In a materials laboratory, an acrylic block is placed near a sealed gamma source for one minute. The source is then returned to a lead-lined container in a separate store.
A detector stays in a fixed position beside the block and counts for 60 s each time.
| Measurement | Count |
|---|---|
| Background, before the source is brought into the laboratory | 21 |
| Beside the block before exposure | 20 |
| Beside the block during exposure | 608 |
| Beside the block straight after exposure | 22 |
Which statement correctly explains these readings?
The gamma rays made the acrylic radioactive, but this new activity had fallen almost to zero before the final reading.
Radioactive material crossed from the sealed source into the acrylic, but stopped emitting when the source was moved away.
The block was irradiated while the source was nearby; no radioactive material was added to it, so the count returned to background.
Putting the source in the lead-lined container stopped its nuclei decaying, so the detector then measured only background radiation.