Reminders on the radio-activity:
The matter is made of atoms. Each atom has a nucleus composed of protons and neutrons. The core is surrounded by electrons orbiting like satellites. Radioactive materials spontaneously emit radiation Gamma or X and / or beta particles or sometimes alpha.
Beta particles are electrons. Because of their charge, they react strongly with matter. They travel a few centimeters to several meters in the air. Aluminum foil can stop them.
Alpha particles are helium nuclei, or 2 protons and 2 neutrons. They travel a few centimeters in air, a sheet of paper can stop them.
Gamma rays are electromagnetic radiation formed during physical phenomena occurring within the nucleus of the atom. They are capable of producing ions directly or indirectly (atoms or molecules of non-zero electric charge) when they pass through matter. They can travel tens of meters of the air age. Their penetration may be very large, very thick lead and concrete and strongly attenuates.
X-rays Gamma rays are similar to but trained in physical phenomena occurring within the electron cloud of the atom. They are used in medicine and rarely found in nature.