Design and flight performance of the cosmic ray detector BUGS-4

The design features and operational performance from the test flight of the fourth generation of spherical geometry cosmic ray detectors developed at Bristol University (Bristol University Gas Scintillator 4 - BUGS-4) are presented. The flight from Ft. Sumner (NM) in September 1993 was the premier flight of a large (1 m radius) spherical drift chamber which also gave gas scintillation and Cherenkov signals. The combinations of this chamber with one gas and two solid Cherenkov radiators lead to a large aperture factor (4.5 m2 sr), but low (∼3.5g/cm2) instrument mass over the energy sensitive range 1 to several hundred GeV/a. Moreover, one simple timing measurement determined the impact parameter which provided a trajectory (path length) correction for all detector elements. This innovative and efficient design will be of interest to experimental groups engaged in studies of energetic charged particles. Although there were technical problems on the flight, which were compounded by the total destruction of BUGS-4 by fire whilst langding in Oklahoma, there was a period of stable operation during which the instrument was exposed at float altitude (∼125 000 ft) to high-energy cosmic rays. We present the performance of the instrument as determined from the analysis of these data and an appraisal of its novel design features. Suggestions for design improvements in a future instrument are made.