Half-thickness and fractional decay

What the Activity is for

A half thickness for cucumber.

Half-thickness is a useful and accessible introduction to constant fractional decay. Somehow adding extra slices of thickness, and seeing their effect on a count rate, is more direct than allowing extra intervals of time to pass and seeing their effect on count rate.

The choice of exact fruit or vegetable is left to you, to suit your source and counter. However, we recommend using a fruit because this is biological tissue, so making it easier to connect the effects to something that will be of importance to students.

What to Prepare

• a piece of fruit
• a ruler
• a sharp knife
• chopping board
• a beta or gamma source
• a counter connected to a detector

Safety note: Students under the age of 16 years are not allowed to handle radioactive sources. Please take all reasonable precautions to ensure that the source is guarded at all times.

What Happens During this Activity

During this experiment, equal thicknesses of absorber are placed sequentially between the source and the detector. Start by creating these equal thicknesses of the absorber, slicing up the fruit as you discuss the extra material that will be added between the source and the detector as you add each thickness. Here's a tip: slice off the bottom of the fruit before you start, to make sure that it will stay still once in place. You'll need to practise before you start, to match the density of the fruit with the appropriate thickness of slice and the count rate and penetration offered by your source. Ideally the count rate will drop to close to zero after about seven slices are added.

Now set up the counter and the source a fixed distance apart, so that you can later add seven slices between the source and the detector. Talk through the addition of each slice in terms of the extra chance of the ionising radiation colliding with the material in that slice. Each slice adds a constant effect. So adding each slice reduces the current rate of the ionising radiation by a constant fraction. This is the important pattern. There is no completely safe thickness – each additional thickness produces a reducing return.

You can also do a similar experiment with 3 cm waves and textbooks. Each extra textbook reduces the radiation by a constant fraction.

We do suggest this emphasis on the constant – pause – fractional – pause – decay, rather than emphasising the half-thickness. We think it's better to see the half-thickness as a consequence of, and one measure of, the constant fractional decay rather than as a magic number.