The neutron-rich isotopes of cosmic-ray neon and magnesium

A balloon-borne detector flown during the period of minimum activity in the last solar cycle has been used to observe the isotopes of Ne, Na, Mg, and Al in the cosmic radiation, over an energy range approx.390--530 MeV/n. The detector has a mass resolution of 0.4--0.55 amu in this charge range and was calibrated in flight by fast particles. While the Al and Na nuclei are consistent with essentially pure 23Na and 27Al beams, both Ne and Mg show appreciable abundances of neutron-rich isotopes. We find relative abundances corrected to the top of the atmosphere for 20Ne:21Ne:22Ne of 57:17:26 and for 24Mg:25Mg:26Mg of 59:21:20. The neon abundances are in good agreement with those found from three different satellite experiments. The abundances of magnesium are more controversial, and these measurements add evidence that magnesium is also neutron rich. Our ability to reproduce the neon abundances found by others enhances our confidence in the measurement of magnesium, but the statistical weight is still marginal. These abundances, corrected for interstellar propagation, appear significantly different from those of solar-system material, and reflect the special nature of the cosmic-ray sources