Mounting a 10 K cooling device without rotating seals on a four-circle diffractometer

Abstract

Instrumentation has been developed for collection of three-dimensional X-ray and neutron diffraction data at temperatures below the boiling point of liquid nitrogen. A closed-cycle two-stage refrigerator has been mounted on the ϕ shaft of a four-circle goniometer. The sample crystal is attached to the nominal 10 K station. The refrigerator mount functions as a goniometer head without arcs. The x−y−z centering device has a range of ± 5 mm for the z direction along the ϕ axis and ± 2 mm along x and y. Dovetails and eccentrics with narrow tolerances allow a precise firmly lockable centering of the crystal. The whole refrigerator unit undergoes all the normal ϕ, χ and ω movements during data collection without any rotating vacuum seals. These are usually integral vulnerable parts of existing liquid-helium-temperature cryocoolers. When refrigeration is not required, the cryo-unit can be dismounted in a few minutes and replaced by a device for mounting an ordinary goniometer head for room-temperature data collection. Thus, the system is simple, robust and easy to operate, both at room temperature and at cryogenic temperatures. Furthermore, the use of a closed-cycle refrigerator makes the system more economical than comparable cryocooler systems based on flow cryostats.