A superconducting bolometer has been developed for the fast detection of infra-red signals. It has a compound response in which the primary response time is about 5 × 10−4 sec. and the secondary response time about 5 × 10−2 sec. In the primary range a signal of 5 × 10−4 μ watt is equivalent to the noise of the apparatus. The frequency at which the response is down 6 db from the primary maximum value is 3000 c.p.s. and from the secondary maximum value about 140 c.p.s. The bolometer consists of a ribbon of CbN, 200 × 10 × 0.25 mils, attached to the surface of a copper base-plate by means of a coating of Bakelite cement which covers the lower face of the ribbon and is about 0.1 mil thick. This unit is placed beneath a rocksalt window in the vacuum chamber of a cylindrical cryostat, 12″ long × 6″ diameter, which maintains operating temperature for about twelve hours when filled with 1000 cm3 each of liquid nitrogen, and liquid hydrogen kept at the triple point. For best operation this temperature must lie within a certain region of the super-conducting transition zone at about 15°K where dR/dT = 10 ohm deg.−1 and R = 0.2 ohm. The noise level varies with temperature in the transition zone and passes through several maxima and minima with variations in level of more than tenfold. Even at the minima it is at least ten times higher than in either the normal or superconducting states immediately above or below. For most sensitive operation, the temperature is kept constant at one of these minima by means of a precisely adjusted current which passes through a heating coil wound on the copper rod supporting the base plate. Radiation signals produce variations in resistance of the ribbon, observed by means of a bridge circuit, transformer, and amplifier.
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