3.2,2 The Substrate and Sample M ount

A calcium fluoride window (25 mm diameter x 2 mm thick) mounted in a copper mount is used as the substrate. The window sits in a grove in the copper mount and is secured by oversized washers and screws as shown in Figure 3-3.

Th erm ocouple ' mounting

— Silver foil to improve — thermal conduction

A ttachm ent to the cryostat (4 x M 2 .5 screws)

. M 2 screw and w asher to hold the window in place

S crew to mount thermocouple

Calcium fluoride window

Figure 3-3: The sample mount and substrate

The entire copper mount is machined out o f a single piece o f OFHC (oxygen free, high conductivity copper) to avoid any additional interfaces that may affect thermal conduction. The sample mount is attached to the base o f the cryostat heat exchanger by four screws, with thin silver foil sandwiched in between to improve thermal conduction.

Calcium fluoride was chosen as the substrate primarily for the purpose o f transmission spectroscopy. Calcium fluoride has a wide range transmission profile with the high energy cut o ff near 120 nm (10.3 eV), with ~ 50% transmission at Lym an-a (121.6 nm; 10.2 eV), and a low energy cut off near 1000 cm '' (10 pm; 0.12 eV). This means that the same substrate may be used both for UV and IR spectroscopy. Figure 3-4 (a) shows

the infrared transmission spectrum o f the 2 mm calcium fluoride window.

As with most transmitting crystal substrates, calcium fluoride has low thermal conductivity o f 9.71 Wm 'K '' compared to 400 W m ''K '' for copper. This is important to take into consideration when sensitive temperature measurements o f the sample are required. Another important consideration in using calcium fluoride is its low solubility: 0.0017g per lOOg water at 293 K. This is particularly important when water samples are heated to room temperature and above to clean the substrate.

3 Th e Ex p e r im e n t a l Appa r at u sa n d Pr o c e d u r e s 78 100 %T 6 0 0 0 5 0 0 0 4 0 0 0 3 0 0 0 2000 1 0 0 0 4 0 0 7 9 0 0 7 0 0 0 Wa ve n u m b e r [cm 1]

Figure 3-4: Calcium fluoride IR transmission spectra through (a) the substrate, (b) the CF40 port window, (c) the two external windows of the cham ber (d) the

chamber and substrate

3 .3

T

C

S

AND T

C

3.3.1 The Cryostat

The chamber was built around a 250mm long variable temperature flow cryostat (custom built by AS Scientific). The cryostat, built out o f stainless steel and OFHC copper, can be used with liquid helium or liquid nitrogen. It consists o f a heat exchanger and an electrical feedthrough for connections to thermocouples and a 50 Q (1 Amp. max.) coaxial resistive heater.

An OFHC radiation shield is mounted to the lower part o f the heat exchanger to improve base temperatures that can be achieved with liquid helium. So far the base temperature that has been reached using this system is 38 K with liquid helium and 84 K with liquid nitrogen. It is believed that due to the small size o f the chamber there is considerable heat load on the cryostat. Also, the wires to the thermocouple and the heater that are wound around the cryostat are attached to a room temperature electrical feedthrough, which may provide an added heat load on the system. Work is currently

J Th e Ex p e r i m e n t a l Ap p a r at u sa n d Pr o c e d u r e s 79

being carried out to improve the base temperature and a closed cycle helium refrigerator is being purchased for a modified system.

The continuous flow cryostat was initially chosen for flexibility and small size (mounted onto a CF40 flange). Also it is easy to interchange between liquid helium and liquid nitrogen coolants without breaking the vacuum in the main chamber.

3.3.2 Liquid Nitrogen Cooling

Liquid nitrogen flows from a pressurised liquid nitrogen dewar via a custom built transfer line and hose directly into the cryostat insert. A liquid level sensor and a solenoid valve control the flow o f liquid nitrogen into the cryostat. The liquid evaporates and escapes via a port at the top o f the cryostat.

3.3.3 Liquid Helium Cooling

Helium vapour Flow m eter 0 -1 0 0 0 m bar gauge 0 -rin g seal V acuum insulated flexible liquid helium transfer line Exhaust Transfer line cryostat insert Dem ountable coupling with o-ring seal Flow controller box N eedle valve Liquid helium dew ar Diaphragm pump Extension tube coupling Extension tube V acuum cham ber

Figure 3-5: Schematic diagram of the set-up for liquid helium transfer

A flexible coaxial liquid helium transfer line is used for liquid helium cooling. The transfer line supplies the liquid helium to the lower part o f the heat exchanger. The liquid then evaporates and cools the radiation stage about half way up the length o f the cryostat, which in turn cools the radiation shield. The gas evolves from the cryostat via

