8
2.1
Sample flow unit
The dewar leg of the sample flow unit is immersed in the liquid nitrogen in the storage
dewar. A heater at the bottom of the dewar leg boils nitrogen. The cold gas passes up
the flexible stainless steel vacuum-insulated transfer tube into the coldhead. The
coldhead contains a heat exchanger and heater. The gas then passes through the nozzle
onto the crystal, forming the
sample flow
. The temperature of the sample flow can be
regulated to any value between about 90 K and 300 K by the heat exchanger, using a
temperature sensor in the nozzle.
A vacuum valve is situated at the top of the rigid section of the transfer tube, from which
the Cryojet can be pumped to maintain the required thermal insulation.
2.2
Shield unit
The shield unit consists of a rigid vacuum-insulated tube that is inserted into the dewar. A
heater at the bottom boils nitrogen. At the top of the unit there is a heat exchanger, with
a heater and temperature sensor, which heats the nitrogen to room temperature. The
nitrogen flows through the central stub of the shield unit to the coldhead via a length of
polythene tubing. This
shield flow
then flows through the outer nozzle, preventing
atmospheric water vapour from icing up the sample or the nozzle.
2.3
Stand
The Cryojet is supported via a stand that is adjustable to suit the geometry of the user’s
experiment.
2.4
Storage Dewar
A 75 l storage dewar is used to store the liquid nitrogen used for the sample flow and
shield flow. The dewar has a top fitting with five ports as follows, shown in Figure 3.
•
51 mm hole: sample flow unit dewar leg
•
38 mm hole: shield unit
•
Non-return vent valve
•
13 mm bent tube: for filling the dewar with liquid nitrogen
•
stepped straight tube: for fitting level probe or pressure relief valve
2.5
Controller
The controller controls the gas flows and the temperature of the jet.
