Radioactivity is a phenomenon in which unstable atoms emit radiation in the form of particles or electromagnetic waves. The unit of measurement for radioactivity is the CURIE (Ci), named after Marie CURIE, who discovered the element radium. A CURIE is equal to 37 billion radioactive disintegrations per second. The rad (rd) is a smaller unit of radioactivity, equal to 0.01 curies. The rem (roentgen equivalent man) is an even smaller unit, equal to 0.001 curies. Therefore, the term radioactivity unit can refer to any of these three units of measurement.