The entire volume of the SOS spectrometer, from the entrance window in the front of the quadrupole to the exit window at the exit of the second dipole, inside the detector hut, can be evacuated. This in order to minimize the effects of multiple scattering on the SOS performance. As is the case with HMS vacuum can, the entrance and exit are covered by mylar-kevlar composite vacuum windows.
The spectrometer vacuum is maintained by a single Leybold 1000 liter turbo pump and the vacuum is read with a cold cathode gauge. This can be displayed on one of the TV monitors in the Hall C counting house.
Since the exit window of the SOS vacuum system has under 1/3 the stress of the large HMS exit window, and has a very sizable safety margin, no kevlar ``Blast Shield" has been installed. Signs indicating the presence of thin vacuum windows have been posted in the shield house and at the pivot point.