Question

How many years are required for 28 mg of cesium-137 to decay to 3.5 mg of cesium-137?

Answer 1

`28 mg` of cesium-137 will decay to `3.5 mg` of cesium-137 in `90.5` years (3 times the half life)

Explanation:

The half life of cesium-137 is approximately `30.17` years (`+-0.03`)

`log_2 (28/3.5) = log_2 (8) = log_2 (2^3) = 3`

So it will take `3` times the half life for the sample to decay from `28mg` to `3.5mg` (one eighth).

`3 * 30.17 ~~ 90.5`

This video shows a sample of Cs-137 being tested by a hand-held detector.