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 $^{10}$Be/$^9$Be isotopic flux ratio
(thanks to the 2 My
lifetime of $^{10}$Be) 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 $^{10}$Be/$^9$Be 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 $^{10}$Be/$^9$Be 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 $^{10}$Be/$^9$Be 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 $^{10}$Be decay in cosmic rays; this should improve the knowledge of the $H$ parameter.