Parallel decrease of the number of nuclei and activity
The activity and the mass of a radioactive body diminish at the same rate (the mass has been expressed in terms of the number of atoms in order for it to be placed on the same scale). The radioactive body chosen is iodine 123, and the initial activity (of 1 millicurie) is equivalent to that of a thyroid scintigraphy. The half-life of iodine 123 is of 13.2 hours, which means that the decay takes place over a few days. It takes about 2 weeks for the activity to drop from 37 million becquerels to 1 becquerel, and 23 days for the last nucleus of iodine 123 to disappear.
IN2P3