The normal operating mode was a continuous celestial scan, spinning approximately once every 20 min about the spacecraft z-axis, which was nominally pointed at the Sun.

[1] HEAO 3 included three scientific instruments: the first a cryogenic high-resolution germanium gamma-ray spectrometer, and two devoted to cosmic-ray observations.

The experimental package contained four cooled, p-type high-purity Ge gamma-ray detectors with a total volume of about 100 cm

, enclosed in a thick (6.6 cm average) caesium iodide (CsI) scintillation shield in active anti-coincidence[2] to suppress extraneous background.

[7] The experiment was proposed, developed, and managed by the Jet Propulsion Laboratory of the California Institute of Technology, under the direction of Dr. Allan S. Jacobson.

The HEAO C-2 experiment measured the relative composition of the isotopes of the primary cosmic rays between beryllium and iron (Z from 4 to 26) and the elemental abundances up to tin (Z=50).

They determined charge and mass of cosmic rays to a precision of 10% for the most abundant elements over the momentum range from 2 to 25 GeV/c (c=speed of light).

The primary data base has been archived at the Centre Etudes Nuclearires de Saclay and the Danish Space Research Institute.