Alpha particles are easily absorbed. A household smoke alarm measures the movement of alpha particles across a small gap. If smoke enters the detector, it will absorb the alphas and the detector will measure a drop in the number getting across the gap. This drop in measurement will trigger the alarm to sound.
Americium-241 is the most common material used as the alpha emitter in a domestic smoke alarm.
Radiation is absorbed as it passes through materials. This allows it to be used to gauge the thickness of a material by measuring the amount of radiation that passes through.
This is commonly used with beta minus emitters in industry, for paper milling and the production of aluminium foil.
If the foil is too thick it absorbs more beta particles. The detector receives less beta particles and then sends a signal to the rollers to increase the force on the foil, making it thinner.
If the foil is too thin it absorbs less beta particles. The detector receives more beta particles and then sends a signal to the rollers to decrease the force on the foil, making it thicker.