Measurement of the isotopic composition of cosmic-rays at 3 GeV/n using a new geomagnetic method

The French-Danish C2 instrument on board the HEAO 3 satellite has been used to measure the mean masses of N, Ne, Mg, Al, Si, S, Ca, Mn, and Fe in the Galactic cosmic rays, at an average kinetic energy of 3.0 GeV/n. The technique takes advantage of the effect of the geomagnetic field upon the cosmic rays fluxes. A new method is described, in which the geomagnetic field effect is quantitatively computed. Using a simple exponential path length propagation model, the inferred isotopic source ratios Ne-22/Ne-20 and (Mg-25 + Mg-26)/Mg-24 are found significantly greater than the present solar system ratios by factors 2.93 (+0.84, -0.77) and 1.85 (+0.47, -0.35). The (Si-29 + Si-30)/Si-28 source ratio is found to marginally exceed the local Galactic ratio by a factor 1.83 (+0.94, -0.74). The elemental N-14/O source ratio, derived from the estimated N-15 fraction at earth, is 6.7 (+2.2, -2.3) percent, in agreement with the value derived independently from the elemental N/O measurement. These results are compared with low energy measurements, and their implications regarding the sources of cosmic rays are discussed.