Editing Explorer 11 (section)

=== Phoswich-Cherenkov Counter Telescope ===
This experiment was designed to search for high-energy gamma rays (greater than 50 [[Electronvolt|MeV]]) from the [[celestial sphere]]. The basic detector scheme consisted of a sandwich of NaI and CsI scintillating crystals (20 g/cm<sup>2</sup>), viewed by a single [[photomultiplier]], and a Lucite Cherenkov counter, viewed by two photomultipliers. It was completely surrounded by a shield of scintillating plastic viewed by five photomultipliers. The sandwich detector provided high-atomic-number material for the pair production process, i.e., energetic gamma rays were converted into [[electron]]-[[positron]] pairs. The electrons and positrons then entered the Cherenkov counter, which detected particles traversing its volume in only the downward sense. The simultaneous electric signals from its two PMTs indicated that one or more charged particles had traversed the telescope. The signals or lack of signals from the surrounding plastic shield at the instant the telescope had been triggered indicated whether the triggering was caused by an uncharged or charged particle. Also, the "last" and "total" components of the signals from the photomultiplier that viewed the sandwich detector afforded a method of distinguishing neutron and gamma-ray induced events. The experiment performed normally from launch until 17 November 1961.<ref name="Experiment1">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1961-013A-01|title=Experiment: Phoswich-Cherenkov Counter Telescope|publisher=NASA|date=28 October 2021|access-date=4 November 2021}} {{PD-notice}}</ref>
[[File:Explorer 11 artist vision.gif|thumb|Explorer 11 in orbit]]
The Explorer 11 telescope, developed at [[Massachusetts Institute of Technology]] (MIT), used a combination of a sandwich scintillator detector along with a [[Cherenkov detector|Cherenkov counter]] to measure the arrival directions and energies of high-energy gamma rays. Since the telescope could not be aimed, the spacecraft was set in a slow spin to scan the celestial sphere. Due to a higher than planned orbit that carried the spacecraft into the detector-jamming radiation of the [[Van Allen radiation belt]], and an early failure of the on-board tape recorder, only 141 hours of useful observing time could be culled from about 7 months during which the instrument operated. During this time thirty-one "gamma-ray signature" events were recorded when the telescope was pointing in directions well away from the [[atmosphere of Earth]], which is a relatively bright source of gamma rays produced in interactions of ordinary cosmic ray [[proton]]s with air atoms. The celestial distribution of the thirty-one arrival directions showed no statistically significant correlation with the direction of any potential cosmic source. Lacking such a correlation, the identification of the cause of the thirty-one events as gamma rays of cosmic origin could not be established. The results of the experiment were therefore reported as upper limits that were significantly lower than the limits obtained from previous balloon-borne experiments.

An improved gamma-ray telescope, also developed at MIT, was flown on the [[Orbiting Solar Observatory 3]] ([[OSO 3]]), which was launched in 1967. It achieved the first definitive observation of high-energy cosmic gamma rays from both galactic and extragalactic sources. Later experiments, both in orbit and on the ground, have identified numerous discrete sources of cosmic gamma rays in our [[galaxy]] and beyond.