Cryogenically cooled detectors

Cryogenically cooled detectors employ the low-noise GaAs Schottky barrier Mott diodes. Between 140 and 220 GHz they exhibit 400 K noise equivalent temperature at a lower limit junction temperature of 20 K, below which the performance degrades. The noise temperature is around 1000 K at 300 K junction temperature. Sensitivity of a Schottky barrier mixer diode ranges from about 2.75 VmW" to 1 VmW over the range 90-325 GHz.In comparison the helium-cooled InSb bolometer used by the present authors (Section 3.4.1) can provide double sideband noise temperatures of 200-300 K in the region 100-300 GHz and sensitivity of 5-6 VmW . ... 

If the interferometer mirror speed is such that the optical velocity is 0.316 cm s (HeNe laser frequency of 5 kHz), 4000-cm radiation is modulated at 1.25 kHz (see Eq. 2.11). Thus, the response time of a detector for FT-IR spectrometry must be less than 1 ms. Although several cryogenically cooled detectors have response times this low, the only mid-infrared detectors that have an appropriate combination of high speed, reasonably good sensitivity, low cost, good linearity, and operation at or near room temperature are the pyroelectric bolometers. 

Elemental composition C 12.14%, 0 16.17%, Cl 71.69%. Phosgene can be analyzed by GC using FID or a balogen-specific detector or by GC/MS. Ambient air may be collected in a metal container placed in an argon bath or condensed into any other type cryogenically cooled trap. Alternatively, the air may be collected in a Tedlar bag. The sampled air may be sucked by a condensation mechanism into tbe GC column. 

Operation of the column oven at 50°C or lower has been a problem in earlier chromatographs because of the difficulty of completely isolating the column oven from other heated components, such as the detector, injection port, and splitter, and still having a usable oven. The processor controller described overcomes this problem by mixing controlled amounts of room air into the column oven and can control very adequately at temperatures of about 30°C without cryogenic cooling. A further advantage of the processor controller is that the processor normally also can handle the temperature control of the other heated zones—inlet, detector, valves, and so on. 

This author does not know of any application where just the dichroism or the dichroism and mechanical properties are measured dynamically, similar to the dynamic X-ray or birefringence experiments. This is probably due to the experimental difficulty. Specifically one would need a fairly high speed detector which generally requires cryogenic cooling and is fairly costly. Secondly, the sensitivity of the dichroism method is not considered to be better than a few percent. Since only small dynamic strains are involved in such an oscillatory experiment, the sensitivity required must be better than this. This latter problem could probably be overcome by time averaging. 

Solid-state MMW sources with both cryogenically cooled and room temperature detectors are discussed, along with their application to conventional and dielectric perturbation based methods of quantification. Post-detection signal processing, smoothing, filtering and spectral profile fitting are described, which extract the full value from data obtained by this sensitive analytical technique. 

PMTs have a relatively large detection area making them well suited to most monochromators. Their dark noise per unit detection area is the lowest of any detector (lower than even cryogenically cooled CCDs). Their quantum efficiencies are in the range of 1-40% in the ultraviolet and visible spectral regions, but fall off rapidly in the near-infrared. PMTs have nanosecond response times that are useful for time-resolved Raman measurements. 

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