where ν ν is the frequency of assaults on the barrier per second, S is the preformation probability of the cluster at the nuclear surface, and P s is the penetrability of the external barrier. In alpha-like theories S is an overlap integral of the wave function of the three partners (parent, daughter, and emitted cluster). In a fission theory the preformation probability is the penetrability of the internal part of the barrier from the initial turning point R i to the touching point R t. Very frequently it is calculated by using the Wentzel-Kramers-Brillouin (WKB) approximation.