Superconducting bolometer

Nevertheless the heat capacity of a carbon resistor was not so low as that of crystalline materials used later. More important, carbon resistors had an excess noise which limited the bolometer performance. In 1961, Low [61] proposed a bolometer which used a heavily doped Ge thermometer with much improved characteristics. This type of bolometer was rapidly applied to infrared astronomy as well also to laboratory spectroscopy. A further step in the development of bolometers came with improvements in the absorber. In the early superconducting bolometer built by Andrews et al. (1942) [62], the absorber was a blackened metal foil glued to the 7A thermometer. Low s original bolometer [61] was coated with black paint and Coron et al. [63] used a metal foil as substrate for the black-painted absorber. A definite improvement is due to J. Clarke, G. I. Hoffer, P. L. Richards [64] who used a thin low heat capacity dielectric substrate for the metal foil and used a bismuth film absorber instead of the black paint. 

The detailed investigation of superconducting (Al) bolometers is due to Clarke and Richards [64,65], This bolometer had a very low heat capacity and reached the thermal fluctuation noise limits. This bolometer needed a much more complex electronics than those using Ge thermistors. 

Even larger values can be realized with materials around their critical temperature Tc for the transition from the superconducting to the normal conducting state. In this case, however, one always has to keep the temperature at Tc. This can be achieved by a temperature feedback control, where the feedback signal is a measure for the rate dQ/dt of energy transfer to the bolometer by the excited molecules. 

Naugle and Porter [2.83] have proposed using the increase in inductance of a superconducting thin film as the transition temperature is approached from below to detect radiation. Their analysis reveals high sensitivity for a granular aluminum thin film superinducting bolometer operated at 2 K. 

Fig. 3.8- Performance of cooled bolometers NEP of carbon, germanium, silicon, and thallium selenide semiconducting bolometers and tin, aluminum and titanium superconducting bolometers. Details in Table 3.3, composite bolometers in Table 8.1 (p. 303), 50% absorption has been assumed for the composite bolometers. The solid line is Coron s [3.42J estimate of the best attainable performance of a Ge bolometer in the absence of higher temperature background radiation...

The quantity AR/AT becomes exceedingly large at the critical temperature Tc of superconducting materials. If the bolometer is always kept at this temperature by a temperature control, the incident radiation power P can be very sensitively measured by the magnitude of the feedback control signal used to compensate for the absorbed radiation power [4.100. 102]. 

A bolometer is operated at the temperature T = 8K between superconducting and normal conducting states, where dR/dT is 10 2/K. The heat capacity is //= 10 J/K and the dc electrical current 1mA. What is the change Ai of the heating current in order to keep the temperature constant when the bolometer is irradiated with 10 W ... 

D. Bassi, A. Boschetti, M. Scotoni, M. Zen Molecular beam diagnostics by means of fast superconducting bolometer. Appl. Phys. B 26, 99 (1981)... 

J. Clarke, P.L. Richards, N.H. Yeh Composite superconducting transition edge bolometer. Appl. Phys. Lett. B 30, 664 (1977)... 

A.T. Lee et al. Superconducting bolometer with strong electrothermed feedback. Appl. Phys. 

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. 

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