How can Radioactivity be used to Control the Thickness of a Material?

The amount of which passes through a material can be detected and used to control the thickness of the material.

The manufacture of aluminium foil (for cooking) is a good example. A radioactive source is placed above the foil and a below it.



Thickness Control using Radioactivity

Some of the radiation is by the foil and some radiation passes through to the detector. The thicker the foil, the radiation passes through to the detector. The amount of radiation arriving at the detector is monitored by the computer. The computer sends a signal (called feedback) to control the between the rollers. The gap between the rollers controls the thickness of the foil.


How does the Feedback Control the Thickness of the Material?



If the foil is too thick, radiation arrives at the detector. The computer sees that the level of radiation is too low and sends a signal to the rollers to make the gap . The smaller gap makes the aluminium foil thinner until the amount of radiation at the detector to its normal level.


The thickness of paper can be controlled in the same way.


What Radioactive Source is used for Thickness Control?

The radioactive source is a emitter. This has the right amount of penetration to be useful for thickness control of aluminium foil.

A beta emitter with a half-life of years would be used so that

1. The count rate would stay almost constant each day.

2. The radioactive source would not have to be replaced very often.

If an emitter was used, all of the particles would be stopped by the foil and none would reach the detector. The computer would not be able to control the foil's thickness.

If a emitter was used, all of the would pass through the foil and reach the detector. The computer would not be able to tell if the foil was too thick or too thin.