Closed-cycle helium refrigeration

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... 

From the exit slit, the beam is directed to either of two detectors by the movable mirror behind the exit slit. Typical detectors are InSb, a photovoltaic indium antimonide detector operated at 77 K, and He Ge, a photo-conductive mercury-doped germanium detector maintained at 15 K by a closed-cycle helium refrigerator... 

Data acquisition and least square fitting of the Moessbauer spectrum were done in a microprocessor based computer (Promeda). The source used was a 220 mCi - Co Rh. All the isomer shifts in this paper are given relative to a-Fe at room temperature. The coal used in the study was IL6 (pyritic sulfur 2.0 and organic sulfur = 2.0 %) and the solvent was a SRC-II heavy distillate 850+. A specially designed reactor for Moessbauer measurements was used. The temperature stability was 1%. The stoichiometries of the pyrrhotites were determined by Moessbauer and x-ray diffraction methods (10). All the samples were also measured at low temperatures (13 K) for better resolution of all the components, using a closed-cycle helium refrigerator. 

The synthetic principle described above for reactions in matrices is also employed in preparative work. For this purpose, highly reactive species, such as metal atoms, SiF2, SiO, BF, BCl, etc. are co-condensed with a sub.strate at -196 °C. After warming up and separation from excess substrate and by-products, numerous new compounds have been synthesized on a gram scale (Timms, 1976). If the substrate cannot be condensed at -196 °C (e.g., CO), a closed-cycle helium refrigerator can be used to maintain a temperature of 30 K on the cold surface. In this way, Ru(CO)s has for the first time been prepared on a gram scale (Godber et ah, 1986). 

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... 

More recent developments of laser ablation techniques coupled with closed-cycle helium refrigerators greatly facilitated its application. Thus, matrix isolation technique has now been utilized to obtain structural and bonding information of a number of inorganic and coordination compounds. Some important features and applications are described in this and the following sections. 

Detection of radiation at radio telescopes is done using superconducting mixers, which are kept at 4.2 K with a closed-cycle helium refrigerator. Radiation from the sky, Vsky, is combined inside the mixer with a source of radiation at the telescope called the local oscillator (or Vlo)- This technique, called heterodyne mixing is needed because signals from space cannot be... 

Fig. 1. Schematic representation of vacuum furnace closed-cycle helium refrigeration system used for metal vapor microsolution optical spectroscopy, as well as conventional metal vapor-matrix isolation experiments. (A) NaCl or Suprasil optical window, horizontal configuration (B) stainless steel vacuum shroud (C) NaCl or Suprasil optical viewing ports (D) cajon-rubber septum, liquid or solution injection port (E) gas deposition ports (F) vacuum furnace quartz crystal microbalance assembly. With the optical window in a fixed horizontal configuration, liquid or solution sample injection onto the window at any desired temperature in the range 12-300 K is performed in position 1A, metal deposition is conducted in position IB, and optical spectra are recorded in position 1C see Procedure).

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. 

A very convenient cooling apparatus is the closed-cycle helium refrigerator, in which repeated compression and expansion of gaseous helium takes place. The sample is placed in a vacuum and attached... 

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. 

Fig. 5 Cold 22-pole ion trap machine employed by Gerlich and coworkers for spectroscopic studies of cold N2 ions [70]. Quadrupoles are used for parent mass selection (QMS I) and product analysis (QMS II). The ion trap is cooled by a closed cycle helium refrigerator and helium is used a buffer gas. Copyright (1999), with permission from Elsevier...

The X-ray diffraction studies at 13 K were performed by a Weissenberg-type low-temperature IP system equipped with closed cycle helium refrigerator. The oscillation photographs were taken around the >-axis. Figure 5.39(a) shows X-ray reflection spots in the (101) plane. At 300 K no satellite reflections were observed. Around 100 K very faint extra diffuse reflections were observed and at 80 K clear satellite reflections were observable. The satellite reflections grew at low temperature. Figure 5.39(b) shows P, Q satellite... 

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