Instrumentation

X-Ray and EUV Spectroheliograph (2 to 400 A)

The objective of this experiment was to determine the distribution of matter and temperature in the corona above solar active regions and determine how this matter changes during solar flares. Four distinct instruments were used. The first was a grazing-incidence spectrometer with a spectral resolution of 1.0 A, used to cover the range 120 to 400 A. The various discrete wavelengths were detected by three Bendix electron multipliers mounted on a moving carriage. Second, a long X-ray spectroheliograph with a bandpass of 2 A was used to cover the range 8 to 15 A. The third instrument was a short X-ray spectroheliograph, used to cover the range 1.7 to 8.0 A. Both X-ray spectroheliographs used the balanced filter method. Fourth, a polarimeter using the scattering technique was used to cover the 20 to 40 keV range. The spatial resolution of the EUV spectroheliograph was 10 x 20 arc seconds. The spatial resolution of X-ray spectroheliographs was 20 x 20 arc seconds. The short EUV detector failed in March 1972. The medium EUV detector sensitivity started dropping during October 1972, and by March 1973 was 60 percent of the original value. The long EUV detector degraded to the point that it no longer produced useful scientific data as of May 1973. Only real-time data were obtained after May 18, 1973 when the spacecraft tape recorder failed.

White-Light Coronagraph and Extreme Ultraviolet Corona

This experiment was designed (1) to study the morphology of the corona in white light and the extreme UV in relation to active phenomena, such as plages and flares in the lower solar atmosphere, and (2) to correlate the white light corona with the extreme UV corona and with solar and interplanetary magnetic fields. The instrumentation was located within the pointed section of the spacecraft and consisted of (1) a white-light coronagraph for use in the pointed mode to record the outer corona of the sun from approximately 3 to 10 solar radii in the visible band of 3900 to 6500 A and (2) an extreme UV coronagraph for use in the raster mode to record the upper chromosphere and lower corona fully to two solar radii and partially to five solar radii in the band from 170 to 550 A. The white-light instrument was a modified Lyot coronagraph that artificially eclipsed the sun with a spar-mounted external occulting disk assembly mounted approximately 76 cm in front of the instrument. The faint outer corona could then be observed against the black sky of space. The image was stored in a SEC Vidicon tube with 256 raster lines, each having 256 picture elements. The distance between picture elements was 1.25 arc-min. The extreme UV coronagraph required no occultation device since the solar disk was not an overwhelming source of extreme UV radiation. There were four open-to-vacuum channel photomultiplier detectors in the image plane behind pinhole apertures in an aperture plate. The assembly was scanned across the solar image in a raster mode. The central aperture detector had a spatial resolution of 20 arc-s. The remaining aperture detector combinations were offset, excluding the disk, and had a resolution of 60 arc-s. In a large raster mode, the scanned areas overlapped. The experiment operated normally until March 1972, when it became partially operable. The extreme UV coronagraph degraded until it became useless in September 1973.

Cosmic X-Ray Experiment

The UCSD cosmic X-ray instrument was a sensitive detector mounted in the rotating wheel section of the spacecraft so that it viewed the celestial sphere in 6 months. The objectives of the experiment were (1) to locate accurately known and newly detected X-ray sources, (2) to measure the intensity of the sources, and (3) to analyze the spectrum of the sources over the range of 7 to 500 keV. The experiment capabilities were (1) a full conical look angle of 6.5 deg, (2) a spatial resolution of plus/minus 0.2 deg, (3) a sensitivity of 5.E-4 photons/(sq cm-s), (4) an energy resolution provided by the use of 126 channels for the 7-500 keV range, and (5) a maximum detection rate of 3.12 photons/s. The X-ray detector was a 4-in diameter by 3/8-in thick NaI(Tl) scintillation crystal viewed by a 3-in. photomultiplier tube (PMT). The detector was surrounded by a thick CsI(Na) scintillation crystal shield with 10 holes bored through it along the optical axis to define the field of view of the detector. The shield scintillator was viewed by six PM tubes. Light pulses in the NaI crystal caused by X-rays that had passed through the holes in the shield had relatively slow rise times and had intensities proportional to the energy of the photons. The corresponding proportional current pulses out of the PM were recognized as valid events and processed by the data system. X-rays or particles that passed through the CsI shield caused light pulses with fast rise times and corresponding pulses in the shield PM tubes. Pulses from the shield PM tubes were used to electronically reject simultaneous pulses from the detector PM. In this way X-rays passed through the collimating holes were processed as useful data.

