Beryllium Radioactive Isotopes as a Probe to Measure the Residence Time of Cosmic Rays in the Galaxy and Halo Thickness: A “Data-Driven” Approach

Abstract

Cosmic rays are a powerful tool for the investigation of the structure of the magnetic fields in the Galactic halo and the properties of the inter-stellar medium. Two parameters of the cosmic ray propagation models, the Galactic halo (half) thickness, H, and the diffusion coefficient, D, are loosely constrained by current cosmic ray flux measurements; in particular, a large degeneracy exists, with only H/D being well measured. The 10Be/9Be isotopic flux ratio (thanks to the 2 My lifetime of 10Be) can be used as a radioactive clock providing the measurement of cosmic ray residence time in a galaxy. This is an important probe with which to solve the H/D degeneracy. Past measurements of 10Be/9Be isotopic flux ratios in cosmic rays are scarce, and were limited to low energy and affected by large uncertainties. Here a new technique to measure 10Be/9Be isotopic flux ratio, with a data-driven approach in magnetic spectrometers is presented. As an example, by applying the method to beryllium events published via PAMELA experiment, it is now possible to determine the important 10Be/9Be measurement while avoiding the prohibitive uncertainties coming from Monte Carlo simulations. It is shown how the accuracy of PAMELA data strengthens the experimental indication for the relativistic time dilation of 10Be decay in cosmic rays; this should improve the knowledge of the H parameter.