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?

Abstract

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\ensuremath{\gtrsim}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 \ensuremath{\sim}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 \ensuremath{\beta} being nearly constant, \ensuremath{\sim}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, \ensuremath{\beta} being as high as \ensuremath{\sim}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.Our all-particle spectrum deviates significantly from that of the proton satellites beyond 50 TeV/particle, while both agree rather well with each other in the lower energy range. When we investigate individual heavy components, we find that all their respective fluxes multiplied by ${\mathit{E}}_{\mathit{P}}^{2.5}$ (${\mathit{E}}_{\mathit{P}}$ is the primary energy per particle) show a decreasing tendency around \ensuremath{\sim}10 TeV/particle and beyond, no indication of recovery being observed as the energy gets even higher. This means that the heavy components, paricularly iron, do not increase so drastically as to cover the excess in the ``knee'' region. Extrapolation of our all-particle spectrum up to ${10}^{15}$--${10}^{16}$ eV/particle indicates a milder ``knee'' shape than that found by the air shower experiments. If the break is as sharp as hitherto reported, then it will suggest either (i) there is a sharp break which might be due to a drastic advent of new components (other than heavy primary nuclei), or a drastic change in nuclear interaction, or (ii) the break just appears to be sharp due to a \ensuremath{\sim}20% (or more) systematic overestimation in converting the air shower sizes into the primary energy values in the ``knee'' region.