Radiation is a form of energy that is emitted from the nucleus of an unstable atom. It is composed of high-energy particles or waves and is classified into three main types: alpha, beta, and gamma radiation. Any substance that contains unstable atoms is considered to be radioactive and emits these types of radiation. The intensity of the radiation decreases as it moves away from the radioactive material.
This is because the particles or rays become more scattered. The thicker the substance, the more radiation will be absorbed.