Closed cycle cryostat

If one intends to use Ne or H2 as a host gas, one needs to take recourse to cryostats that are able to attain temperatures of 4 K or less. Until recently, ow cryostats, where liquid He is guided in a controlled fashion to the cold end, were usually required for such experiments. However, two-stage 4 K closed-cycle cryostats have recently become available that appear to obviate the need for liquid He. ° The author knows of two prominent matrix-isolation research groups who have been using such devices for a little while now and have no complaints about them. 

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... 

Two providers currently offer closed cycle cryostats for matrix isolation with 0.5-1 W of cooling power at 4 K. The first is Janis Reserach (http //www.janis.com/p-a4k.html), which uses hardware from Sumitomo, the other is Cryomech (http //www.cryomech. com/cryostats.html). 

The technique of matrix isolation has been shown to produce highly characteristic spectra of individual components of complex samples combination of MI spectroscopy with separation techniques promises to increase further the analytical capabilities of the technique. While our research to date has emphasized FTIR and molecular fluorescence spectrometry, MI as a sampling procedure is not limited to these two forms of spectrometry. For example, some interesting preliminary analytical results by MI Raman spectrometry recently have been described (32). It should also be stressed that the cryogenic procedures required for the vast majority of MI spectral studies are neither difficult nor unduly expensive except in very special cases, closed-cycle cryostats requiring no cryogenic liquids (and no prior experience in low-temperature techniques) are entirely satisfactory for MI... 

The main components of FLN instruments for analysis are a cryogenic system, a tunable laser, a medium resolution spectrometer, and a recording system. The cryogenic system consists of a liquid He optical cryostat with a He circulation system. In some cases, commercially available closed-cycle cryostats can be used. The optical system consists of a sample holder, optics for the exciting beam input, and a high aperture collector of the fluorescent radiation. 

Most previous attempts to obtain X-ray diffraction data at very low temperatures (< 80 K) have used custom built systems with closed cycle helium refrigerators mounted on large, robust four circle diffractometers. In order to remove the inherent disadvantages of these systems - cost, single application, absorption and scattering of the windows - we have built an open flow system from mainly off-the-shelf components which uses liquid helium as the cryogen. This is not the first open flow helium system [19, 20] but is the first that is mainly off-the-shelf and is mountable on any diffractometer. It is based on an ADP Helitran ESR cryostat with modifications to the nozzle assembly and to the direction of the gas flow. The lowest temperature is estimated to be <30K. At the current price for liquid helium in... 

Standard optical metal or metal-glass cryostats are commonly used in low-temperature studies of TSL and TSC. Less expensive facilities for liquid N2 temperatures have been designed, and closed cycle cryotips may be employed as well. TSR requires dynamic temperature cycles. Therefore, it is generally advantageous to design the sample holder-dewar arrangement as small as possible to enable quick turnaround times. 

The vacuum chamber is interfaced to an ion implanter (200 kV) from which ions with energy up to 200 keV (400 keV for double ionizations) can be obtained. The IR spectrometer is positioned (by a moveable optical bench) in a way such that the IR beam is transmitted, through a hole of the sample holder, by the substrate. The sample holder is mounted at an angle of 45° both with the ion beam and with the incoming IR beam and spectra can be acquired in situ without tilting the sample. Samples can also be put in thermal contact with a closed-cycle helium cryostat whose temperature can be varied between 10 and 300 K. The spectra shown in this work have been taken with a resolution of 2cm and a sampling of 0.5cm . ... 

Fig. 3.16 The FDS at LANSCE (a) schematic and (b) cut-away view. Key filter, detectors, shielding, closed cycle refrigerator to cool filter, shielding, sample (usually annular or cylindrical), cryostat for sample, incident beam tube. Reproduced from [20] with permission from Elsevier.

Cryostat. A cryostat designed for MBssbauer measurements of species isolated in a low temperature matrix was cooled down by a Cryomini D closed-cycle helium refrigerator (Osaka Sanso Kogyo Co. Ltd.) (Fig. 5). The sample holder in the cryostat was maintained at a desired temperature between 15 and 100 K by means of a 40-flanganin heater and a DTC-2 digital temperature controller... 

Nowadays, the standard cryostats are closed-cycle helium refrigerators. They are commercially available. We use either the Displex Closed-Cycle System CSA from Air Products or the Closed-Cycle Compressor Unit RW 2 with Coldhead Base Unit 210 and Extension Module ROK from Leybold. These systems can run for thousands of hours with minimal maintenance. The sample holder can be cooled to temperatures from room temperature to about 10 K. A typical example of a closed-cycle helium matrix apparatus is shown in Figure 1.1. 

Luminescence Spectroscopy. Photoluminescence measurements were performed with the aid of a Fluorolog3 spectro-fluorometer Fl3—22 (Horibajobin Yvon) equipped with double Czerny—Turner monochromators, a 450 W xenon lamp and a R928P photomultiplier with a photon counting system. Cooling down to 10 K was achieved by a closed cycle He cryostat (Janis Research). All emission spectra were corrected for the photomultiplier sensitivity and all excitation spectra for the intensity of the excitation source. To avoid any contamination of water on the sample s surfaces, we carried out the measurements in silica ampules with extreme purity which show no luminescence of the ampules itself. Reflection spectra were recorded on a Cary 5000 UV—vis—NIR spectrophotometer (Varian), which were corrected for both the lamp intensity and the photomultiplier sensitivity. 

The helium flow-through cryostat uses liquid helium that is pumped from a container and sprayed onto the bottom of the cold tip. In this case, temperatures of 3.5 K can be reached. The cooling process takes a long time (typically 1 h), both with the closed-cycle helium refrigerator and the flowthrough cryostat. Several samples should preferably be cooled simultaneously. 

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