Age of cosmic ray nuclei and their propagation in space

Abstract

Information on the origin, age and propagation of cosmic ray nuclei can be obtained from a detailed study of the intensities of Li, Be, and B nuclei in the primary radiation observed in the vicinity of the earth. While at any given energy, the absolute total flux of these nuclei, (Li+Be+B), at the top of the atmosphere will depend almost exclusively on the amount of matter traversed by the radiation, their relative intensities will depend on the extent to which the long lived Be¹⁰ isotopes, produced in spallation reactions in space, have decayed during the life time (or age T) of the cosmic rays. In the present work the dependence of the ratio of the flux of Be nuclei to that of B nuclei on the age of the cosmic rays has been taken advantage of to set a reasonable lower limit to the age of the cosmic ray nuclei; from an experiment performed with nuclei having a geomagnetic cutoff rigidity corresponding to 7.5 GeV/n it is found that T \gtrsim5 ×107 years. This result is then used together with the total amount of matter traversed by cosmic ray nuclei (2.5 g/cm²), obtained from other experiments, to deduce the average gas density of regions traversed by the cosmic rays as \lesssim 0.03 hydrogen atoms/cm³. This very low value of the average density suggests that cosmic ray nuclei are not confined to the disc or spiral arms of our galaxy but reside for the major fraction of their life time in regions of much lower density such as the intergalactic space and the halo of our galaxy, if it exists.