Cryostat probe

For cryosectioning, tissue samples are quickly frozen with or without freezeembedding medium (e.g., Tissue Tek Miles Laboratories) and stored at 80°C until analysed. Optionally, aldehyde prefixation can also be used for tissue and organ probes before snap-freezing. Cutting of frozen tissue blocks is performed with a cryostat (a microtome mounted in a freezing cabinet). 

NASA has formally invited its European counterpart to work on a continuation of the HST and the ESA has accepted. This in no way compromises its own cryostatic project, FIRST (the Far InfraRed Space Telescope). Designed to supersede the highly successful ISO (Infrared Space Observatory), this new space observatory will probe the Universe in the far infrared and submillimetre ranges. It should be recalled that the ESA contributed some 15% of the costs... 

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. 

Switch on the pulsed EPR spectrometer with the two microwave (mw) sources for the pump and probe pulses. Cool the sample cryostat down to 50 K and insert the frozen sample tube. Overcouple the resonator to a Q-value of approximately 50 and adjust the mw frequency to the center of the resonator frequency. 

Figure 3.5. (b) Cutaway diagram of the cryostat, showing the essential parts of the magnet and probe. (Courtesy of Bruker Instruments, Inc.)... 

For the manufacture a cylindrical silicon single crystal is first sawed to produce the outer windows. In a second step the probe room is drilled out After final polishing of the inner and outer windows the cell can be fixed with only one lead gasket to the cryostat body. It is essential that the cell frame is made from Vacodil 36. This steel has nearly the same coefficient of expansion as silicon and allows the gas-tight connection between the cell and the cryostat body (see Fig. 1). 

The ceramic material obtained by this method (sample C) is compared to those obtained by solid state reactions using either a carbonate (A) or nitrate (B) as the source of barium. Micrographs for the three samples are shown in Figure 1. Note that sample C is homogeneous with a particle size of about 1 micron whereas about 100 and 20 micron particle sizes are observed for samples A and B respectively. Resistivity measurements on the same set of samples, were performed using a standard four-probe method with silver paint contacts in an exchange gas cryostat with a Si-diode thermometer. A Tc of 91K with a transition width of 0.5K is observed for sample C whereas samples B and A exhibit widths of 2 and IK respectively. The Meissner effect was observed to be 35, 65 and 50% for samples A, B and C respectively. 

The single crystals were placed in a cryostat and were kept at cryogenic temperature for the photo-induced and time-resolved absorption measurements. Both pump and probe beams polarized along the b-crystal axis were exposed onto the afc-plane of the crystals. As regards the photoinduced absorption measurements, we used an Ar -laser (Lexel, Model 95) and a CW dye laser (Spectra-Physics 375) for excitation. A probe beam was derived from a 40 W tungsten halogen lamp, dispersed by a monochromator (JASCO, CT-25C) in the spectral range of... 

For temperature-variation studies, some form of thermal control of samples is required, and Figure 5 describes a commercial cryostat capable of use between 77 and 373 R, which the authors have found very convenient for experiments with polymer samples (39). This cryostat is designed for use with 1-cm-square cross-section fluorescence cuvettes, and some modification of the probe Is required for use with polymers. For polymers in solution requiring outgasslng, the cuvette shown In Figure 6.a was employed. 

---