the atom goes up one In the periodic table?

OO Correct again. But the nucleon number...

RR Stays the same! Because the total number of protons and neutrons hasn't changed. What about gamma decay?

OO Electromagnetic radiation.

RR I asked about the process, not the radiation. I'd guessed it was electromagnetic because It's part of the spectrum, after X-rays. Hence my earlier question.

OO Oh, we are being clever!

RR Explain gamma decay. Please.

OO Too complicated. Perhaps when you're...

RR Don't you dare say "older"!

OO OK. How about this? Alpha particles don't go far in air and can be stopped by a sheet of paper because they are big and slow and keep colliding with things, giving up their kinetic energy. Beta particles go further because they are small and fast. They need a sheet of aluminium to stop them. Gamma ray are like X-rays but even more so. They go through just about anything. Perhaps not a nice thick piece of lead. Does that make sense?

RR Perfectly. Do you want me to explain it to you?