3 Th e Ex p e r im e n t a l Ap p a r at u sa n d Pr o c e d u r e s_______________________________________ 8 0

the transfer line and is pumped away by the diaphragm pump (Oxford Instruments H4- 312). The diaphragm pump also serves the purpose of drawing the liquid helium out of the unpressurised liquid helium dewar. Figure 3-5 shows a schematic diagram o f the set-up used for liquid helium transfer. The helium exhaust port on the liquid helium transfer line is connected to the diaphragm pump by 10 mm PTFE tubing, via a flow controller box (Oxford Instruments VC31) which contains a pressure gauge and a needle valve. The needle valve is used to regulate the flow o f helium in the transfer system by monitoring the flow rate on a flow meter that is situated at the exhaust end of the diaphragm pump. All connections to the pump and the flow meter box are made using 1 0 mm push-on fittings.

For liquid helium cooling, typically a 100/ dewar o f liquid helium is used. The transfer line consists of two sections that can be decoupled to facilitate helium transfer (See Figure 3-5). The end o f the dewar leg o f the transfer line is equipped with an M8 thread

so that an extension tube can be added to extend the leg to the base o f the dewar for efficient helium transfer. The transfer is carried out by first attaching the cryostat insert o f the transfer line to the cryostat and switching on the diaphragm pump. The dewar leg of the transfer line is then inserted into the dewar until the tube touches the liquid. By closing the vent valve and the relief valve on the dewar the cold helium vapour can be felt escaping through the other end o f the transfer line. The flexible dewar end o f the transfer line is then coupled to the transfer line insert on the cryostat to allow cold helium gas to circulate through the cryostat before lowering the transfer line to the base o f the dewar. The dewar leg is then raised approximately 1 cm off the base o f the dewar to prevent clogging of the transfer line due to any precipitates (e.g. dirt or water ice) that may be present at the base of the dewar. It is recommended that the helium flow rate does not exceed 3x10^ mbar 1 s '\ It is also important that all parts o f the transfer line and the cryostat are dry and this should be routinely checked prior to helium transfer to avoid blockage due to ice formation.

The transfer line is insulated with a vacuum jacket (pressure ~ 10'^ mbar). In order to maintain maximum insulation it is necessary to pump out the transfer line once every 4-

3 Th e Ex p e r im e n t a l Ap p a r at u sa n d Pr o c e d u r e s 81

3.3.4 The Copper-Constantan Thermocouple

The thermocouple was calibrated by taking voltage measurements with a digital voltmeter at four different temperatures. This was carried out by placing the thermocouple in its mount into boiling water, ice and liquid nitrogen and referred to the room temperature measurement. This was then compared to standard calibration data to obtain the correction for the room temperature reference junction. Each measurement was repeated three times, in a different order to check for stability during temperature cycling. The results are shown below:

Table 3-2: Calibration of the Cu-Constantan thermocouple

M e d iu m T(K ) M easu red V o lta g e (m V ) C a lib ra tio n V o lta g e (m V ) H2O B o ilin g P oin t 3 7 3 3 .4 ± 0 . 1 4 .2 7 9 R o o m T em p eratu re 297 _0.1 ± 0 . 1 0 .9 5 1 H2O M e ltin g P oin t 273 _{- 0 . 9 ± 0 . 1} 0.000 L iq u id N itr o g en 7 7 - 6 . 5 ± 0 . 1 -5.539

There is an average linear shift o f about 0.9 mV corresponding to the room temperature voltage reading from calibration data taken with the reference junction at 0 ®C. Errors in temperature readings are estimated to be within 5 K o f the real value.

3.3.5 The RhodiumTron Temperature Sensor

Figure 3-6: The rhodium -iron sensor

A 27 Q rhodium-iron resistive temperature sensor (Oxford Instruments PHZ 0002) is used to measure the temperature near the sample. The sensor consists o f a four-wire assembly as shown in Figure 3-6. The sensing wires are encapsulated to protect them and prevent errors in measurement due to possible piezoresistive effects. The sensor

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capsule is mounted into a hole at the base o f the cryostat. A three point calibrated sensor is used here, although a 30-point calibrated sensor also exists, but the latter is considerably more expensive. The sensor is capable of measuring the temperature in the range o f 1.5 to 500 K. A typical excitation current o f approximately 1 mA is applied and the resistance is measured. A temperature controller, described in the next section, is used to interpret the resistances and display the corresponding temperature in selected units.

The temperature measured by the rhodium-iron sensor is only approximate as far as the sample is concerned and is limited by its proximity to the sample. Due to the bulk of the sensor it is not possible to mount it near the sample. As a future modification, additionally, a gold/iron-chromel thermocouple will be fitted to the substrate itself which should give a better estimate of the sample temperature.