As they change into more stable forms, they release invisible waves of energy or particles. This is called ionizing radiation. Radioactivity is the emitting of this radiation. Ionizing Radiation Ionizing radiation is energy that can ionize, or electrically charge, an atom by stripping off electrons. But despite their energy, they can travel only a few inches in the air.
Alpha particles lose their energy almost as soon as they collide with anything.
A sheet of paper or your skin's surface can easily stop them. They can travel in the air as much as 100,000 miles per second for a distance of about 10 feet. Beta particles can pass through a sheet of paper, but may be stopped by a thin sheet of aluminum foil or glass. Gamma radiation is very penetrating and is best shielded by a thick wall of concrete, lead, or steel. The radioactivity of a material decreases with time.