The RADEX Geiger counter RD1503 is a particularly sensitive detector of Beta particles, and X and Gamma rays (called ionizing rays). It measures the Beta particles and ionizations created by X and Gamma rays to indicate the 'quantity of energy' transmitted to matter over a certain amount of time.
The measurement units which are used are micro Rems per hour (µRem/h) or micro sieverts per hour (µSv/h). They correspond to the dose received by the human body over a period of one hour.
Matter is made up of atoms. Each atom consists of a nucleus which is surrounded by neutrons which gravitate around it like satellites. Radioactive substances spontaneously produce Gamma or X and/or Beta particles or sometimes Alpha particles.
Beta particles are electrons. Because of their charge, they react very strongly to matter. They can travel from a few centimetres to a few metres through the air. A sheet of aluminium can stop them.
Alpha particles are helium nuclei (2 protons and 2 neutrons). They can travel a few centimetres through the air. A sheet of paper can stop them.
Gamma rays are electromagnetic rays formed during physical phenomena which take place within the nucleus of the atom. Gamma rays can produce ions directly or indirectly (ions are atoms or molecules with an electric charge which is not nil) while they travel through matter. They can travel dozens of metres through the air. A thick quantity of lead and concrete can significantly reduce their impact.
X rays are similar to Gamma rays but are formed by physical phenomena occurring within the electronic structure of the atom. They are used for medical purposes and are rarely found in nature.