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

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 $\lambda =55\mathrm{}°$N and at a mean atmospheric depth of 8.5 g/${\mathrm{cm}}^2$; tracks with zenith angles less than 30° 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±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±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±0.11 which corresponds to a traversal of 12.2±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.