Alpha particles, beta particles, and gamma-rays originate in nuclei and can be detected using a simple hand-held counter like the one shown at the right in the photograph below. The sources shown in the photograph are an 241Am alpha particle source (second from left in front), a 90Sr beta particle source (left front), and a natural uranium ore source (primarily 238U) for gamma-rays (left at rear). The uranium ore is encased in plastic.
If something is placed between the radioactive source and the detector, the particles might be stopped. The photograph also shows three possible "stoppers" of nuclear radiation: a lead brick (right rear), a thin sheet of lead (right front), and a piece of paper (on the lead sheet).
The question is which "stopper" will work for each of the sources:
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