Magnet, spectrometer cooling

The best energy resolution for electrons is obtained using silicon semiconductor detectors, with the possible exception of magnetic spectrometers. Silicon detectors may be surface-barrier or Si(Li) detectors. The surface-barrier detectors operate at room temperature, while the Si(Li) detectors give best results when cooled to liquid nitrogen temperatures. The energy resolution of semiconductor detectors is determined by the electronic noise alone. It deteriorates as the area and the sensitive depth of the detector increase. For commercial detectors the full width at half maximum (FWHM) ranges from about 7 to 30 keV. 

Some instruments, such as nuclear magnetic resonance (NMR) spectrometers that use superconducting magnets, require cooling by cryogens such as liquid nitrogen and liquid helium. Some freezers use liquid nitrogen to maintain samples at low temperatures. 

Flexible superconducting tapes provide promise of uses for superconductors in motors, generators, and even electric transmission lines. Meanwhile, superconducting magnets cooled to the temperature of liquid helium already are in use. High-field nuclear magnetic resonance (NMR) spectrometers have become standard instruments in chemical research laboratories, and the same type of machine (called an MRI spectrometer) is used for medical diagnosis in hospitals worldwide. 

FIGURE 3.15 Schematic diagram of a Fourier transform NMR spectrometer with a superconducting magnet.The probe is parallel with the z axis of the magnet, which is cooled with liquid helium surrounded by liquid nitrogen in a large Dewar flask. 

Photo of a modem 300-MHz NMR spectrometer. The metal container at right contains the superconducting magnet, cooled by a liquid helium bath inside a liquid nitrogen bath. The electronics used to control the spectrometer and to calculate spectra are at left and in the background. 

Fig. 1. 249.9-GHz FIR-ESR spectrometer. A, 9-T magnet and sweep coils B, phase-locked 250-GHz source C, 100-MHz master oscillator D, Schottky diode detector E, resonator and modulator coils F, 250-GHz quasioptical waveguide G, power supply for main coil (100 A) H, current ramp control for main magnet I, power supply for sweep coil (50 A) J, OC spectrometer controller K, lock-in amp for signal L, field modulator and lock-in reference M, Fabry-Perot tuning screw N, vapor-cooled leads for main solenoid O, vapor-cooled leads for sweep coil P, He bath level indicator Q, He transfer tube R, bath temperature thermometer S, " He blow-off valves. [From Lynch et al. (1988), by permission of the AIP.]...

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