Isotopic Composition of the Low-Energy Helium Nuclei in the Primary Cosmic Radiation

Abstract

The isotopic composition of low-energy helium nuclei in the primary cosmic radiation has been determined by using the "constant sagitta" scattering method on tracks of helium nuclei stopping in a nuclear-emulsion stack flown at a geomagnetic latitude $\ensuremath{\lambda}=55\ifmmode^\circ\else\textdegree\fi{}$N and at a mean atmospheric depth of 8.5 g/${\mathrm{cm}}^{2}$; tracks with zenith angles less than 30\ifmmode^\circ\else\textdegree\fi{} were accepted. The ratio of ${\mathrm{He}}^{3}$/(${\mathrm{He}}^{3}$+${\mathrm{He}}^{4}$) for the same energy per nucleon (between 200 and 400 Mev) is found to be 0.41\ifmmode\pm\else\textpm\fi{}0.09 at flight altitude. The correction for production of secondary ${\mathrm{He}}^{3}$ in the residual atmosphere is calculated to be 4%. If one assumes that no ${\mathrm{He}}^{3}$ nuclei are present at the source, the observed ratio corresponds to a traversal of 14\ifmmode\pm\else\textpm\fi{}3 g of interstellar matter by the low-energy helium nuclei. The value of ${\mathrm{He}}^{3}$/(${\mathrm{He}}^{3}$+${\mathrm{He}}^{4}$) corresponding to the same magnetic rigidity (between 1.3 and 1.6 Bv) is found to be 0.36\ifmmode\pm\else\textpm\fi{}0.11 which corresponds to a traversal of 12.2\ifmmode\pm\else\textpm\fi{}3.5 g of interstellar matter. The observed ratio may indicate the presence of ${\mathrm{He}}^{3}$ at the source of cosmic rays, or may be a reflection of local production within the solar system.