Nuclear radiation is the emission of energy in the form of particles or waves from the nuclei of unstable atoms. The different types of nuclear radiation are classified based on their mass, charge, and penetrating power.

Alpha particles are positively charged helium nuclei, consisting of two protons and two neutrons. They are the heaviest and least penetrating type of nuclear radiation, and can be stopped by a sheet of paper or a few centimeters of air.

Beta particles are high-energy electrons or positrons (positively charged electrons). They are less massive than alpha particles and can penetrate deeper into matter, but can be stopped by a few millimeters of aluminum or a few meters of air.

Gamma rays are high-energy photons, which are packets of electromagnetic radiation. They are massless and have the greatest penetrating power of all types of nuclear radiation, and can only be stopped by thick layers of lead or concrete.

X-rays are also a type of electromagnetic radiation, but they have lower energy than gamma rays and are less penetrating. They can be stopped by a few centimeters of aluminum or a few meters of air.