Measurement of Low-Energy Cosmic-Ray Antiprotons at Solar Minimum

The absolute fluxes of the cosmic-ray antiprotons at solar minimum are measured in the energy range 0.18 to 1.4 GeV, based on 43 events unambiguously detected in BESS '95 data. The resultant energy spectrum appears to be flat below 1 GeV, compatible with a possible admixture of primary antiproton component with a soft energy spectrum, while the possibility of secondary antiprotons alone explaining the data cannot be excluded with the present accuracy. Further improvement of statistical accuracy and extension of the energy range are planned in future BESS flights. p's was reported (7,8) based on four events detected in the low-energy range of 0.3 to 0.5 GeV. This measurement (BESS '95) extends the range to 1.4 GeV, and was conducted at a period close to solar minimum. The BESS detector (9) is shown in Fig.1. The thin superconducting coil (10) (4 g/cm 2 thick including the cryostat) produces a uniform axial magnetic field of 1 Tesla. The rφ-tracking in the central region is performed by fitting up to 28 hit-points, each with 200 � m reso- lution, in the JET- and IDC-drift chambers, resulting in a magnetic-rigidity (11) resolution of 0.5 % at 1 GV/c. Tracking in the z-coordinate is done to an accuracy of 300 � m by fitting points in IDC measured with vernier pads and points in the JET chamber measured using charge- division. The continuous and redundant 3-dimensional tracking with the drift chambers, all equipped with multi- hit capacity, enables us to recognize multi-track events and tracks having interactions and scatterings, thus min- imizing the background. The dE/dx of the particle in the JET chamber is obtained as a truncated mean of the integrated charges of the hit-pulses which compose the track. The TOF scintillator hodoscopes, newly built for '95 flight, measure the time-of-flight of particles with a resolution of 110 ps as compared to previous 280 ps. The first-level trigger is provided by a coincidence be-