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Hard X-Ray Experiment (HXX)

NSSDCA ID: 1974-070A-03

Mission Name: ANS
Principal Investigator:Prof. Jonathan E. Grindlay


This experiment was designed to detect cosmic X-ray emissions in the energy range from 1 to 30 keV. The principal scientific objectives of the experiment were (1) to gather spectral data with an energy resolution of 20%, (2) to detect silicon emission lines at about 1.8 and 2.0 keV at an energy resolution of 0.15%, (3) to study periodic and random intensity variations of sources over a time range of 4 ms to several minutes, (4) to obtain data on X-ray light curves, and (5) to define positions of sources with a precision approaching 1 arc-min. The experimental package contained three major components: (1) a collimator assembly, (2) a large area detector (LAD) unit for measuring 1- to 30-keV X-rays, and (3) a Bragg-crystal spectrometer tuned for detection of the silicon lines. The LAD and Bragg spectrometer detectors were very sensitive, being able to detect 3E-3 photons/(sq cm-s). X-ray incident on the front face of the package passed through the collimator assembly onto either the LAD or a series of four Bragg crystals that were oriented at about 45 deg with respect to the incident beam. The collimation in front of the LAD was a combination of a fine collimation (10 arc-min FWHM) and coarse collimation (3 deg FWHM). The two counters composing the LAD were pointed in directions that differ by about 4 arc-min. This allowed for the possibility of guiding on strong X-ray sources in the event guide stars were not available. Also, it made for a uniform response of the combined output of the detectors within this 4 arc-min range. Each detector consisted of an argon-filled proportional counter with a 9.4-mg/sq cm beryllium window. The effective collection area of each counter was about 40 sq cm, after correction for the collimator transmission, and each had a detection efficiency in excess of 10% from 1.8 to 20 keV. The output from the LAD counters was processed by a 15-channel logarithmic pulse-height analyzer, all channels of which were recorded in memory either every 4 s or 64 s. Higher time resolutions of 1 to 4 ms were possible through the use of a scheme that recorded the time of arrival of the first six events occurring each second in the LAD. In addition, a single channel analyzer was used to record the integrated counts in the 1.3- to 7-keV range in 1-, 4-, or 16-s intervals. Only the coarse collimator fed X-rays onto the four Bragg pentaerythritol (PET) crystals. The diffracted X-rays were then detected by two argon-filled proportional counters with 4.7-mg/sq cm beryllium windows. When projection effects and peak reflectivity of the crystal were accounted for, the effective detection area of each counter was 6 sq cm within the 2-eV resolution of the crystal. The output from a Bragg detector was filtered by an eight-channel logarithmic pulse-height analyzer operating in the energy interval from 1 to 4.2 keV. For both the LAD and Bragg detectors, effective non-X-ray event rejection was accomplished by pulse-shape discrimination of the proportional counter signals. For additional details on this instrument, see Astrophys. J., Letters, v. 201, p. L127, 1975.

Alternate Names

  • HXX

Facts in Brief

Mass: 8 kg
Power (avg): 2 W

Funding Agency

  • NASA-Office of Space Science (United States)


  • Solar Physics: X-Ray

Additional Information

Questions and comments about this experiment can be directed to: Coordinated Request and User Support Office



NameRoleOriginal AffiliationE-mail
Dr. Herbert W. SchnopperOther InvestigatorDanish Space Research Institute
Prof. Jonathan E. GrindlayPrincipal InvestigatorHarvard College Observatory
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