The others are radioactive solids. Science teaches us that everything is made of tiny little particles called atoms. They are too small to be seen even under a powerful microscope. During radioactive decay, two types of tiny electrically charged particles are given off, travelling very fast. They are called alpha and beta particles.
Some radioactive materials are alpha emitters, and others are beta emitters.
In addition, highly energetic rays called gamma rays are often emitted. Gamma rays are not material particles at all, but a form of pure energy very similar to x-rays, travelling at the speed of light. Gamma rays penetrate through soft tissue just as light shines through a window. Beta particles have less penetrating power, travelling less than two centimeters in soft tissue. Alpha particles have the least penetrating power, travelling just a few micrometers in soft tissue, equivalent to a few cell diameters. . . .