Decay withe time of iodine-131and Caesium-137
This graph shows the evolution of the activity of the two main radioactive atoms dispersed during the Chernobyl accident, iodine-131 and cesium-137. One sees the effect of their very different periods (half-lives). Because of its short period of 8 days, iodine has been very active and has prevailed in the early days, but disappeared after 3 months. Cesium is less active because of its 30 years period but has still barely declined since the accident. (Note: logarithmic scales are required to represent the very large variations of activity and the large time scale).