In industry

To control the thickness of metal or paper

Radioactive isotopes are used in industry to control the thickness of metal or paper as it is rolled into thin sheet.

An emitter is placed on one side of a sheet and a detector on the other.

If the thickness of the sheet remains constant the activity will not change.

If there is a change in thickness, the activity increases or decreases.

This can trigger the rollers to squeeze harder or less hard to maintain the correct thickness.

A beta source is used because beta radiation can penetrate paper or thin aluminium, but the amount of penetrating will vary sufficiently as thickness changes.

The source should have a long half-life so that the count rate remains almost constant each day and so that it does not need to be replaced too frequently.

Graphic shows how radiation absorbed by aluminium foil is used to gauge the thickness of a material by measuring the radiation that passes through. The foil is between an emitter and a detector.

To check for leaks in water pipes

Water supplies can be contaminated with a gamma-emitting radioactive isotope to find leaks in pipes.

Where there is a leak, contaminated water seeps into the ground, causing a build-up of gamma emissions in that area.

The build-up of gamma emissions can be found using a Geiger-Muller tube.

This makes it easier to decide where to dig to find the leak without having to dig along the whole length of the pipe.

Gamma rays concentrated at the pipe leak, underneath the a normal residential road.

The isotope used for this purpose must:

  • be a gamma emitter to penetrate the ground and road surface;
  • have a half-life of at least several days to allow the emissions to build up in the soil but not too long so that exposure is limited not be poisonous to humans as it will form part of the water supply.