Cesium is listed in the Nuclear Table; its atomic number is Z = 55. You will find that the half-life of Cesium-137 is 30 years. Thus, if I have 4 kg at first, I will have half, or 2 kg, at the end of a 30-year period.
When another 30 years goes by, the 2 kg is reduced by half again, and so I have 1 kg. After a total of 60 years, the initial 4 kg is reduced to 1 kg.