Vacuum cryostats

Although work below liquid nitrogen temperature necessitates a precision-engineered vacuum cryostat, it is quite easy to work between 78 K and 300 K without a vacuum by using styrofoam insulation. Standard coolants are liquid nitrogen (b.p. 77-3 K), liquid hydrogen (b.p. 20-4 K), and liquid helium... 

The window material used in vacuum cryostats is usually beryllium, aluminium, or aluminised mylar. It should be noted, however, that commercial beryllium and aluminium often contain sufficient iron to give a detectable Fe (14 keV) resonance, and this has been known to cause problems when working with this isotope. 

Most low-temperature infrared spectroscopy has been performed using thin films. Some membrane proteins, such as bacteriorhodopsin, lend themselves easily to thin film use. The use of thin films affords some advantages, such as small volumes (tens of microliters), good compatibility with vacuum cryostat mounting, and excellent thermal contact with the cold stage. 

MTDATA, NPB, and TPD samples were 0.76, 4.11, and 5.13 pm, respectively. After coating, the samples were immediately loaded inside a vacuum cryostat with a pressure less than 10 torr for V, DISCLC, and AS measurements. 

Fig. 7. The effect of Brj intercalation on the temperature dependence of the resistivity of a bulk SWCNT sample. Curve a, pristine material curve b, saturation-doped with Br2 curve c, after heating in the cryostat vacuum to 4. 0 K for several hours [3. ].

Muller (1951, 1956) developed this instrument, which for the first time enabled extensive details of the atomic structure of a solid surface to be seen directly. Figure 1.1 illustrates schematically the basic construction of a FIM. The specimen is prepared in the form of a fine wire or needle, which has been chemically or electrochemically polished to a sharp point with an end radius typically 50-100 nm. It is mounted along the axis of a vacuum chamber, about 50 mm from a phosphor screen (perhaps 75 mm in diameter). The specimen is mounted on an electrical insulator within a cryostat, and it can be raised to a high positive potential (3-30 kV) by means of the leads attached. 

Sometimes, to get data from 4 K up to room temperature, the simplest and more economical way is to let the cryostat warm if the thermal insulation is good and the vacuum chamber is kept under pumping, the warm-up time can be several days. If the experiment thermal time constant is much shorter, data at practically constant temperature can be recorded. 

Of course all surfaces that are at such low temperatures must be kept out of contact with the ambient environment. This is achieved by a detachable and rotable vacuum shroud that surrounds the two expander stages and the sample, all of which must be kept under high vacuum while they are cold to avoid collisional heat transfer. By default, evacuation of the assembly occurs through vacuum ports mounted on the main body of the expander, but in some cases it is advantageous to have extra ports on the vacuum shroud itself. Furthermore, the first expansion stage of closed-cycle cryostats, where a temperature of 35 10 K is attained, is usually fitted with... 

A major problem with functional inlet systems is that they are often quite bulky, which makes it difficult to move the cryostat from the sample deposition line to different spectrometers, or from there to stationary sources of radiation. If only one form of spectroscopy is applied in a given study, then the vacuum shroud may be mounted with all parts attached to it in the sample chamber of the corresponding spectrometer. In this case the expander, on which the sample holder is attached, must be rotated within the vacuum shroud to switch the sample from... 

The sample preparation vacuum line (often called spray-on line ) should allow for (a) controlled mixing of the host gas with the substrate (or with other components that are added to the matrix) by manometric techniques and (b) the controlled release of the gas (mixture) toward the inlet system of the cryostat. These conditions are met by a vacuum line that incoporates a storage bulb for the gas (mixture), inlets for attachment of evacuable containers that allow degassing of the substrate prior to its mixing with the host gas, pressirre gauges that cover suitable ranges, a needle valve that allows the controlled release of the gas, possibly via a flowmeter, and interfaces to the bottles that contain the host gas(es), and to the inlet system that is attached to the vacuum shroud of the cryostat. 

In order to avoid having to shut down the whole vacuum system each time the cryostat is vented after an experiment, it is advisable to place a small slide or butterfly valve between the turbopump and the expander port. When the system is reevacuated, the turbopump must be stopped before opening the valve, otherwise the ambient pressure shock wave that results might deform the delicate rotor and... 

Second, contraptions used for external generation of reactive intermediates are often quite bulky, which may impede the mobility of the experiment. Such contraptions may render it difficult to investigate a sample by several different kinds of spectroscopy. In principle it is possible, with the aid of a gate valve mounted on the vacuum shroud, to construct devices that allow one to retract and detach the external source of a reactive intemediate after a matrix has been built, but the implementation of this strategy is technically quite challenging because high vacuum must be maintained at all times within the cryostat. 

Here the mechanical properties are not so important because the external windows are not subjected to big temperature variations, and they do not need to be clamped down hard because the vacuum inside the cryostat holds them in place. 

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