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Low-Energy Gamma-Ray and Hard X-Ray Sky Survey (A-4)

NSSDCA ID: 1977-075A-04

Mission Name: HEAO 1
Principal Investigators:Dr. Walter H. G. Lewin
Principal Investigators:Prof. Laurence E. Peterson


This experiment measured point and diffuse sources of X-rays and gamma rays in the 10-keV to 10-MeV range. The instrument consisted of seven NaI(Tl)/CsI(Na) Phoswich scintillators surrounded by eight large CsI(Na) scintillators that provided shielding and defined the fields of view. There were three detector types. The intermediate-energy detectors had an energy range of 10 to 200 keV, an area of 225 sq cm, CsI shielding of 2 in., and a field of view (FWHM) of 1 deg x 20 deg. The slat collimators of the intermediate-energy detectors were positioned at 60 deg relative to the scan direction, allowing point source determination to 1 deg over the approximately 40-deg-wide band scanned during each spacecraft rotation. The point-mode detectors had an energy range of 0.1 to 5 MeV, an area of 180 sq cm, CsI shielding of about 4 in., and a field of view (FWHM) of 20 deg. Sources detected were identified with low-energy sources by spectral similarity with measurements made by the intermediate-energy detector at about 100 keV. The diffuse-mode detectors had an energy range of 0.2 to 10 MeV, an area of 125 sq cm, CsI shielding of about 6 in., and a field of view (FWHM) of 10 deg. Point sources measured by the diffuse-mode detectors were related to those with similar spectra in the point-mode detectors. Each of the detectors was equipped with a pulse-shape analyzer and discriminator that detected and vetoed CsI(Na) events. The combination of shield upper-and lower-level discriminators (nominal settings of 5 MeV and 0.1 MeV) used for detector anticoincidence was selectable by command. Event time was nominally known to 0.1-s accuracy. This could be improved to 5 ms or 2E-5 s by command. The instrument also contained three particle monitors, which measured proton and electron fluxes in three energy ranges. There was a high-resolution timing system that measured cosmic gamma-ray bursts. For more details, see R. Rothschild, Earth and Extra. Sci., v. 3, pp. 189-196, 1979.

Alternate Names

  • HEAO1/Low-EnergyGamma-RayandHardX-RaySkySurvey_A-4_

Funding Agency

  • NASA-Office of Space Science (United States)


  • Astronomy: Gamma Ray

Additional Information

Questions and comments about this experiment can be directed to: Dr. Cynthia Y. Cheung



NameRoleOriginal AffiliationE-mail
Prof. Laurence E. PetersonPrincipal InvestigatorUniversity of California, San
Dr. Walter H. G. LewinPrincipal InvestigatorMassachusetts Institute of
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