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Misconceptions about radioactivity

Nuclei disappear when they decay

When a nucleus decays it becomes more stable, perhaps by losing some matter, but it doesn't just disappear.

This misconception seems to be perpetrated by some of the models we use for demonstrating half-life, particularly the use of a large number of dice that are thrown and then all the sixes removed.  The dices are normally only removed so you can count and keep track of them but the obvious conclusion drawn by many students is that radioactive decay causes nuclei to disappear.

Half-life is half the time for the radioactivity to disappear

Half-life is correctly defined as the time it takes for radioactivity to decrease by a half.

It's understandable when students get their words confused.  The key point to get across is that radioactivity never really disappears completely, or if it does, you have no idea when that will be.

You get contaminated by radiation

There are two important ideas that you need to distinguish between: contamination and irradiation.  Students tend not to use these words but they implicitly confuse the ideas.

You need to make the distinction between

  1. radioactive stuff - like the dust from a nuclear explosion
  2. nuclear radiation - like alpha, beta and gamma

Radioactive stuff can move long distances if it's blown by the wind or carried in the water.  When some radioactive stuff comes to rest on something we don't want it to, like your head, we say that thing has been contaminated.

Nuclear radiation can't travel long distances in practice.  Alpha particles get stopped by a few centimetres of air, beta by a few metres, and gamma rays, even though they can travel a long way without being absorbed, spread out so much as to be harmless after, say, a few tens of metres.

When nuclear radiation strikes something and transfers its energy to it we say that thing has been irradiated.

So after a nuclear accident your clothing can be contaminated with radioactive dust and the radiation can irradiate your skin.

Radiation makes things radioactive

Many students have the impression that radiation acts like a communicable disease.  When radiation hits something that thing becomes radioactive itself.  This then gives out radiation, passing the radioactive disease on like a spreading virus.

In fact, when something absorbs nuclear radiation it doesn't itself become radioactive.  The only exception to this is if you use a neutron source, which are very rare in nature and normally have to be made.  In this case a minuscule fraction of the nuclei can sometimes absorb a neutron becoming unstable and giving off a particle to settle down.

As far as students are concerned this is never the case with alpha, beta and gamma radiation.

Beta particles are electrons so come from an atom's electron shells

A beta particle is an electron that's created when a neutron changes into a proton.  So a beta particle comes from a change in the nucleus, not from the electrons surrounding it.

Alpha particles cause more ionizations than beta particles

Most books say that alpha radiation is 'more ionizing' than beta radiation but this is perhaps a little misleading.

If we have an alpha and beta particle of the same energy then they'll tend to cause roughly the same number of ionizations before coming to rest (say, 100 000 in air).  This is because both particles have the same amount of energy to 'spend' on ionization and the cost of ionization is more of a function of what's being ionized (i.e. an air molecule) than what's doing it.

But alpha particles tend to lose their energy in a very short distance because they are much more likely to ionize a given atom at a given distance than the equivalent beta particle.

So alpha particles cause ionizations that are concentrated in a fairly short distance whereas with beta particles they are more spread out.

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