Pre-formation of α-clusters in natural α-decay

A recent investigation into the fundamental mechanism of α-decay with an α-cluster as the starting point has clearly revealed that α-formation and α-emission is one and the same process and that tunneling is not necessary for α-decay. This α-decay without tunneling (ADWT) study provides reliable expressions for the α-decay radius and for the absolute α-preformation probability Pα. ADWT values of Pα for even-even nuclei are qualitatively a decreasing function of the masses of nuclei and quantitatively lie in the range 0.6-0.08 in very good agreement with the experimental values. For naturally α-active nuclei, Pα lies in the range 0.17-0.08 and exhibits shell-effect and deformation effect in the form of a distinct shell-closure at N=126 and enhanced values for nuclei off a shell-closure region, respectively. This is also in accord with experimental observations. The observed trend of Pα values is rather puzzling in that heavy nuclei with low Pαs are spontaneously α-active whereas light and intermediate nuclei with high Pαs are not. A further investigation into the role of α-clusters in the intra-nuclear mechanism of α-formation and α-emission marks out, on the one hand, Coulomb repulsion as the cause of α-decay rather than as a barrier to it, and, yields, on the other hand, a necessary and sufficient condition for spontaneous α-activity of a nucleus. This condition clearly explains why low Pαs and not high Pαs favour natural α-activity. (author)