def calculate_ldf_value(self, r, n_electrons=None, s=None):
        """Calculate the LDF value

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

        """
        if n_electrons is None:
            n_electrons = self._n_electrons
        if s is None:
            s = self._s
        return self.ldf_value(r, n_electrons, s)