docs: 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.")