Physics GCSE: Radioactivity

Half-life

What is half-life?

Radioactive substances will give out radiation all the time, regardless of what happens to them physically or chemically. As they decay the atoms change to daughter atoms, until eventually there won't be any of the original atoms left.

Different substances decay at different rates and so will last for different lengths of time. We use the half-life of a substance to tell us which substances decay the quickest.

Half-life - is the time it takes for half of the radioactive particles to decay.

It is also the time it takes for the count-rate of a substance to reduce to half of the original value.

We cannot predict exactly which atom will decay at a certain time but we can estimate, using the half-life, how many will decay over a period of time.

The half-life of a substance can be found by measuring the count-rate of the substance with a Geiger-Muller tube over a period of time. By plotting a graph of count-rate against time the half-life can be seen on the graph.

Here's an example:

This would also work if you plotted the number of parent atoms against time.

The longer the half-life of a substance the slower the substance will decay and the less radiation it will emit in a certain length of time.

The following radioactive substances contain 1000 unstable atoms. Below is a small test for you to try.

Click on the up and down buttons get to the number of unstable atoms remaining after the length of time shown.

Using radioactivity

Different radioactive substances can be used for different purposes. The type of radiation they emit and the half-life are the two things that help us decide what jobs a substance will be best for. Here are the main uses you will be expected to know about:

1. Uses in medicine to kill cancer - radiation damages or kills cells, which can cause cancer, but it can also be used to kill cancerous cells inside the body. Sources of radiation that are put in the body need to have a high count-rate and a short half life so that they are effective, but only stay in the body for a short period of time. If the radiation source is outside of the body it must be able to penetrate to the required depth in the body. (Alpha radiation can't travel through the skin remember!)

2. Uses in industry - one of the main uses for radioactivity in industry is to detect the thickness of materials. The thicker a material is the less the amount of radiation that will be able to penetrate it.

Click on the buttons to change the thickness of the metal:

3. Alpha particles would not be able to go through metal at all, gamma waves would go straight through regardless of the thickness. Beta particles should be used, as any change in thickness would change the amount of particles that could go through the metal.

They can even use this idea to detect when toothpaste tubes are full of toothpaste!

4. Photographic radiation detectors - these make use of the fact that radiation can change the colour of photographic film. The more radiation that is absorbed by the film the darker the colour it will go when it is developed. This is useful for people working with radiation, they wear radiation badges to show them how much radiation they are being exposed to.

5. Dating materials - The older a radioactive substance is the less radiation it will release. This can be used to find out how old things are. The half-life of the radioactive substance can be used to find the age of an object containing that substance.

There are three main examples of this:

i) Carbon dating - many natural substances contain two isotopes of Carbon. Carbon-12 is stable and doesn't disintegrate. Carbon-14 is radioactive. Over time Carbon-14 will slowly decay. As the half-life is very long for Carbon-14, objects that are thousands of years old can be compared to new substances and the change in the amount of Carbon-14 can date the object. 

ii) Uranium decays by a series of disintegrations that eventually produces a stable isotope of lead. Types of rock (igneous) contain this type of uranium so can be dated, by comparing the amount of uranium and lead in the rock sample.

iii) Igneous rocks also contain potassium-40, which decays to a stable form of Argon. Argon is a gas but if it can't escape from the rock then the amount of trapped argon can be used to date the rock.