Muon Veto Insert - expansion section

The expansion section, or `barrel' comes above the fibreglass tube and above the cryostat proper. It makes room for the light guides to split apart, and so make room for vacuum seals, photomultiplier housings, and the sample rod plug on the main sealing flange.

The muon light guide transforms adiabatically to a circle as it bends, whereas the veto counter's guide becomes circular abruptly after it finishes bending. The circular sections then pass through the main end flange via sliding seals.