Radioactivity accompanies the Earth since its creation. We have lived in a sea of radiation – light, thermal, ultraviolet and also ionization one. We can sense light and thermal radiation; however, the ionization one is invisible for our senses, even though it is omnipresent. 
   

  
Natural radioactive materials are present in soil, in floors and walls of our houses, schools and offices, in the food we eat and drink, and in the air we breathe. Our bodies themselves contain natural radioactive elements. Cosmic rays have flown to us permanently from the space.

We have been also subject to artificial ionization radiation – from TV displays, from medical apparatuses, from radioactivity released from the industrial use of radioactive materials.

Radioactivity is a technical term for a physical phenomenon related to the decay of atom nuclei, initiated either naturally or artificially, and associated with emission of particles or electromagnetic radiation. Nuclei can be transformed either by alpha, beta, or gamma transmutation.

  
Notwithstanding the fact that radioactivity cannot be detected by human senses, we are able to find it quite easily and very accurately – that means to detect or measure it. Radioactive substances have a very important property – their activity is reduced with time. The time needed for the transmutation of a half amount of nuclei present at the beginning is called halftime of transmutation. Nuclear transmutation is a statistical phenomenon and its probability is the same for all time intervals.