The activity of a radioactive material is measured by the number of disintegrations that occur in it every second. The activity also measures the number of radiation emitted per second. The more a substance is active, the more radiation it emits. The alpha or beta activity is equal to the number of alpha or beta decays. The gamma activity is in proportion of the average number of gamma emitted by disintegration. Previously, the activity was compared to a standard, the activity of one gram of radium, while today we consider the rates of decay or radiation.