Thermal radiation microwave region

The circular atom microwave spectroscopy experimental set-up is sketched on Fig. 1-a. A thermal beam of Li atoms crosses three sections of the apparatus the excitation, the microwave interaction region and the detection zone. The whole set-up is protected from room temperature thermal radiation by a liquid nitrogen cooled shield (which can be replaced in a later stage of the experiment by a liquid helium cooled one). 

MWR involves measuring the power in the microwave region of the natural thermal radiation from body tissues to obtain the so-called brightness temperature of the tissue under observation. Brightness temperature Tb is defined as... 

Radiofrequeney (RF)/mierowave radiation is a potential hazard that does not have good warning properties. Therefore, baseline data should be obtained for all routine operations with a potential for RF/mierowave radiation exposure above applieable standards, and baseline RF surveys of new equipment may be required. As a practical matter, a lower frequeney limit needs to be drawn to assess when baseline surveys are neeessary. Since the body is fairly transparent to RF frequencies in the kilohertz region, and standard RF/mierowave radiation meters have a lower limit of 300 or 500 kHz (depending on the meter), 500 kHz is sometimes used as a cutoff for the lower limit for thermal effeets. Most RF/microwave equipment used in semiconductor manufacturing operate at or above a frequency of 13.56 MHz. This equipment includes plasma etchers and ashers, sputtering units, mold pre-heaters, microwave ovens, and plasma enhanced CVD units. 

One of the most fruitful application of laboratory microwave spectroscopy over the last twenty years is the analysis of the molecular content of interstellar clouds. These clouds contain gas (99% in mass) which has been mostly studied by radioastronomy, and dust, whose content has been analysed mostly by IR astronomy. The clouds rich in molecular content are dense or dark clouds (they present a large visual extinction), with a gas density of 10 -10 molecules cm", and temperatures of T < 50K. At these low temperatures only the low-lying quantum states of molecules can be thermally (or collisionally) excited, i.e. rotational levels. Spontaneous emission from these excited states occurs at microwave wavelengths. In some warm regions of dense clouds (star formation cores) the absorption of IR radiation produces rotational emission in excited vibrational states. Other rich chemical sources are the molecular clouds surrounding evolved old stars, such as IRC-i-10216, and called circumstellar clouds. 

Chen, E.M.A., Nimmo, R Thermal and orbital evolution of Tethys as constrained by surface observations. In Lunar and Planetary Institute Conference Abstracts, vol. 39, p. 1968 (2008) Cheung, A.C., Rank, D.M., Townes, C.H., Thornton, D.D., Welch, W.J. Delection of water in interstellar regions by its microwave radiation. Nature 221, 626 (1969)... 

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