Franz, The gap I was remembering from the beam line simulation is the gap inside the second sweep magnet that is just downstream of the secondary collimator. That sweep magnet gap is indeed 50 mm. It was made that large to allow room for the vacuum pipe that starts just after the secondary collimator and carries the collimated photon beam down to the gluex target entrance, which currently has an interior diameter of 44 mm. The pair spectrometer vacuum box is inserted into that photon beam pipe, but it does not need to have the same height as the beam pipe interior diameter, currently 44 mm. The secondary collimator diameter is only 10 mm. My guess is that the beam pipe downstream of the secondary collimator should be roughly twice that to avoid scraping from secondaries. This would say that a 20mm gap through the pair spectrometer would be fine, assuming that it is aligned with ~1mm accuracy. Richard Jones
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