A LOW TEMPERATURE SYSTEM FOR AUTOMATIC SINGLE CRYSTAL DIFFRACTOMETRY

An automated low temperature system, compatible with the PAILRED and other automatic diffractometers, has been developed and put into operation. Temperatures from ambient to -180°C can be maintained within ±0·2 degC for the hundreds of hours required for the collection of complete three-dimensional intensity data. Frost formation has been eliminated by surrounding the diffractometer with an enclosure within which the moisture content is 1 p.p.m. The precautions necessary to achieve this degree of dryness are discussed. An electropneumatic proportional control system maintains temperature by continuously adjusting both the flow rate of gaseous nitrogen through a pneumatic cryogenic valve and the boiling rate of liquid nitrogen. Temperature is sensed by a thermocouple close to the crystal and compared with a pre-selected temperature. Control action is proportional to the deviation signal. An all metal flexible transfer line conducts the nitrogen to the crystal. This line is continuously evacuated by small VacIon pumps producing a low temperature vacuum of 30 ntorr. Periodic transfer of liquid nitrogen from large commercially available reservoirs to the working Dewer is completely automatic and produces no significant upset of the established temperature. The method of temperature calibration by means of solid-liquid equilibration of organic fluids sealed in capillary tubes is discussed. Characteristic results for organic crystals are presented.