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Lesson 3: Half-life part 1


In this lesson we’ll see that radioactivity tends to decrease over time and that this has problems as well as benefits.  We’ll see that for some jobs we want the radioactivity to stop quickly.  For others we choose a substance that’s radioactive for a long time.

Try Why Do Astronauts Float by Julian Hamm

We won’t explain the exact meaning of half-life or why radioactivity decreases over time just yet.

Half-life gives you an idea of how long a substance will be radioactive for

Some substances lose their radioactivity quickly.  Some take a long time.  Radioactivity always decreases with time but sometimes it appears to increase because other radioactive substances are produced.

In theory the radioactivity of a source never reaches zero but in it does.

The longer the half-life the longer the material will be radioactive for.  We’ll define half-life more precisely in lesson 15.

Radioactivity depends on half-life and how big the sample is

The shorter the half-life the more radioactive a sample of radioactive material will be.

Also the bigger the sample the more radioactive it is.  In absolute terms doubling the amount of radioactive material doubles the radioactivity but this is often not what you'd find if you tried to measure it.

The activity above uses a logarthmic scale for time so we can fit very short and very long times on the same scale.

The most dangerous half-life is a few tens of years

After five half-lives only about 3% of the starting radioactivity will remain.  This is 5 days if the half-life is 1 day, say, or 10 000 years if the half-life is 2000 years.

You'd think that the longer the half-life the more risk there'd be because it stays radioactive for so long.

But you have to remember that long half-life means not very radioactive in the first place.

Radioactive materials with a short half-life are very radioactive but stop being radioactive quite quickly.  Those with a long half-life stay radioactive for hundreds or thousands or millions of years but the radioactivity is low.

The most dangerous radioactive substances have a half-life of a few tens of years.  They can be dangerous in quite small quantities and stay radioactive for more than a human life-time.

Tracers are 'open' sources and have a short half-life

Sometimes radioactive sources are introduced directly into the environment.  These are called ‘open’ sources.  For example, when used as a gamma tracer for finding leaks in an oil pipeline or when injected into a patient for medical imaging.

Because you don’t have too much control over what happens to open sources you want them to have a short half-life.  This means they stop emitting radiation quite quickly.

Short half-life means initially very radioactive so small quantities are normally enough to give the the required amount of radioactivity.

If you don't want radioactivity to change much you need a long half-life

When the radioactive material stays in sealed container, we say the it's a 'closed' source.  We have more control over what happens to a closed source so we don’t mind if the half-life is longer.

We do have to be extremely careful about disposing of closed sources after the machine they are used in reaches the end of its life.

With our thickness gauge example we need the half-life to be quite long so that the beta emissions don’t change much from day to day.  Similarly with the smoke detector we choose an alpha emitter that has a long half-life so the detector will work for a long time.

As well as thinking about the type of radiation and half-life, cost and availability are also important when selecting a radioactive material.

Back to Summary of Radioactivity and Atomic Physics Explained