Cosmic X-Ray Sources in the Range 1.5 to 9 A

The purpose of this MIT experiment was to survey the entire sky for cosmic X-ray sources in the energy range 1 to 60 keV with an angular resolution of about 1 deg and perform spectral analysis in five broad bands. Each portion of the sky was viewed several times during each year of operation. Two multicompartmented proportional counters equipped with honeycomb collimators (3.5-sq deg solid angle) were mounted in one segment of the OSO wheel section, with the centers of their fields of view oriented 15 deg above and 15 deg below the spacecraft equator. X-rays were detected in one or another of four compartments depending upon their energy. Low-energy photons were stopped in the first compartment, higher-energy photons penetrated to the second compartment, and photons of even higher energies penetrated through the first and second compartments to the third and fourth compartments. The energy bands were logarithmically equispaced. A separate single compartment counter with a thin aluminum window detected photons between 1.0 and 1.5 keV. Counts from each compartment were stored in one of 256 accumulators corresponding to a division of the spacecraft spin into 256 sectors. Inflight calibration was provided by periodic exposure to a radioactive source.

Hard Solar X-Ray Monitoring

The UCSD solar X-ray experiment was designed to accomplish two principal objectives: (1) to study, with good temporal and energy resolution, the solar X-ray emission over the energy interval 2-300 keV and (2) to monitor the local radiation environment (cosmic rays, trapped protons and electrons, and cosmic and local X-rays), thereby allowing for a clean interpretation of the primary results. The charged particle data from solid-state detectors were read out each 15.36 s. The instrument was located in the rotating wheel section of the spacecraft. The three detector systems in the instruments were: (1) a collimated proportional counter (2-15 keV), (2) a NaI(Tl) scintillation counter (10-300 keV), and (3) three silicon surface-barrier, charged-particle devices. The proportional counter consisted of an aluminum collimator (20 by 90 deg), a 2-mm-thick Be window, and an aluminum-lined counter filled to one atm with xenon and carbon dioxide in a nine to one ratio. The detector was provided with a weak Fe(55) source for inflight calibration. The basic element in the hard X-ray detector was a 1-cm thick by 3.5-cm diameter NaI(Tl) scintillator directly coupled to an RCA photomultiplier tube (PMT). A two-segment CsI(Na) anticoincidence shield surrounded the aluminum cylinder subassembly. Each segment of the shield was polished, wrapped in aluminum foil to provide efficient light reflection to an end-mounted PMT, covered by a layer of lead foil, and placed in an aluminum housing. The shield-detector unit had a 90-deg response in the wheel-rotation plane and a plus/minus 10-deg response in the perpendicular direction. The detector output was analyzed and loaded into nine energy-channel counters. Inflight calibration for the system was provided by monitoring the outputs from a series of suitably placed Am(241) sources.

Solar Gamma-Ray Monitor

This University of New Hampshire experiment monitored the solar high-energy photon (gamma-ray) spectrum from 0.3 to 10.0 MeV for intensity, time variation, and possible line emission, particularly during flares. In addition, special attention was paid to lines at 0.51 MeV, 2.22 MeV, 4.43 MeV, and 6.14 MeV, which indicate the production of positrons, neutrons, and excited nuclei in the solar atmosphere. The instrumentation consisted of a high-resolution, gamma-ray scintillation spectrometer mounted in the wheel section. The detector was a NaI(Tl) scintillation crystal viewed by a photomultiplier. It was surrounded by a CsI(Na) anticoincidence shield and photomultiplier array, which also provided directional sensitivity. Gamma rays were detected in 377 energy channels from 0.3 to 9.5 MeV by a pulse-height analyzer. For correlation purposes in the data analysis, a secondary system made up of a thin NaI(Tl) scintillation crystal and a photomultiplier was used for detecting X-rays from 7.5 to 120 keV. X-ray pulses were divided by a four-channel pulse-height analyzer. High-energy solar neutrons (energies greater than 30 MeV) could be identified from a study of the large energy loss events in the central detector, and their correlation was

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