The activity of a radioactive sample is the number of nuclear disintegrations per second, which is equal to the first-order rate constant times the number of radio-active nuclei present. The fundamental unit of radioactivity is the curie (Ci), where 1 Ci corresponds to exactly 3.70 x 10^10 disintegrations per second. This decay rate is equivalent to that of 1 g of radium-226. Calculate the rate constant and half-life for the radium decay. Starting with 1.0g of the radium sample, what is the activity after 500 years? The molar mass of Ra-226 is 226.03 g/mol.