The Design and Construction of Rapid-Response Thermocouples for Use as Radiation Detectors in Infra-Red Spectrographs

Abstract

The design and construction of a rapid‐response vacuum thermocouple for use in an infra‐red spectrograph is described. The thermocouple is made by condensing 600–1000A thick layers of Bi and Sb, which overlap slightly to form the hot junction, on a 400–500A thick film of Formvar. This film is stretched over a glass form having an opening under the hot junction. The hot junction is blackened with gold evaporated in H₂ at 2–4 torr. (1 torr=1 mm Hg.) The thermocouple is intended for use with radiation interrupted at frequencies between 1 and 5 hertz. (1 hertz=1 cycle per second.) It is shown that the signal‐to‐noise ratio of the thermocouple, defined as the ratio of the interruption frequency component of its output to the square root of its resistance, is the only criterion of its goodness, and that in an infra‐red spectrograph a single junction thermocouple has a greater signal‐to‐noise ratio than a multijunction thermopile. An important operation in the thermocouple construction is the use of a glow discharge before condensation of each metal to reduce the thermocouple resistance by factors of 5 to 10. Such a thermocouple has a resistance of between 20 and 50 ohms, a d.c. sensitivity of 6–7 microvolts per 10⁻⁴‐watt cm⁻², and an output under radiation interrupted at 7 hertz of not less than one‐half the d.c. value.