Noise from liquid nitrogen

The optimum operating temperature for minimum noise from the FET was found to be slightly higher than the operating temperature of the Ge(Li) detector. A thermal heater made from a 100-ohm carbon resistor was therefore installed next to the FET and operated at a current that heated the transistor to a temperature just above that of liquid nitrogen. 

A photoconductive detector is a semiconductor whose conductivity increases when infrared radiation excites electrons from the valence band to the conduction band. Photovoltaic detectors contain pn junctions, across which an electric field exists. Absorption of infrared radiation creates electrons and holes, which are attracted to opposite sides of the junction and which change the voltage across the junction. Mercury cadmium telluride (Hg,. Cd/Te, 0 < x < 1) is a detector material whose sensitivity to different wavelengths is affected by the stoichiome-try coefficient, x. Photoconductive and photovoltaic devices can be cooled to 77 K (liquid nitrogen temperature) to reduce thermal electric noise by more than an order of magnitude. 

Noise associated with the detector elements themselves (dark or thermal noise) can be reduced by decreasing the temperature, typically from —20 to —70° C although cooling to liquid nitrogen temperatures essentially eliminates dark noise, it is typically impractical for applications outside of the laboratory. The theoretical value for relative signal-to-noise reduction is a factor of 2 improvement for every 6.3°C reduction in temperature, although this can be... 

Because of delamination, each detector has a few so-called bad pixels which can be seen very clearly in the live FPA image. Their intensity and the signal-to-noise ratio (SNR) differ from those of other pixels. New FPA detectors must not have more than 3% bad pixels. Similar to conventional single-element MCT detectors, the detector pixels function at low temperature, which can be attained by cooling with liquid nitrogen. During cooling, there is a thermal contraction of the detector pixels and, because the detector consists of layers of different materials (with different expansion coefficients), delamination can occur. However, the... 

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. 

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