def get_sigma(database_file_name='', e_min=np.NaN, e_max=np.NaN, e_step=np.NaN, t_kelvin=None):
    """retrieve the Energy and sigma axis for the given isotope

    :param database_file_name: path/to/file with extension
    :type database_file_name: string
    :param e_min: left energy range in eV of new interpolated data
    :type e_min: float
    :param e_max: right energy range in eV of new interpolated data
    :type e_max: float
    :param e_step: energy step in eV for interpolation
    :type e_step: float
    :param t_kelvin: temperature in Kelvin
    :type t_kelvin: float

    :return: {'energy': np.array, 'sigma': np.array}
    :rtype: dict
    """

    file_extension = os.path.splitext(database_file_name)[1]

    if t_kelvin is None:
        # '.csv' files
        if file_extension != '.csv':
            raise IOError("Cross-section File type must be '.csv'")
        else:
            _df = get_database_data(file_name=database_file_name)
            _dict = get_interpolated_data(df=_df, e_min=e_min, e_max=e_max,
                                          e_step=e_step)
            return {'energy_eV': _dict['x_axis'],
                    'sigma_b': _dict['y_axis']}
    else:
        raise ValueError("Doppler broadened cross-section in not yet supported in current version.")
retrieve the Energy and sigma axis for the given isotope

    :param database_file_name: path/to/file with extension
    :type database_file_name: string
    :param e_min: left energy range in eV of new interpolated data
    :type e_min: float
    :param e_max: right energy range in eV of new interpolated data
    :type e_max: float
    :param e_step: energy step in eV for interpolation
    :type e_step: float
    :param t_kelvin: temperature in Kelvin
    :type t_kelvin: float

    :return: {'energy': np.array, 'sigma': np.array}
    :rtype: dict