was recorded by all three counters. In addition, charged particle coincidences between the crystal sandwich and Cherenkov detectors were recorded. In this mode, directional information was obtained. The solid-angle-area factor of the telescope was about . The look direction of the telescope was identical to the symmetry axis of the spacecraft. For a beam incident parallel to the look direction of the telescope, the detection efficiency fell to 0 at 15° from this direction. The energy thresholds for each detector were as follows: (1) scintillation plastic (upper portion), electrons -350 keV, protons -3.5 MeV, (2) scintillation plastic (lower portion), electrons -400 keV, protons -35 MeV, (3) crystal sandwich, electrons -400 keV, protons -75 MeV, and (4) Cherenkov, electrons -15 MeV, protons -350 MeV. The accumulation time for the charged particle data was approximately 30 seconds. A single scaling circuit in Explorer 11 permitted one channel at a time to be monitored. During the 7 months in which the instrument was turned on and working in orbit, only 141 hours (3%) were considered useful observing time. During this time, the telescope was monitored for gamma rays and charged particles. Phoswich-Cherenkov Counter Telescope This experiment was designed to search for high-energy gamma rays (greater than 50 MeV) from the celestial sphere. The basic detector scheme consisted of a sandwich of NaI and CsI scintillating crystals (20 g/cm2), 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. The Explorer 11 telescope, developed at Massachusetts Institute of Technology (MIT), used a combination of a sandwich scintillator detector along