docs: def collision_rate(Temperature, element, isotope):
      r"""This function recieves the temperature of an atomic vapour (in Kelvin),
      the element, and the isotope of the atoms, and returns the angular
      frequency rate of collisions (in rad/s) in a vapour assuming a
      Maxwell-Boltzmann velocity distribution, and taking the cross section
      of the collision to be
  
          sigma=pi*(2*r)**2
  
      where r is the atomic radius. colission rate returned is
  
          gamma_col=2*pi* ( sigma * v * n )
  
      where v is the average velocity of the distribution, and n is the
      number density of the vapour.
  
      A few examples (in Hz):
      >>> print collision_rate(25 + 273.15, "Cs", 133)/2/pi
      9.0607260277
  
      For cesium collisions become important for temperatures above 120 Celsius.
      >>> print collision_rate(120 + 273.15, "Cs", 133)/2/pi
      10519.235289
  
      """
      atom=Atom(element,isotope)
  
      sigma=pi*(2*atom.radius)**2
      v=speed_average(Temperature,element,isotope)
      n=vapour_number_density(Temperature,element)
      return 2*pi*sigma*v*n