Stability (or rate of decay) of a radioisotope is measured in half-life.
The decay of an unstable nucleus is a random event and is independent of chemical or physical conditions.
The half-life of a radioisotope is the time taken for the sample's activity to fall by half, and can be easily calculated.
The mass of a radioisotope falls from 1.6g to 0.1g in 2 hours. What is the half-life of this radioisotope?
If a 1g sample of a radioisotope with a half-life of 3 days has an activity of 32 counts sec-1, how long would it take for the activity of a 2g sample to fall to 8 counts sec-1?