11) C

The detector 2 graph shows us that the background radiation is 20 counts.

In order to find the half life, we need to find the initial count rate of the source.

This is = 220 – 20 = 200

The half life of the activity would be the time take for the count rate to become (200/2) + 20 = 120

This is 2.4 hours.

  • α – radiation stops after 1-2 cm of air. Therefore, this radiation cannot be α.
  • ß – radiation stops after 1m of air. Since detector 2 is 1m away and does not pick up any radiation, the type of radiation must be ß.
  • γ – radiation is not stopped by air.

Type of radiation = γ

Half – life = 2.4 hours