with full regard to conductive and radiative leaks using eg neck-tube plugs and super-insulation, helium evaporation rates as low as 0.01 ~ h -1 can be obtained.
An interesting balance between the two cooling methods which combines the advantages of both, has recently been developed.9, lo In this design the compact nature and simple operation of the CF is linked with the low 4.2 K consumption of an RC by having the CF unit located directly above the helium storage vessel, thus effectively eliminating the transfer tube. MOssbauer spectroscopy is particularly suited to this cryogenic arrangement. A greater number of such efficient applications are possible if the experimenter is prepared to adapt his apparatus around this arrangement eg by raising apparatus (spectrophotometer table, electromagnet etc) to accommodate a storage vessel below.
Conclusions
Comparison between a cooling system which employs continuous helium flow and the equivalent helium reservoir cryostat shows that a CF unit is more suited to short-term experiments < 10 h of relatively low cooling power requirements < 1 W with the RC suited to longer term experiments. The CF unit becomes relatively more efficient for long-term experiments at higher temperatures T > 20 K. The continuous flow method of cooling suits awkward, restricted geometries which are not generally amenable to reservoir cryostats.