Flow cryostats

It is more convenient to use the enthalpy of the cold gas evaporating from the LHe in a continuous gas flow cryostat (Fig. 5.4). The flux and hence the temperature of the experiment can be regulated by a heater or a needle valve. 

Accurate measurements of the frequency-resolved transverse spin relaxation T2) of Rb NMR on single crystals of D-RADP-x (x = 0.20, 0.25, 0.30, 0.35) have been performed in a Bq field of 7 Tesla as a function of temperature. The probe head was placed in a He gas-flow cryostat with a temperature stability of 0.1 K. To obtain the spin echo of the Rb - 1/2 -o-+ 1/2 central transition we have used the standard (90 - fi - 180y -ti echo - (2) pulse sequence with an appropriate phase-cycling scheme to ehminate quadrature detection errors and unwanted coherences due to pulse imperfections. To avoid sparking in the He gas, the RF-field Bi had to be reduced to a level where the 7T/2-pulse length T90 equalled 3.5 ps at room temperature. 

Fig. 7) was operated routinely for many years on the neutron diffractometer DIO at the Institut Laue Langevin and was used for studies on, for example, Bechgaard salts at temperatures close to the superconducting transition [87,88]. Since 1990, 1.6 K can be reached routinely with an improved design and a dilution cryostat has been tested successfully in the laboratory which fits the Euierian cradle and is capable of 70 mK (C. M. E. Zeyen and S. Pujol, private communication, 1993). The helium flow cryostat of Argoud and Muller [80] is also well adapted to four-circle geometry and can be installed even on a small Euierian cradle. 

Figure 7 Continuous-gas-flow cryostat of Ref. 86, with its special helium siphon installed on the Eulerian cradle of the neutron diffractometer DIO at the Institut Laue Langevin (Grenoble, France). The arrows indicate the two rotating Johnstons the circles indicate the corners with magnetic spacers.

The EMR studies were performed in the range of MFs from 1000 to 5500 G using X-band EPR spectrometer Bruker EMX-8/2. The commercial gas-flow cryostat was used to achieve temperature in the range of -100° - 90° C. Ferrofluid spectra in a quartz flat cell and PVP films in a quartz tubes. Microwave frequency power did not exceed 0.1 mW. 

Furthermore, the complex relaxation requirements of a double resonance technique mean that low temperatures are required in all cases. In general, temperatures below that of liquid nitrogen are needed. These are obtained either with a variable temperature flow cryostat using liquid helium ( 4-40 K) or with a liquid helium immersion dewar ( 4 K, or 2 K for pumped (superfluid) helium). 

Optical absorption spectroscopy in the temperature regime between about 10 K and 300 K is usually performed using conventional optical absorption spectrometers (Cary, Phillips) and temperature variable helium gas flow cryostats. In order to initiate the polymerization reaction the diacetylene monomer crystals were excited into the So -> Sj absorption edge at about 310 nm using either a xenon high pressure arc or the 308 nm line of an excimer laser (ArF). 

Fig. 23. A Mossbauer spectrometer coupled to an Ion implanter. 1 = transducer, 2 = absorber 3 = source, 4 = He flow cryostat. The beam comes from the right, the detector is situated at the left...

Mossbauer spectra were collected as a function of temperature at room pressure, and as a function of pressure at room temperature. This was accomplished using a continuous flow cryostat for temperatures from 4.2 to 293 K, a vacuum furnace for temperatures... 

Fig. 4.6. Cross-section of an optical continuous-flow cryostat (CF 204 of Oxford Instruments), with the extremity of the removable transfer tube inserted, but without sample holder. The evacuation valve at the top is masked by the sample port. The optional windows on the radiation shield can be replaced by metallic irises to reduce the field of view. This cryostat can be fitted with one or two more optical windows at 90° from the main optical axis for additional excitation, and also with a down-looking window. The arrows indicate the direction of the flow of liquid or gaseous helium. Reproduced with permission from Oxford Instruments...

Luminescence was excited with the 488.0 nm and 514.5 nm lines of an Ar+ ion laser. The sample crystals were again placed in the He gas flow cryostat. The emitted light was dispersed with a Spex 500 M monochromator and detected with a Ge diode (Applied Detector Corporation ADC403L). This detector is sensitive in the 600 nm to 1800 nm wavelength range and its... 

The low temperature measurements used an helium continuous-flow cryostat, allowing work at fixed temperature down to 15 K. Most of the experiments were performed at 30 K + 5 K. 

Fig. 1.37 A membrane DAC (after reference 47) in a helium-flow cryostat (1) DAC, (2) gas feedthrough, (3) optical objective, (4) window, (5) bellows, (6) X-V -Z- micropositioners, (7) copper sleeve, (8) thermal exchanger, (14) optical window. (Reprinted with permission from Rev. Sci. Instr., 64, 203, (1993).)...

The samples were grown by MBE at 700 C (for details see [4]). The samples had 10 layers containing quantum dots. For the irradiation with 2.4 MeV protons at RT, a Van de Graaff accelerator was used. The passivation with atomic hydrogen at 250"C for 30 min was undertaken in a CVD reactor. The PL measurements were performed at a Bruker IFS 66v FTIR spectrometer equipped with a Ge detector. The samples were placed in a helium gas flow cryostat at temperatures from 5-300K. For the excitation in a wide power range we used the 488 nm line of an Ar laser. 

A schematic drawing of the typical X-band ( 9.35 GHz) ODMR spectrometer used to obtain the results reviewed in this chapter is shown in Figure 7.4. The sample is inserted in the quartz dewar of an Oxford Instruments He gas flow cryostat, at the centre of the optically-accessed microwave cavity, which is positioned between the poles of a conventional electromagnet. 

For EL-. (7-, and C7ph-detected resonances (see Section 1.4.3), thin copper wire leads were inserted into the quartz dewar of the lie gas flow cryostat to provide bias. The cr- and cTpu-detected resonances were recorded by measuring the microwave-induced changes in the current resulting in similar changes in the voltage across a standard resistor connected in series with the LED. 

For measuring the resistivity of the films as a function of temperature, a continuous flow cryostat (Oxford Inst. Model CF-1204) with a temperature controller was used. The details of the sample preparation and measurement have been described previously (3). A temperature controlled silicon diode ( 0.1 K) was mounted next to the sample for accurate temperature measurement. X-ray powder diffraction patterns were obtained using a computerized Phillips powder diffractometer (Type AFD 3520) with Ni-filtered CuKa radiaticm. 

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