Explorer 11 was the first gamma-ray detection satellite flown, weighing
in at 82 pounds. It was launched on 27 April 1961 and the instrument
aboard was designed to detect gamma-rays above 50 MeV.
Over a period of 23 days, 9 hours of data from "pointing into space" was
obtained. In this data were found 22 events from gamma-rays and 22,000
events due to charged cosmic rays.