We have an Oxford Cryostream cooler for Low temperature setup which uses liquid nitrogen to cool the crystal. It also keeps the crystal under an inert atmosphere. The combination of these two functions help overcome the problems that can arise when the crystals:
Liquid Nitrogen is sucked out of an open Dewar, along a flexible transfer line into the Coldhead where the liquid is evaporated to gas. The area inside the Coldhead is cooled to about 80K as the cold gas passes along one path of a heat exchanger. This means that the Cryostream has created a cold environment a matter of centimeters from the crystal instead of at the bottom of a storage Dewar which could be more then a meter from the crystal.
For more details, go to the Oxford Cryostream homepage.