code: def lnLambda(self,r,v):
          
          if self._lnLambda:
              lnLambda= self._lnLambda
          else:
              GMvs= self._ms/v**2.
              if GMvs < self._rhm:
                  Lambda= r/self._gamma/self._rhm
              else:
                  Lambda= r/self._gamma/GMvs
              lnLambda= 0.5*numpy.log(1.+Lambda**2.) 
          return lnLambda
docs: NAME:
             lnLambda
          PURPOSE:
             evaluate the Coulomb logarithm :math:`\\ln \\Lambda`
          INPUT:
             r - spherical radius (natural units)
             v - current velocity in cylindrical coordinates (natural units)
          OUTPUT:
             Coulomb logarithm
          HISTORY:
             2018-03-18 - Started - Bovy (UofT)