The decay rate R at a certain time t and the number of undecayed nuclei N at the same time are related by R = λN
The decay rate of a sample is experimentally measurable quantity and is given a specific name: activity. The SI unit for activity is becquerel, named after the discoverer of radioactivity, Henry Becquerel.
1 becquerel is simply equal to 1 disintegration or decay per second.
There is also another unit named “curie” that is widely used and is related to the SI unit as:
1 curie = 1 Ci = 3.7 × 1010 decays per second
= 3.7 × 1010 Bq.
