The activity of a radioactive isotope is the number of nuclei in the isotope that disintegrate per second. The activity of an isotope is measured in becquerel (Bq). 1 Bq is 1 disintegration per second.The activity of a radioactive isotope is proportional to its mass and decreases exponentially over time. The activity can be measured using a Geiger tube and Geiger counter and is proportional to the count rate or number of counts per second.