Liquid helium cooled, far infrared, carbon bolometer detector

Abstract

A low temperature carbon bolometer was designed and constructed as a thermal detector of far infrared radiation. The emphasis of this project was to construct such a device using the simplest and least expensive techniques. As with former experimenters in this field the sensitive element was made from the resistive material of a commercial carbon composition resistor and ground to a very thin slab. Contrary to usual practice, with carbon devices, the element was suspended in an evacuated space supported only by two fine electrical wires joined to each end. For simplicity these wires were cemented to the carbon slab using silver point thus avoiding the customary use of plated or evaporated metal contacts. The bolometer element was housed in a non resonant reflecting cavity to improve detection efficiency. The whole detection apparatus formed a low temperature cryostat surrounded by superfluid helium. Much of the project involved methods of preventing the liquid helium from leaking into the evacuated cryostat. A theoretical model to describe the thermal and electrical properties of a bolometer detector was developed. Tests were carried out to support the validity ofthis model including sensitivity measurements. Theoretical models of noise from numerous optical, thermal and electrical sources was also developed and contributions from each were calculated and compared with the levels measured. The noise equivalent power was slightly better than the room temperature operating folay cell but was not as low as has been previously achieved with similar low temperature detectors. The sensitivity reached 10 4 volts per watt.