A detector of radioactivity shows background radiation to be 20 counts per minute with no radioactive sources nearby. A radioactive source which emits alpha particles and beta particles is brought very close to the detector and the detector reading increases to 280 counts per minute. A sheet of paper is then placed between the source and the detector, and the detector reading drops to 60 counts per minute. How many counts per minute were caused by alpha particles and by beta particles from the source?

Answer:

3) α = 220, ß = 40

Background radiation = 20 units
α + ß = 280 – 20 = 260
We know that when the paper is placed, all α radiation will be blocked.

So,
ß + background radiation = 60
ß + 20 = 60
ß = 40
α + ß = 260
α + 40 = 260
α = 220