γ Rays of 1 to 10 MeV from the Crab and Cygnus Regions

Abstract

As part of a study of techniques for γ-ray astronomy in the MeV energy range, several balloon payloads have been launched by the Southampton University group from Palestine, Texas. The following results are from a payload launched on September 23, 1972, which floated at a depth of 8 g cm⁻² for 7 h from 0600 GMT. The payload consisted of an anticollimation telescope using two 12 cm by 5 cm NaI(Tl) crystals side by side, each viewed by a single 12 cm phot omul tiplier, and protected from charged particles by a plastic scintillator box. These detectors were made directional for low energy γ rays by a lead occulting disk, 18 cm in diameter and 10 cm thick, placed 50 cm above the crystals, the axis of the whole assembly being pointed so as to track a celestial source continuously. The occulting disk alternated between positions over each detector at intervals of 5 min. In-flight energy calibration was carried out using a ²²³Th γ-ray source. In addition to the γ-ray detectors, the payload contained a thin NaI(Tl) detector sensitive to 100 keV X rays. The null results obtained on some celestial sources indicating that the movement of the lead disk did not, by itself, affect the counting rate of a detector.