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 radioactive nuclei present. the fundamental unit of radioactivity is the curie (Ci) where 1Ci corresponds to exactly 3.7x10^10 disintegrations per second. this decay rate is equivalent to that of 1g of radium-226. Calculate the rate constant and half life for the radium decay. starting with 2.6g of the radium sample, what is the activity after 450yr? The molar mass of Ra is 226.03 g/mol.