Golay Cell Research Articles

Far-Infrared Vacuum Grating Spectrometer*

A far-infrared grating spectrometer has been developed specifically to investigate gases and solids in the 200–1000μ region. The monochromator is a ƒ/2 Czerny–Turner system and three gratings with constants of 250, 500, and 1000μ are employed. Filtering of unwanted radiation is accomplished by using metallic-mesh reflection filters and a conical light pipe is used for the concentration of the light onto a Golay detector window. This instrument is calibrated by using the pure rotation spectrum of water vapor or methylchloride (CH3Cl).

The Cooled Germanium Bolometer as a Far Infrared Detector

A direct comparison of the signal/noise ratios between a Golay detector and a cooled germanium bolometer is described. The Perkin-Elmer 301 far infrared spectrophotometer was used in the comparison. The spectral range 90 μ to 250 μ was covered.

Vibration Insulating Mounting for a Golay Cell

First Page

Lunar Brightness Distribution at 1.5 mm Wave-length

WE have recently measured the microwave brightness temperature over the lunar disk using a new 1.5 m aperture Cassegrain telescope with both cooled indium antimonide and Golay cell detectors. The measurements cover a fairly broad band centred at 1.5 mm and with a mean wave-length deviation of about 0.2 mm1. The results of solar scans show that the angular response function of the telescope when used with either detector is of approximately Gaussian form. The results of two series of lunar drift curve measurements are shown in isothermal form in Figs. 1 and 2. The results have been corrected for the time constant of the detector circuit but not for the angular response function of the telescope.

Versatile Amplifier for Precise Radiometry

An amplifier is described that is suitable for the precise measurement of chopped radiation at very low intensity. It can be operated with all types of detectors used in such measurements, including thermopiles, bolometers, Golay cells, photomultipliers, photoconductive cells, and photoemissive cells, and in all cases the minimum detectable signal is determined only by the inherent noise of the detector. Comprehensive descriptions of the circuits, their principle of operation, and performance figures are given. Chopping frequencies between 5 cps and 1 kc can be used, and the amplifier gain is stabilized and defined by the application of feedback. It is shown that an electrical signal of 10−6 V rms from a 22-Ω source can be measured to a precision of 0.1%, while with very small signals such as 10−9 V rms, the precision is limited only by the amount of noise present at the detector.

Spectroscopy at extreme infra-red wavelengths III. Astrophysical and atmospheric measurements

Observations of solar, lunar and atmospheric radiation between 1 and 4 mm wavelength are described. The optical arrangement employed parabolic reflecting telescopes with Golay cells as detectors and gauzes or objective gratings for wavelength selection. The atmospheric results include measurements of transmission through various types of cloud and also the detection of an absorption line at about 1·6 mm which is almost certainly the 3 -2 -2 2 pure rotation line of water predicted by Van Vleck. More generally the atmospheric measurements confirm earlier experimental results showing that the absorption due to the tails of submillimetre absorption lines is greater than that predicted by Van Vleck. Some direct laboratory measurements have been made of pressure broadening of the water vapour line at 0.538 mm. Combined solar and lunar measurements give an effective solar temperature of 5800 °K at 1·3 mm and 6700 °K at 2·5 mm wavelength and a value of δ = 0.36 for the ratio of thermal to electromagnetic attenuation coefficients for lunar surface material.

Far Infrared Spectrometer with a Variable Depth Grating

This paper describes the construction of a far infrared vacuum spectrometer. A variable depth grating was used as a dispersive medium, and the mounting is of the Littrow type. The detecting element is Golay cell. The spectral range now available is from 200 to 1500 µ in wavelength. The results of performance test and the discussion on the variable depth grating are also presented.

