Composition and energy spectra of cosmic-ray primaries in the energy range10(13)-10(15) eV/particle observed by Japanese-Russian joint balloon experiment

We report experimental results obtained by the emulsion chambers on board o f the long duration balloon. We have been carrying out the trans-Siberian-c ontinental balloon flight since 1995, and the results from 1995 to 1996 exp eriments are presented here. Total exposure of these two years amounts to 2 31.5 m(2) h at the average altitude of similar to 32 km. The energy range covers 10-500 TeV for proton-primary, 3-70 TeV/n for heliu m-primary, and 1-5 TeV/n for Fe-group (Z = 26-28), though statistics of hea vy components is not yet enough. Our preliminary data show that the spectra of the proton and the helium have nearly the same power indices similar to 2.80, while those of heavier ones become gradually harder as the mass gets heavier, for instance the index is similar to2.70 for CNO-group and similar to2.55 for Fe-group. It is remarkable that a very high energy proton with multi-PeV is detected in 1995 experiment, and the estimated flux of this event coincides with a s imple extrapolation from the energy spectrum with the power index 2.8 obser ved in the range 10-500 TeV. It indicates that there is no spectral break a t around 100 TeV, in contrast to the maximum energy predicted by the curren t shock-wave acceleration model. This evidence requires some modification o n the acceleration and/or propagation mechanism. Also we present all-particle spectrum and the average primary mass in the e nergy range 20-1000 TeV/particle. Our preliminary data show no drastic chan ge in mass composition over the wide energy range, at least up to 1 PeV/par ticle, though the statistics is not yet enough to confirm it concretely. The flight performance and the procedure of the analysis, particularly the energy determination methods and the detection efficiency calculation are a lso given. (C) 2001 Elsevier Science B.V. All rights reserved.