Helium cooled bolometer

Optothemial spectroscopy is a bolonietric method that monitors the energy in a stream of molecules rather than in the light beam. A well collimated molecular beam is directed toward a liquid helium cooled bolometer. There will be energy... 

It passes through the sample absorption cell made of Pyrex with polyethylene windows and is detected with a liquid helium cooled bolometer. One of the lasers is frequency-modulated at 1 kHz and the detector output is processed with a lock-in amplifier, as shown. Far-infrared rotational spectra of CO, HC1 and HF have been recorded [67], and as an example of the excellent sensitivity achieved, we refer the reader to the spectrum of the OH radical [68] shown later in this chapter. Evenson s spectrometer operates over a wide range of the far-infrared region up to 9 THz, with excellent frequency stability. 

Figure 3.12 Schematic drawing of the helium-cooled bolometer. Heat filters cut out unwanted radiation and present a cold image to the bolometer...

The Gunn oscillator frequency-lock became progressively more unstable as its slewing rate increased, whether caused by rapid frequency stepping or by the application of FM. We found ourselves limited in practice to a stepping time of 50-100 ms and an FM deviation of 10 MHz at a rate of 1 kHz. This was considered reasonably satisfactory in that a complete scan could be completed in a few seconds and that our signal detection rate 2 kHz was close to the frequency at which manufacturers, e.g. Millitech, specified their detector performance. The same FM rate was used whether our detector was a Schottky barrier mixer diode or the helium-cooled bolometer. 

In addition to the expansion of time resolution, the expansion of spectral range to far IR is also desirable, since metal-adsorbate vibrations and surface vibrational modes occur in this range. The available spectral range is now limited by the prism (>900 cm" for Si and >700 cm" for Ge), the low intensity of the light source, and the low sensitivity of detectors. Since the absorption coefficient of Si fortunately decreases in the far-IR region, ATR-SEIRAS in the far-IR wiU be possible with the use of a synchrotron far-IR facility and a helium-cooled bolometer [142]. 

Fig. 1.30 Optothermal spectroscopy in a molecular beam with a helium-cooled bolometer as detector and two optical systems, which increase the absorption path length...

This opportunity will be extended to the far-IR when fast detectors are available. Indeed the typical cut-off frequency of a liquid-helium cooled bolometer is presently below 500 Hz, even if much faster detectors based on superconducting devices are being developed. 

The detectors are generally Schottky diodes or InSb helium-cooled bolometers in the millimetre and submillimetre wave region. 

Whereas it is usual to require many hours of signal averaging to obtain a reasonable quality spectrum in the far I.R. using a Golay detector, this same spectrum can be obtained in minutes with a well prepared liquid Helium cooled bolometer. 

Chanin, G. (1972). Liquid helium-cooled bolometers. In Infrared Detection Techniques for Space Research, ed. V. Manno J. Ring. Dordrecht, Holland D. Reidel Publ. 

Coron, N., Dambier, G., Leblanc, J. (1972). A new type of helium-cooled bolometer. 

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