docs: def __get_sigmas(self):
          """will populate the stack_sigma dictionary with the energy and sigma array
          for all the compound/element and isotopes"""
          stack_sigma = {}
          _stack = self.stack
  
          _file_path = os.path.abspath(os.path.dirname(__file__))
          _database_folder = os.path.join(_file_path, 'reference_data', self.database)
  
          _list_compounds = _stack.keys()
          for _compound in _list_compounds:
              _list_element = _stack[_compound]['elements']
              stack_sigma[_compound] = {}
  
              for _element in _list_element:
                  stack_sigma[_compound][_element] = {}
                  _list_isotopes = _stack[_compound][_element]['isotopes']['list']
                  _list_file_names = _stack[_compound][_element]['isotopes']['file_names']
                  _list_isotopic_ratio = _stack[_compound][_element]['isotopes']['isotopic_ratio']
                  _iso_file_ratio = zip(_list_isotopes, _list_file_names, _list_isotopic_ratio)
                  stack_sigma[_compound][_element]['isotopic_ratio'] = _list_isotopic_ratio
  
                  # _dict_sigma_isotopes_sum = {}
                  _sigma_all_isotopes = 0
                  _energy_all_isotpes = 0
  
                  for _iso, _file, _ratio in _iso_file_ratio:
                      stack_sigma[_compound][_element][_iso] = {}
                      _file = os.path.join(_database_folder, _file)
                      _dict = _utilities.get_sigma(database_file_name=_file,
                                                   e_min=self.energy_min,
                                                   e_max=self.energy_max,
                                                   e_step=self.energy_step)
                      stack_sigma[_compound][_element][_iso]['energy_eV'] = _dict['energy_eV']
                      stack_sigma[_compound][_element][_iso]['sigma_b'] = _dict['sigma_b'] * _ratio
                      stack_sigma[_compound][_element][_iso]['sigma_b_raw'] = _dict['sigma_b']
  
                      # sigma for all isotopes with their isotopic ratio
                      _sigma_all_isotopes += _dict['sigma_b'] * _ratio
                      _energy_all_isotpes += _dict['energy_eV']
  
                  # energy axis (x-axis) is averaged to take into account differences between x-axis of isotopes
                  _mean_energy_all_isotopes = _energy_all_isotpes / len(_list_isotopes)
                  stack_sigma[_compound][_element]['energy_eV'] = _mean_energy_all_isotopes
                  stack_sigma[_compound][_element]['sigma_b'] = _sigma_all_isotopes
  
          self.stack_sigma = stack_sigma