Calculations Using the First Order RateEquation:r = k[N]Since the rate of radioactive decay is first order we can say:r = k[N]1,where r is a measurement of the rate of decay,k is the first order rate constant for the isotope, andN is the amount of radioisotope at the moment when the rate is measured.The rate of decay is often referred to as the activity of the isotope and is often measured in Curies(Ci),one curie = 3.700 x 1010atoms that decay/second.By knowing the amount of radioisotope and the activity of the sample, the rate constant can bedetermined.