Isotope analysis using the geomagnetic method

The data recorded by the C2 instrument on board the HEAO3 satellite improve our understanding of isotopic composition of heavy relativistic cosmic ray nuclei. The C2 counters and hodoscope combination is suitable for an accurate determination of momentum and arrival direction of individual cosmic rays. The next step in data analysis is to assign a cut-off rigidity value to each arrival direction with the help of a magnetic field model and to build the average transmission function for each element. This method is very useful for organizing data collected in a finite cut-off rigidity range, but in spite of its conceptual simplicity it does not allow to easily figure out how systematic and statistical errors propagate. Indeed the geomagnetic method does not separate isotopes in the usual sense: it is based on a comparison of the shape of the transmission to that of reference element. In this paper we describe an analysis of the observed transmission based on its comparison to expectation. For this we have selected oxygen nuclei with directions where the rigidity cut-off is > approximately 7 GV and can be considered sharp. We describe this selection and the cut-off assignment. We present the observed transmission and compare it to that calculated under the assumption of sharp cut-offs. This calculation takes into account the finite resolution of the counters. This allows to estimate to estimate the level of systematic errors. It is also shown that under sharp cut-off selection the transmission function for a single isotope is weakly dependent on charge and does not depend on the cut-off distribution, thus ensuring an unambiguous comparison to the reference element. In conclusion we use this technique to derive a tentative abundance of Neon isotopes