It is known that a radioactive source emits a single type of radiation. Detectors are placed in the two positions shown. The graph shows how the readings change over time. Which type of radiation does the source emit, and what is its half-life?

Answer:

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