Which traditional unit is used to measure the activity of a radioactive substance?

The traditional unit used to measure the activity of a radioactive substance is the Curie (Ci). The Curie was originally defined based on the activity of one gram of radium-226, which emits a specific amount of radiation. One Curie corresponds to 3.7 x 10^10 disintegrations per second. This historical unit set the groundwork for measuring the radioactive decay of substances, making it a widely recognized standard in both scientific and industrial contexts.

In contrast, while the Bequerel (Bq) is also a unit of radioactivity, representing one disintegration per second, it is a modern SI unit that has largely replaced the Curie in scientific literature. The other units mentioned—Sievert (Sv) and Gray (Gy)—are associated with the biological effects of radiation (dose) and the absorbed dose of radiation, respectively, rather than measuring the activity of radioactive materials.