Question

How can alpha particles be detected?

Answer 1

Zinc sulfide can detect alpha particles.

Ernest Rutherford used a screen covered with zinc sulfide in his famous gold-foil experiment which led to the discovery of the nucleus. The zinc sulfide would emit light when hit by alpha particles.

Here is an article about Rutherford's experiment.
http://galileo.phys.virginia.edu/classes/252/Rutherford_Scattering/Rutherford_Scattering.html

Hope this helps!