def METHOD_NAME(self):
    if self.domain == "negative":
        x = pybamm.standard_spatial_vars.x_n
    elif self.domain == "positive":
        x = pybamm.standard_spatial_vars.x_p
    inputs = {"Through-cell distance (x) [m]": x}
    Domain = self.domain.capitalize()
    return pybamm.FunctionParameter(
        f"{self.phase_prefactor}{Domain} particle radius [m]", inputs
    )