GB786516A - Radiation detectors - Google Patents

Abstract

786,516. Infra-red radiation, detecting. DISTILLERS CO., Ltd. Feb. 23, 1956 [March 12, 1955], No. 7305/55. Class 40 (3). A detector for the determination of the energy content of cyclically interrupted radiation of a certain frequency comprises two adjacent chambers 1, 3, at least one of them having a wall, e.g. 2, transparent to the radiation, and a passage 4 connecting the two chambers. Means are provided for filling the chambers with a gas which absorbs energy at the frequency concerned and the volume of the connecting passage is made extremely small, e.g. one millionth part of the volume of one of the chambers. An electrically heated element 7 is mounted in the passage and means are provided to measure the variations in temperature of this element due to the cooling effect of the gas flow past it resulting from pressure variations caused by the reception of radiation intermittently by one or both chambers. Where the capacity of the chambers, which are preferably equal in size and shape is from two to three cubic inches the passage may be rectangular in cross-section and 0.4 inches long by 0.003 inches wide with a depth of 0.003 inches. Square or circular section passages may also be used. The element should have a heat capacity approximately equal to that of the gas immediately surrounding it and low compared with the heat capacity of the passage as a whole. It is preferred to determine its temperature variations by resistance thermometry and it should therefore have a high resistance temperature variation. Suitable elements comprise nickel oxidecobalt oxide flakes and platinum or nickel wires. A platinum wire 3Á in thickness, 0.4 inches long, and supported symmetrically in the passage is suitable for a passage of the dimensions given. It should be under sufficient tension to maintain it taut. If the variations in resistance are to be detected by means of a bridge circuit a second and identical element 8 not exposed to the gas flow is mounted in one of the chambers and shielded from radiation. This then forms a reference arm of the bridge. A shield or shields are also necessary to prevent radiation penetrating the connecting passage and one such shield is shown at 5. A second passage of long time constant compared with the first may be provided between the chambers to stabilize the instrument against external fluctuations in temperature. Instead of a bridge-type of detector circuit (Fig. 6, not shown), the element 30 (Fig. 11), may be connected in series with a D.C. source 28 and an isolating resistor 29. An alternating current amplifier and indicator can then be coupled across it via a capacitor 39. The detector is suitable for use in a chopped double beam infra-red gas analyser of the general type described in Specification 683,952.