Photons from the bremstraahlung process at the interaction region travel in the vacuum inside the photon beam pipe. The beam pipe ends at 92.3m away from the interaction region by Exit Window
 where fraction of the bremstraahlung photons convert to e+e-pair.

  Window is made of Al 86.22%; Si 10,8%; Fe 0,21%; Cu 2,5%; Mg 0,27%.

  The window thickness was measured to high accuracy ( results ).

  This allows us to calculate the properties of exit window from the properties of  constituents in the mixture,

  The results are:
  density: 2.82gcm^-3
  thickness: 9.883mm
  X0 of mixture is 23.215g/cm^-2 = 8.232cm
  This means that thickness of window is 0.12X0

  Attenuation length of photons at 20GeV is 10.7cm
  It means that 8.8% of photons with this energy converts to e+e-pairs.

   The data for attenuation were taken from  program XCOM which can be found at http://physics.nist.gov/PhysRefData .

 Disclaimer: Results given at this page are preliminary and might change in the future,