The charged particle detector had four measuring systems: two particle telescopes that operated in interplanetary space, and two that measured the intense trapped electrons and protons inside theJovian radiation belts.
During the interplanetary phase of the mission, before and after encounter with Jupiter, the experiment sought to identify the chemical elements hydrogen, helium, lithium, beryllium, boron, carbon, and nitrogen, and to separate hydrogen, deuterium, helium-3 and helium-4 in an attempt to differentiate between particles coming from the Sun and those from the Galaxy. The instrument was also used to determine how the streams of high energy particles from the Sun travel through interplanetary space. The main telescope of seven solid-state detectors measured the composition of cosmic rays from 1 to 500 million electron volts (MeV), and a three-element, lowenergy telescope measures 0.4 to 10 MeV protons and helium nuclei.
Two types of sensors were used to cope with the extremely high intensities of trapped radiation in the magnetosphere of Jupiter. A solid-state electron current detector, operating below -40 degrees C, was designed to detect those electrons above 3 MeV that generate the decimetric radio waves emitted from Jupiter. A trapped proton detector contains a foil of thorium which undergoes nuclear fission when impacted with protons above 30 MeV, but is insensitiveto electrons.