What is the smallest unit of activity in radioactive measurements?

The smallest unit of activity in radioactive measurements is the Becquerel (Bq). This unit is defined such that one Becquerel corresponds to one disintegration or decay event per second. This definition highlights the fundamental characteristic of radioactivity, which is the rate at which unstable nuclei decay. The Becquerel is part of the International System of Units (SI) and serves as a standard unit of measurement for radioactive activity, making it essential for scientific communication and consistency in the field of nuclear science.

In contrast, the Curie (Ci) is an older unit of radioactivity that has a much larger value, equal to 3.7 x 10^10 disintegrations per second, making it less suitable for precise measurements when compared to the Becquerel. The Rutherford (Rd) is another unit of radioactive activity, but it is not commonly used in contemporary practice relative to the Becquerel. Finally, the Sievert (Sv) is a unit of measurement for biological effects of radiation, relating to the health impact rather than the activity itself, thus it is not applicable to measurements of radioactive decay directly. Hence, the Becquerel stands out as the smallest and most frequently used unit for expressing the rate of radioactive decay.