Two metal trays, X and Y, are involved in separate incidents at a research facility.
Tray X is placed near a sealed gamma source. The source remains sealed throughout and is then returned to a shielded store.
Tray Y is involved in a different incident. It is moved away from the incident area and its surface is then washed.
A detector is placed the same distance from each tray. The count rate is recorded in each condition.
| Condition | Count rate in counts per minute |
|---|---|
| Background, with no tray or source present | 20 |
| Tray X while near the sealed gamma source | 760 |
| Tray X after the source is returned to the store | 21 |
| Tray Y in the incident area | 690 |
| Tray Y after being moved, before washing | 675 |
| Tray Y after washing | 84 |
Using the readings, explain which tray was irradiated and which tray was contaminated. Suggest which tray, if either, needs further cleaning before it is handled.