Search

The need for detectors in spectroscopic research in both the laboratory and in space has led in recent years to the development of a variety of spectrally selective detectors which may be put in two broad categories: (1) photoionization chambers having relatively narrow bandpasses within the region of 1050 to approximately 1500 Å; (2) multiplier phototubes having high work function photocathodes falling into three major groupings which restrict the sensitivity to regions below approximately 3000 Å, 2000 Å or 1000 Å. This paper reviews the characteristics of vacuum u.v. ion chambers having fill gases such as nitric oxide, acetone, carbon disulfide or ethylene oxide with windows of LiF, CaF2or BaF2. Their quantum yields range from approximately 10–50 per cent and may be operated in gas multiplication mode with gains of 103at 600–800 V. The characteristics of photomultipliers and phototubes having alkali telluride, copper iodide or other cathodes with lithium fluoride, sapphire, quartz or no window are described. Some of the photomultipliers studied have quantum efficiencies as high as 30 per cent, exhibit long wavelength rejection rations of many orders of magnitude over only several hundred Å. The discussion of u.v. photodetectors is mostly confined to work that has been done either at the Goddard Space flight Center or in association with it in connection with astrophysical and geophysical research.

The solar spectral irradiance outside the earth’s atmosphere was determined by Langley’s method of extrapolation to zero air mass, from measurements taken on Mount Lemmon at an elevation of 8025 ft near Tucson, Arizona, during October, 1951. The spectrum was produced and the energy scanned by a Leiss quartz double-monochromator, detected by a 1P21 photomultiplier, amplified, and presented on a strip chart recorder. About twenty-five spectra were recorded from sunrise to noon, with band widths ranging from 10 A at 3030 A to 170 A at 7000 A. The equipment was calibrated frequently by recording the spectrum of a standard tungsten lamp. Compared with earlier work performed in this field, our results agree best with those of Pettit. There is good agreement with the direct measurements from a rocket obtained by Purcell and Tousey in 1954 and with the Sacramento Peak ultraviolet observations by Stair and Johnston in 1955. The change of solar intensity with air mass showed that the attenuation of the atmosphere above Mount Lemmon was approximately 15% higher than that for a Rayleigh atmosphere in the region 3400 A to 4650 A, where there is no absorption due to ozone. A discussion is included which emphasizes the importance of clear and constant atmospheres which are necessary to obtain accurate values of solar spectral irradiance outside the earth’s atmosphere by the Langley method. The solar illuminance computed from spectral data was 12 700 lumens/ft^2.

A brief account is given of the historical development of Goddard Space Flight Center in relation to optical studies. This is followed by short accounts of current activity in the uv and soft x-ray spectral regions, in lasers and holography, in vision, photography, and testing. Finally there is an optimistic word about the future of optics at GSFC and in NASA as a whole.

The ultraviolet spectral response characteristics of several 1P28 photomultipliers constructed in quartz envelopes, and of conventional 1P28 photomultipliers in glass were measured using a vacuum grating monochromator equipped with a thermocouple for measuring the absolute energy at the exit slit. The radiation source was a hydrogen discharge tube. The quartz multipliers were found to respond to 1550A and the glass multipliers to 1750A. The limits and response near the limits were determined by the absorption of the respective envelopes. Each type responded to a small extent at wavelengths below the enevlope transmission limits, because of fluorescence of the envelope. The response at 2537A varied over the same range for both types.

Photographs of the constellation Cygnus taken in the daytime from altitudes above 100 kilometers indicate that the day sky brightness in the wave-length region from 3600 to 7000 angstroms is only slightly brighter than the night sky viewed from the ground. No diffuse cloud of particles was apparent in the vicinity of the rocket payload, but discrete particles must be considered in the design of instruments for rockets and satellites. The resultant data and reports of star sightings from manned spacecraft indicate similar optical environments for both types of vehicles, that is, discrete particles and relatively low levels of background brightness, only slightly brighter than the night sky as an upper limit.

Under astronomically favorable circumstances, photographs do not reveal excess light near the triangular libration points of the earth-moon system. We find that the visible surface brightness of anomalous dust populations, if these populations do exist, is less than 10-9candela per square centimeter.

The ultraviolet spectral response characteristics of several 1P28 photomultipliers constructed in quartz envelopes, and of conventional 1P28 photomultipliers in glass were measured using a vacuum grating monochromator equipped with a thermocouple for measuring the absolute energy at the exit slit. The radiation source was a hydrogen discharge tube. The quartz multipliers were found to respond to 1550A and the glass multipliers to 1750A. The limits and response near the limits were determined by the absorption of the respective envelopes. Each type responded to a small extent at wavelengths below the enevlope transmission limits, because of fluorescence of the envelope. The response at 2537A varied over the same range for both types.

Laboratory and space research have both benefited from the new spectrally selective ultraviolet photodetectors. Large band-gap photoemitters, such as cesium telluride, having high quantum efficiencies in the ultraviolet and very low yields at longer wavelengths are now available. An image tube with such a cathode combines the advantages of spectrally selective photoemission with the flux-integrating property of photographic emulsion. Observations can thus be made at selected ultraviolet wavelength regions, either photographically or visually, even in the presence of strong long wavelength backgrounds. Visual measurements with a Cs-Te image converter, having a sapphire faceplate and a green phosphor, were made with the following results: (a) cathode quantum efficiency is 0.016, which is expected to improve in future tubes; (b) minimal detectable converted 2537 A irradiance is 4×10-5ergs cm-2sec-1; and (c) tube background is undetectable. Mercury spectra obtained with this tube demonstrate the expected high rejection of visible light. Tables are included which list the brightnesses of several extended ultraviolet and visible sources of interest in the space sciences. The luminance of visible sources; the radiance of ultraviolet sources; and the relative brightness, on a photon basis, of spectrally converted ultraviolet sources along with visible sources are tabulated.