<Dd> N is number of atoms of the parent isotope in the sample at time t (the present), given by N (t) = N e, and </Dd> <Dd> λ is the decay constant of the parent isotope, equal to the inverse of the radioactive half - life of the parent isotope times the natural logarithm of 2 . </Dd> <P> The equation is most conveniently expressed in terms of the measured quantity N (t) rather than the constant initial value N . </P> <P> The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature . This is well - established for most isotopic systems . However, construction of an isochron does not require information on the original compositions, using merely the present ratios of the parent and daughter isotopes to a standard isotope . Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition . </P>

What element is used to determine the age of rocks