Superheterodyne Radiometers for Use at 70 Gc and 140 Gc

In this paper four different millimeter wave equipments, which have been made for plasma diagnostic work, are described. They are: 1) A straightforward 70-Gc superheterodyne radiometer with an over-all noise factor of 13 db; 2) An early 140-Gc radiometer, with second harmonic mixing, which has an over-all noise factor of about 25 db; 3) A later and more sensitive 140-Gc radiometer which contains a fundamental local oscillator, VX 3352 mixer crystals and a 408-Mc IF amplifier commencing with an Adler tube; 4) A very simple 140-Gc transmission measuring equipment containing a 1-watt source and a crystal video receiver which has a tangential sensitivity of ---42 dbm. The last part of this paper discusses the minimum temperature changes which can be detected, at short millimeter wavelengths, with various types of superheterodyne radiometers, the Golay cell, the barretter, the crystal video radiometer, the 1.5/spl deg/K carbon bolometer and the 1.5/spl deg/K InSb photoconductive detector. The performances expected from straight traveling-wave tube radiometers and traveling-wave masers at short millimeter wavelengths are also considered. The Appendices are devoted to mixer crystal performance in the millimeter and submillimeter regions, a theory of second harmonic mixing and the voltage sensitivity of a forward biased detector crystal.

Far-Infrared Interferometer for the Measurement of High Electron Densities*

The average electron concentration of a gas-discharge plasma can be inferred from the phase change suffered by an electromagnetic wave in its passage through the medium. With available microwave generators, the maximum electron density that can be measured is approximately 3×1014 cm−3. On the other hand, interferometers that operate at optical wavelengths become insensitive at electron densities less than approximately 1016 cm−3. An infrared interferometer is described here which operates at 0.03 cm wavelength and is able to detect densities above 5×1013 cm−3 in a plasma column 1 cm thick. In the design, incoherent radiation from a mercury arc lamp is split by a diffraction grating and the plasma placed in one of the diffracted beams. The beams are then recombined and fringe shifts measured by a Golay detector.

The detection of sub-mm radiation

The characteristics of an ideal detector are discussed and are compared with the practical results achieved with the following types of detectors: superheterodyne and video receivers using point-contact rectifiers; InSb photoconductive detectors, both wide-band and tunable; thermal detectors, including the Golay cell, and carbon, germanium and superconducting tin bolometers. The practical performance achieved is considerably inferior to that which is ideally possible. The extension of microwave techniques is limited by practical difficulties in constructing the detectors and other components required, but the performance obtained from the infra-red systems which require cooling with liquid helium is limited by that of the LF amplifiers used with them.

Design of interference filters for the observation of infra‐red emission from atmospheric carbon dioxide by an earth satellite

AbstractDetailed proposals are made for the design of a high‐order Fabry‐Perot type interference filter in which the transmission bands of the filter are exactly matched to 15 lines of the R‐branch of the CO2 spectrum near 15 μ. The striking energy advantages of such an interferometer are discussed and it is shown that use of an optically worked high‐index germanium spacer leads to a further considerable increase in light grasp. It is concluded that technical problems are not severe and a calculation of the energy that can be collected by the proposed system suggests that a detector of area 9 mm2 with a noise‐equivalent‐power of 10−10 watts (i.e. comparable with a standard Golay cell) can provide adequate sensitivity.

Radio-astronomy, Radiometry and Infra-red Techniques

The year 1932 was one which had no great apparent significance for navigators, and yet it saw the commencement of two new lines of research which today, after an interval of more than a quarter-of-a-century, promise important contributions to the safety of navigators both at sea and in the air.The two lines of research were superficially quite unrelated, but fundamentally they relied upon the same principle—the detection of radiant energy emitted by objects solely as a result of their temperature. The first of these small beginnings was the discovery by K. G. Jansky that radio waves could be detected from extra-terrestrial sources: the second was the commencement by the U.S. Signal Corps Engineering Laboratories of an intensive study of infra-red devices with the object of obtaining night vision without illumination of the field of view.From Jansky's discovery has sprung the whole science of radio astronomy, which has revolutionized our ideas about the universe, and brought in its wake, as one practical benefit, the radio sextant. From the work of the U.S. Signal Corps there resulted a number of very useful infra-red components, including the pneumatic detector, better known as the Golay cell.

