docs: def Nasv(macs,T):
      ''' 
      Returns
      -------
      Na*<sigma v>
          for MACS [mb] at T [K].
          
      '''
  
      Na = avogadro_constant
      k  = boltzmann_constant
      vtherm=(2.*k*T/mass_H_atom)**0.5
  
      s  = macs*1.e-27
      Nasv = s*vtherm*Na
      return Nasv