Superconducting solenoidal spectrometer for a balloon-borne experiment

The BESS detector is a new type of balloon-borne spectrometer which utilizes various technologies recently developed for collider experiments. The principal scientific objectives include a measurement of cosmic-ray antiproton spectrum, search for anti-nuclei in cosmic radiation, and precise measurements of cosmic-ray primaries. A thin superconducting solenoidal coil produces a uniform magnetic field of 1 T. Cylindrical drift chambers are located inside and outside the coil and perform continuous tracking. The momentum resolution is 0.5% at 1 GeV/c. i.e., the maximum detectable rigidity is 200 GV. Scintillation counter hodoscopes, placed above and below the solenoid, provide timing and dE/dx measurements and trigger generation. The timing resolution is 80 ps/counter. This cylindrical configuration achieves a large geometrical acceptance of 0.35 m² sr which is essential to detect rare cosmic-ray particles. In order to cope with high trigger rate and large data size, intelligent trigger circuits and a fast and parallel data acquision system are also developed. In 1993, the first scientific flight of the BESS apparatus was carried out and the first mass-measured antiprotons were detected at energies below 500 MeV.