A mechanically refrigerated gas stream (to -120°C) and some useful accessories

A system for cooling a stream of dry air to -120°C is described. Mechanically refrigerated cooling units are used to maintain the cold baths in which heat exchangers are immersed. Adapters for converting a precession camera to a flat-plate oscillation-rotation camera and for mounting crossed polaroids on the precession camera are described. These devices are particularly useful for examining and aligning crystals grown in situ at low temperatures. They can also be used for aligning crystals on the precession camera even when monochromated radiation is used.