  Lifetime values for alpha decay in even-even nuclei with $Z=84-98$ and
$N=128-152$ have been calculated in the superasymmetric fission model. The
interaction between the alpha particle and the daughter nucleus has been formed
in the double folding approach using a density dependent NN interaction. The
densities have been obtained using the Relativistic Mean Field formalism. The
spectroscopic factors for the decays have been deduced and are shown to vary
smoothly as a function of effective numbers of valence nucleons, $N_p$ and
$N_n$ chosen with a suitable core. The implication of such a smooth behaviour
has been discussed.
