Halflife Page

THE HALF-LIFE OF RADIOACTIVE ELEMENTS

Radioactive Decay

When an unstable nucleus of an atom emits radiation it loses energy and becomes stable (no longer radioactive).

If the radioactivity from a sample of a radioactive element were measured using a Geiger Muller tube the number of clicks heard over a period of time would drop. This is because each time an unstable nucleus emits radiation and becomes stable there are less unstable nuclei left in the sample.

We say that the sample of radioactive element has decayed.

The animation below shows the decay of a radioactive sample of Carbon-14 to stable Nitrogen by the emission of beta particles.

You cannot predict which unstable nucleus will emit. It is a random process.

How is Radioactive Decay Measured?

The unit of radioactive decay is the Becquerel (named after Henri Becquerel - the discoverer).

The symbol for the Becquerel is Bq.

1 Bq = 1 unstable nucleus decaying (emitting radiation) per second.

If a radioactive sample has a radioactivity of 8 Bq it means that every second eight unstable nuclei in the sample decay.

When using a Geiger Muller tube we talk about the activity of a sample and the count rate.

If a Geiger counter clicks 50 times every minute we say the count rate is 50 counts per minute.

If a radioactive sample gives a high count rate we say the sample has a high activity.

Over a period of time the count rate will fall. We say the activity of the sample drops.

What is the Half-life of a Radioactive Element?

The activity of different radioisotopes falls at different rates.

Some radioisotopes take only a few seconds for nearly all the unstable nuclei to decay. Others take days, weeks, months, years and even billions of years to decay.

To compare the activity of different radioisotopes we talk about the half-life of a radioisotope.

The half-life of a radioisotope is the time it takes for half of the unstable nuclei present to decay.

The first picture shows a sample of Carcon-14 at 0 years.

The second picture shows the sample of Carcon-14 after 5730 years.

This shows that it takes 5730 years for half of the unstable nuclei of Carbon-14 atoms in the sample to decay to stable Nitrogen atoms.

We say that the half-life of Carbon-14 is 5730 years.

The picture below shows how long it takes Phosphrous-32 to decay into Sulphur-32.

You can see that every 14 days half of the unstable nuclei of the Phosphrous-32 atoms decay into stable Sulphur-32 atoms. We say the half-life of Phosphrous-32 is 14 days.

The picture below shows how long it takes Iodine-131 to decay into Xenon-131.

You can see that every 8 days half of the unstable nuclei of the Iodine-131 atoms decay into stable Xenon-131 atoms. We say the half-life of Iodine-131 is 8 days.

Table showing the half-life of some common radioisotopes.
Radioisotope	Half-life
Radon-222	4 days
Strontium-90	28 years
Radium-226	1602 years
Plutonium-239	24 400 years
Uranium-235	700 000 000 years
Remember: Radon-222 means a radon atom has a mass number of 222.

Another way to think of half-life is the time it takes for the activity (or count rate) of a radioactive sample to fall by half (for example from 50 counts per minute (CPM) to 25 counts per minute.

A short half-life means the activity falls quickly (for example Radon-222).

A long half-life means the activity falls slowly (for example Uranium-235).

If a graph of Activity (vertical axis) against Time (horizontal axis) is plotted it can be used to determine the half-life of a radioactive isotope.

The graph below shows how the radioactivity of a radioisotope varies with time. The activity (in counts per second) at the start is 80 CPS. When the activity has fallen to 40 CPS it has dropped by half. Using the graph we can see that this takes almost 6 days so the half-life of the radioisotope is 6 days.

The activity never reaches zero. It slows down as time passes but in terms of half-life the number of unstable nuclei just keeps dividing by two. You cannot divide zero by two. You just get zero.

Remember: when plotting graphs like this the level of background radiation detected by the Geiger Muller tube must be subtracted from all the results.

VIEW CONTENTS