The gamma-ray detector was 20 inches high, 10 inches in diameter, and
weighed about 30 pounds. It consisted of a sandwich crystal scintillator
(CsI and NaI) and a Lucite Cerenkov counter, surrounded by a plastic
anticoincidence scintillator. The 2 detectors in coincidence served to
define the solid angle of the instrument to about 17 degrees half-angle.
The satellite could not be actively pointed, and so, was put into a tumble
in order to get a "rough" scan of the entire celestial sphere. By 19 May
1961, the satellite, located between 300 and 1100 miles above the Earth,
began to send sky survey data to the ground. Over the next 4 months, it
provided "nearly 20 miles of data on microfilm". Reconstruction of the data
allowed gamma-ray times of arrival to be determined to 0.1 second, and where
the detector was pointed to about 5 degrees.
Images of both the gamma-ray detector and schematics of its design are
included in the
Explorer-11 images (http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/images/explorer11_images.html) page.
