The Vela 6 nuclear test detection satellites were part
of a program run jointly by the Advanced Research Projects of the U. S.
Department of Defense and the U. S. Atomic Energy Commission, managed by the U.
S. Air Force. The twin spacecraft, Vela 6A and 6B, were launched on 8 April
1970 and placed ~180 degrees apart in nearly circular orbits at a geocentric
distance of ~118,000 km. The orbital period was ~112 hours. Each satellite
rotated about its spin axis with a ~64-sec period. The satellites carried both
X-ray and gamma-ray detectors which could be used for cosmic observations. The
X-ray detector was located ~90 degrees from the spin axis, and so covered the
entire celestial sphere twice per satellite orbit.
The scintillation X-ray detector aboard Vela 6A & 6B consisted of two 1-mm-
thick NaI(Tl) crystals mounted on photomultiplier tubes and covered by a 5-mil-
thick beryllium window. Electronic thresholds provided two energy channels,
3-12 keV and 6-12 keV. In front of each crystal was a slat collimator
providing a FWHM aperture of ~6.1x6.1 degrees. The effective detector area was
~26 cm2. Data were telemetered in 1-sec count accumulations.
Sensitivity to celestial sources was limited by a high intrinsic detector
background. The X-ray detectors failed on Vela 6A on 12 March 1972 and on Vela
6B on 27 January 1972.
The Vela 6 satellites also each carried 6 gamma-ray detectors with a total
volume of 60 cm3, covering the energy range 300-1500 keV. The
gamma-ray detectors continued to provide data until mid-1979. In fact, they
were still working well when tracking conflicts essentially shut the satellite
Data from the Vela 6 satellites was used to look for correlations between
gamma-ray bursts and X-ray events. At least 2 good candidates were found,
GB720514 and GB740723 (Terrell et al. 1982). The four Vela satellites (5A & B,
6A & B) recorded 73 gamma-ray bursts in the ten year interval July 1969 -
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