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Radioactivity

Thickness Control.

The amount of radioactivity 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 detector below it.

Some of the radioactivity is absorbed by the foil
and some passes through to the detector.
The thicker the foil, the less radioactivity passes through it to the detector.
The amount of radioactivity arriving at the detector
is monitored by the computer.
The thickness of the foil is controlled by the gap between the rollers.

If the foil is too thick, less 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 smaller.
The smaller gap makes the aluminium foil thinner
until the amount of radiation at the detector rises up to its normal level.

If the foil is too thin, more radiation arrives at the detector.
The computer sees that the level of radiation is too high
and sends a signal to the rollers to make the gap bigger.
The bigger gap makes the aluminium foil thicker
until the amount of radiation at the detector falls back to its normal level

The radioactive source is a b emitter (beta emitter).
This has the right amount of penetration to be useful
for thickness control of aluminium foil.
A b emitter with a half-life of many years would be used so that
1.  The count rate wouldn't keep changing each day.
2.  The radioactive source would not have to be replaced very often.

If an a emitter (alpha emitter) was used, all of the a-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 g emitter (gamma emitter) was used,
all of the g-waves 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.

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

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Copyright © 2008 Dr. Colin France. All Rights Reserved.