Observation of heavy cosmic-ray primaries over the wide energy range from ∼100 GeV/particle to ∼100 TeV/particle: Is the celebrated ‘‘knee’’ actually so prominent?

We have exposed a new type of emulsion chamber of area 1.53 ${\mathrm{m}}^2$ at an atmospheric depth of 11.7 g/${\mathrm{cm}}^2$ for 22.2 h. The chamber makes extensive use of screen-type x-ray films, which have recorded the tracks due to over 100 000 cosmic-ray heavy primary nuclei of Z≳8. With this experiment we have succeeded in determining the absolute intensities of the heavy primaries over a pretty wide energy range from a few GeV/nucleon up to ∼1 TeV/nucleon, using a single detector and a unified charge-and-energy determination method. In the present paper we give a report of our results on silicon and heavier components, accompanied by a detailed account on our newly adopted energy determination method and a discussion of its accuracy. Our iron flux is in agreement with that obtained by Spacelab-2, the integral spectral index β being nearly constant, ∼1.5, up to a few TeV/nucleon. Of peculiar interest is our silicon flux, which is again consistent with the Spacelab-2 result. The energy spectrum gets softer beyond 100 GeV/nucleon, β being as high as ∼1.95 there. Current interstellar acceleration and propagation models will meet difficulty in explaining this result. We also report about the abundance ratio of the subiron group to iron, which is strongly sensitive to the escape length of cosmic rays in the Galaxy, and find that it decreases in the form of power laws over the wide energy range from a few GeV/nucleon to a few TeV/nucleon, though a quantitative study in connection with a particular propagation model is reserved to the future.