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How can Radioactivity be used to Control the Thickness of a Material?
The amount of radiation 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
A radioactive source is placed above the foil and a detector below it.
Some of the radiation is absorbed by the foil and some radiation
passes through to the detector. The thicker the foil, the less 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 gap 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,
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 increases 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 thickness of paper can be controlled in the same way.
What Radioactive Source is used for Thickness Control?
The radioactive source
is a beta 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 many 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 alpha emitter was used, all of the alpha 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 gamma emitter was used,
all of the gamma
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.
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