A research institute prepares a gamma-emitting tracer for a coastal silt survey. The tracer has an activity of 640 MBq640\ \mathrm{MBq}640 MBq when it is prepared and a half-life of 6 hours.
Four technicians each calculate the activity of the tracer at a different time after it is prepared.
| Technician | Time after preparation in hours | Calculated activity in MBq\mathrm{MBq}MBq |
|---|---|---|
| W | 6 | 160 |
| X | 12 | 320 |
| Y | 18 | 80 |
| Z | 24 | 160 |
Which technician's calculation is correct?
Technician X
Technician W
Technician Z
Technician Y