def ldf_value(self, r, n_electrons, s):
        """Calculate the LDF value

        Given a core distance, shower size, and shower age.
        As given in Fokkema2012 eq 7.4.

        :param r: core distance in m.
        :param n_electrons: number of electrons in the shower.
        :param s: shower shape parameter.
        :return: particle density in m ** -2.

        """
        if s == self._s:
            c_s = self._c_s
        else:
            c_s = self._c(s)
        r0 = self._r0
        alpha = self._alpha
        beta = self._beta

        return (n_electrons * c_s * (r / r0) ** (s - alpha) *
                (1 + r / r0) ** (s - beta))