1. Beta and paper mills

2. Beta sources are ideal for use in paper mills to automatically monitor and adjust the thickness of the paper.
The mill is set up such that the paper sheet passes between a beta source and a detector.
The thickness of the paper will affect the rate of absorption of the beta particles and therefore the amount of beta particles received by the detector.

3. The thicker the paper the lower the count at the detector
The thicker the paper the lower the count at the detector. If this occurs the rollers in the mill are squeezed further together to reduce the thickness.
Likewise, if the paper gets thinner the count at the detector increases. If this occurs the rollers are moved further apart to allow the paper to be thicker.
This process of detection and roller movement is done automatically all the time in the mill so the thickness is kept in the correct tolerances.

4. Explain why a source with a long half life is needed in a paper mill.
If the half life was too short then the activity of the source would fall too quickly and the activity at the detector would change due to the source not the thickness of the paper.
For the mill to work properly for a number of years the activity level of the source needs to be fairly constant and therefore it needs to have a long half life.

5. Explain why radioactive sources producing alpha or gamma radiation aren’t suitable for a paper mill.
A source producing alpha particles is not suitable because the majority of them would not get through the paper to the detector. It would be far too insensitive.
A source producing gamma radiation would not be suitable as the paper would not have any noticeable effect on the radiation. The thickness changes of the paper would not make any detectable difference.