Far Infrared Spectrograph for Use from the Prism Spectral Region to about 1-mm Wavelength

A far infrared spectrograph is described. The mounting is of the Czerny-Turner type. The radiation sources are a heated platinum strip used in the short-wave region and a high-pressure mercury lamp used in the long-wave region. Golay cells with diamond or quartz windows are used as detectors. Holders for filters to eliminate short-wave radiation and for reflection or transmission samples are designed for convenient use. Special chambers provide optical paths for long gas cells and provide holders for small solid samples. All parts of the optical system are in a vacuum tank and are remote-controlled. Spectra are automatically recorded on a strip-chart recorder. Spectra in the region from 18 microns to about 1 mm, obtained with six echelette gratings, are shown.Reproducibility of wavelength settings is one part in 5000. Resolution of better than 0.5 cm−1 throughout the entire spectral region has been achieved.

Observations of Solar and Lunar Radiation at 15 Millimeters

Observations of the radiation from the sun and the moon in a wide wavelength band centered approximately at 1.5 mm are reported. These waves were detected with a Golay detector using a searchlight mirror for a telescope. Measurements of atmospheric extinction are reported and compared with calculations of water vapor absorption which have been made by J. H. Van Vleck. It is found that the measured extinction is greater than the calculated.Observations of the radiation emitted by the moon at different phases are reported.The transmittances of some materials to the 1.5-mm waves from the sun are included.

An Infrared Spectrograph for Use in the 40–150-Micron Spectral Region*

Problems relating to the design of a far infrared spectrograph are discussed. The design of a spectrograph which was constructed is described. A platinum strip coated with thorium-oxide is used as a source. Discrimination against short-wave radiation is provided by quartz, paraffin, turpentine soot, a compensated potassium bromide chopper, reststrahlen plates, and a grating (used as a reflection filter) with groove separation somewhat less than the wavelength being investigated. As a dispersing device, an echelette grating ruled with 180 lines per inch is used. The detector is a Golay pneumatic cell. A low noise vacuum-tube amplifier renders the signal from the detector suitable for operation of a strip-chart recorder. The spectrograph is evacuable.Spectral operating characteristics of the instrument are shown in ammonia and atmospheric water vapor spectra in the region between 45 and 150 microns. Absorption lines separated by less than 1 cm−1 are well resolved throughout this spectral region.

A New Classification System for Radiation Detectors*

In Part I, a classification system for radiation detectors is proposed. The system is based upon the manner in which the noise equivalent power depends upon the time constant and the sensitive area, when the sensitivity is limited either by radiation fluctuations or by internally generated noise.The major part of Part I is devoted to establishing the reference condition for the measuring of the noise equivalent power. In establishing the reference condition of measurement (Section 6), the output of the detector in the absence of a signal is first equalized so that the noise power per unit frequency band width is constant. The reference time constant is then defined in terms of the relative response as a function of signal modulation frequency (Section 7). The equalization is then modified by the addition of an RC low pass filter with a time constant equal to the reference time constant, and the noise equivalent power is measured. The power so obtained is termed the noise equivalent power in reference condition A, denoted by Pm. This procedure has the property that the noise equivalent power of the detector is measured in the presence of noise whose band width is equal to the band width of the detector.In order to establish the reference condition of measurement it is necessary to consider the various sources of noise which are involved in detectors (Section 2), and the various types of time constants (Section 3). The important concept of responsivity-to-noise ratio is defined in Section 4. A general theorem involving the sensitive area and the responsivity-to-noise ratio is established in Section 5.After the question of detectors with non-uniform spectral sensitivity is discussed in Section 8, the classification system is defined in Section 9: A detector is a Type I detector if its noise equivalent power depends upon its sensitive area A and its reference time constant τ in accordance with Pm=A12/k1τ12,where k1 is independent of A and τ. A Type II detector is defined by Pm=A12/k2τ,and more generally, a Type n detector is defined by Pm=A12/knτ12n.The usefulness of the proposed classification is illustrated in Section 10 by its application to sequential scanning systems.In Part II the classification system proposed in Part I is used to determine the type number of each of eight different kinds of detectors. By a detailed analysis of each kind of detector, it is found that all of the detectors studied are either Type I or Type II detectors. The results of the analysis are summarized in Table I.The detectors studied include bolometers (Section 2), thermocouples and thermopiles (Section 3), the Golay detector (Section 4), photographic plates and films (Section 5), vacuum and gas photo-tubes and photo-multiplier tubes (Section 6), and dipole antennas (Section 7). In the case of the dipole antenna it is shown that Johnson noise becomes equivalent to the noise produced by radiation fluctuations.