def _c(self, s):
        """Part of the LDF

        As given in Fokkema2012 eq 7.3.

        :param s: shower age parameter.
        :return: c(s)

        """
        r0 = self._r0
        return (gamma(4.5 - s) /
                (2 * pi * r0 ** 2 * gamma(s) * gamma(4.5 - 2 * s)))