An industrial thickness gauge is used to monitor and control the thickness of aluminium sheets (1.0 mm thick) during a continuous manufacturing process, as shown in the diagram below.
Which radioactive source and half-life combination is most suitable for this application?
An alpha emitter with a half-life of 5 days5\text{ days}5 days
A beta emitter with a half-life of 30 years30\text{ years}30 years
A gamma emitter with a half-life of 30 years30\text{ years}30 years
A beta emitter with a half-life of 2 hours2\text{ hours}2 hours