A factory uses a gamma thickness gauge to monitor a moving sheet of rolled steel.
| Quantity | Value |
|---|---|
| Detector reading at the correct thickness | 1420 counts per minute |
| Background count rate | 40 counts per minute |
| Half-life of the source | 6 years |
The detector and background count rate remain unchanged.
Calculate the target detector reading after 12 years.
After 12 years, the detector records 325 counts per minute.
Explain what this reading shows about the thickness of the steel and how the control system should respond.